The many, many reasons space travel is bad for the human body

Leaving earth upends almost every system inside of us.

After the astronaut Scott Kelly spent a year on the International Space Station, he returned to Earth shorter, more nearsighted, lighter and with new symptoms of heart disease that his identical twin brother did not share. (Mark Kelly, now a U.S. senator, also spent a brief time in space.)

Even their DNA diverged, as nearly 1,000 of Scott Kelly’s genes and chromosomes worked differently. (On the upside, he aged about 9 milliseconds less that year, thanks to how fast the space station circled the Earth.)

Most of these effects cleared up within a few months, but not all — underscoring the potential health hazards of space travel, many of which are unknown. These will ratchet up during ambitious future trips, such as NASA’s planned Artemis mission to the moon and later travel to Mars.

Even a partial list of the likely physical and emotional consequences of deep space travel is daunting.

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Space motion sickness sets in almost immediately. The nausea, dizziness, headaches and confusion can linger for days.

“Puffy Face Bird Leg Phenomenon” develops, as blood and other bodily fluids rush to the upper body in low gravity and stay there, swelling heads and shrinking legs.

Astronauts’ appearances can change as their faces swell. The astronauts may feel congested, as though they have a constant head cold.

Muscles atrophy by as much as 1 percent every week in weightlessness, especially in the legs.

Blood volume drops — and with less blood to pump, the heart weakens and loses its signature heart shape, growing more rounded.

Like any other muscle, the heart doesn’t need to work as hard in microgravity and will begin to atrophy without rigorous exercise.

Doused with radiation, many immune cells die and immunity is lowered. There’s also DNA damage, potentially upping cancer risk.

Inflammation spikes throughout the body, possibly contributing to heart disease and other conditions.

Bones thin by about 1.5 percent a month. Spinal discs harden.

In the head, parts of the eyeball can flatten, causing sharper distance vision and dimmer near vision.

Fluids flood the skull, diminishing smell and hearing.

Gene activity changes, including in the brain. In mice, 54 different genes in the brain worked differently after weeks in space.

Brain cells can be affected by radiation, diminishing memory and thinking (in mice).

Circadian rhythms falter, making insomnia common.

Finally, months or years of solitude — or close confinement with fellow astronauts — can lead to lasting psychological stress.

“Space is just not very hospitable to the human body,” said Emmanuel Urquieta, chief medical officer at the Translational Research Institute for Space Health in Houston, which partners with NASA to study the effects of deep space exploration.

Humans evolved in conditions of plentiful gravity and relatively slight background radiation, he said. Space is the reverse and it upends the operations of almost every biological system inside of us. —

Most of the potential health risks of space travel can be mitigated to some extent, scientists point out. Exercise, for instance, “is quite effective” at helping astronauts maintain muscle mass and bone density, said Lori Ploutz-Snyder, the dean of the University of Michigan School of Kinesiology. She was previously a researcher at NASA, where she led studies of exercise and space travel.

The New Space Age

On the space station, astronauts routinely work out for about an hour most days, she said, using specialized devices to run, cycle and lift weights, despite being weightless. But on lunar and Mars missions, which will involve smaller ships and possibly years-long durations, exercise equipment will need to be shrunk and astronauts’ willingness to keep up with the workouts enlarged.

[ To counter the effect of sitting too much, try the astronaut workout ]

The Earth’s magnetic field shields the relatively close-in space station as well from some of the worst deep-space radiation, but the lunar and Mars missions — higher and farther from Earth — will not enjoy that protection.

The moon and Mars journeys will demand advanced shielding, Urquieta said, together with drugs and supplements that might lessen some of the internal effects of the remaining — and inevitable — radiation. Antioxidants, such as vitamins C and E, could sop up a portion of the damaging molecules released after radiation exposure, while other protective drugs and nutrients are under investigation, he said.

Despite every available precaution and protection, deep space will remain a harsh, unwelcoming place for the human body. But it will also, and always, represent something else for the human imagination, Urquieta said — its endless sweep of sequined darkness sparking our ambitions, dreams and stories.

Which is why, even knowing better than most people the toll such a trip might take on him, he would go into space “in a heartbeat,” he said. “Absolutely. No question. It’s so inspiring. It’s space."

About this story

Additional design and development by Betty Chavarria. Editing by Kate Rabinowitz, Manuel Canales and Jeff Dooley. Copy editing by Wayne Lockwood.

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Spending time in space can harm the human body − but scientists are working to mitigate these risks before sending people to Mars

5 dangers of space travel

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When 17 people were in orbit around the Earth all at the same time on May 30, 2023, it set a record. With NASA and other federal space agencies planning more manned missions and commercial companies bringing people to space, opportunities for human space travel are rapidly expanding.

However, traveling to space poses risks to the human body. Since NASA wants to send a manned mission to Mars in the 2030s, scientists need to find solutions for these hazards sooner rather than later.

As a kinesiologist who works with astronauts, I’ve spent years studying the effects space can have on the body and brain. I’m also involved in a NASA project that aims to mitigate the health hazards that participants of a future mission to Mars might face.

Space radiation

The Earth has a protective shield called a magnetosphere , which is the area of space around a planet that is controlled by its magnetic field . This shield filters out cosmic radiation . However, astronauts traveling farther than the International Space Station will face continuous exposure to this radiation – equivalent to between 150 and 6,000 chest X-rays .

This radiation can harm the nervous and cardiovascular systems including heart and arteries , leading to cardiovascular disease. In addition, it can make the blood-brain barrier leak . This can expose the brain to chemicals and proteins that are harmful to it – compounds that are safe in the blood but toxic to the brain.

NASA is developing technology that can shield travelers on a Mars mission from radiation by building deflecting materials such as Kevlar and polyethylene into space vehicles and spacesuits . Certain diets and supplements such as enterade may also minimize the effects of radiation. Supplements like this, also used in cancer patients on Earth during radiation therapy, can alleviate gastrointestinal side effects of radiation exposure.

Gravitational changes

Astronauts have to exercise in space to minimize the muscle loss they’ll face after a long mission. Missions that go as far as Mars will have to make sure astronauts have supplements such as bisphosphonate , which is used to prevent bone breakdown in osteoporosis. These supplements should keep their muscles and bones in good condition over long periods of time spent without the effects of Earth’s gravity .

Microgravity also affects the nervous and circulatory systems. On Earth, your heart pumps blood upward, and specialized valves in your circulatory system keep bodily fluids from pooling at your feet. In the absence of gravity, fluids shift toward the head.

My work and that of others has shown that this results in an expansion of fluid-filled spaces in the middle of the brain. Having extra fluid in the skull and no gravity to “hold the brain down” causes the brain to sit higher in the skull , compressing the top of the brain against the inside of the skull.

A man wearing a white headset and a suit which has many wires coming out of it and a plastic panel connected to a laptop.

These fluid shifts may contribute to spaceflight associated neuro-ocular syndrome , a condition experienced by many astronauts that affects the structure and function of the eyes . The back of the eye can become flattened, and the nerves that carry visual information from the eye to the brain swell and bend. Astronauts can still see, though visual function may worsen for some. Though it hasn’t been well studied yet, case studies suggest this condition may persist even a few years after returning to Earth.

Scientists may be able to shift the fluids back toward the lower body using specialized “pants ” that pull fluids back down toward the lower body like a vacuum. These pants could be used to redistribute the body’s fluids in a way that is more similar to what occurs on Earth.

Mental health and isolation

While space travel can damage the body, the isolating nature of space travel can also have profound effects on the mind .

Imagine having to live and work with the same small group of people, without being able to see your family or friends for months on end. To learn to manage extreme environments and maintain communication and leadership dynamics, astronauts first undergo team training on Earth.

They spend weeks in either NASA’s Extreme Environment Mission Operations at the Aquarius Research Station , found underwater off the Florida Keys, or mapping and exploring caves with the European Space Agency’s CAVES program . These programs help astronauts build camaraderie with their teammates and learn how to manage stress and loneliness in a hostile, faraway environment.

Researchers are studying how to best monitor and support behavioral mental health under these extreme and isolating conditions.

While space travel comes with stressors and the potential for loneliness, astronauts describe experiencing an overview effect : a sense of awe and connectedness with all humankind. This often happens when viewing Earth from the International Space Station.

The Earth, half-obscured by shadow, as seen hanging in darkness, from the Moon.

Learning how to support human health and physiology in space also has numerous benefits for life on Earth . For example, products that shield astronauts from space radiation and counter its harmful effects on our body can also treat cancer patients receiving radiation treatments.

Understanding how to protect our bones and muscles in microgravity could improve how doctors treat the frailty that often accompanies aging. And space exploration has led to many technological achievements advancing water purification and satellite systems .

Researchers like me who study ways to preserve astronaut health expect our work will benefit people both in space and here at home.

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  • Published: 05 November 2020

Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars

  • Zarana S. Patel   ORCID: orcid.org/0000-0003-0996-6381 1 , 2 ,
  • Tyson J. Brunstetter 3 ,
  • William J. Tarver 2 ,
  • Alexandra M. Whitmire 2 ,
  • Sara R. Zwart 2 , 4 ,
  • Scott M. Smith 2 &
  • Janice L. Huff   ORCID: orcid.org/0000-0003-4236-7698 5  

npj Microgravity volume  6 , Article number:  33 ( 2020 ) Cite this article

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  • Cardiovascular diseases
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NASA’s plans for space exploration include a return to the Moon to stay—boots back on the lunar surface with an orbital outpost. This station will be a launch point for voyages to destinations further away in our solar system, including journeys to the red planet Mars. To ensure success of these missions, health and performance risks associated with the unique hazards of spaceflight must be adequately controlled. These hazards—space radiation, altered gravity fields, isolation and confinement, closed environments, and distance from Earth—are linked with over 30 human health risks as documented by NASA’s Human Research Program. The programmatic goal is to develop the tools and technologies to adequately mitigate, control, or accept these risks. The risks ranked as “red” have the highest priority based on both the likelihood of occurrence and the severity of their impact on human health, performance in mission, and long-term quality of life. These include: (1) space radiation health effects of cancer, cardiovascular disease, and cognitive decrements (2) Spaceflight-Associated Neuro-ocular Syndrome (3) behavioral health and performance decrements, and (4) inadequate food and nutrition. Evaluation of the hazards and risks in terms of the space exposome—the total sum of spaceflight and lifetime exposures and how they relate to genetics and determine the whole-body outcome—will provide a comprehensive picture of risk profiles for individual astronauts. In this review, we provide a primer on these “red” risks for the research community. The aim is to inform the development of studies and projects with high potential for generating both new knowledge and technologies to assist with mitigating multisystem risks to crew health during exploratory missions.

Introduction

Spaceflight is a dangerous and demanding endeavor with unique hazards and technical challenges. Ensuring the overall safety of the crew—their physical and mental health and well-being—are vital for mission success. These are large challenges that are further amplified as exploration campaigns extend to greater distances into our solar system and for longer durations. The major health hazards of spaceflight include higher levels of damaging radiation, altered gravity fields, long periods of isolation and confinement, a closed and potentially hostile living environment, and the stress associated with being a long distance from mother Earth. Each of these threats is associated with its own set of physiological and performance risks to the crew (Fig. 1a ) that must be adequately characterized and sufficiently mitigated. Crews do not experience these stressors independently, so it is important to also consider their combined impact on human physiology and performance. This “space exposome” is a unifying framework that reflects the interaction of all the environmental impacts on the human body (Fig. 1b ) and, when combined with individual genetics, will shape the outcomes of space travel on the human system 1 , 2 .

figure 1

a The key threats to human health and performance associated with spaceflight are radiation, altered gravity fields, hostile and closed environments, distance from Earth, and isolation and confinement. From these five hazards stem the health and performance risks studied by NASA’s Human Research Program. b The space exposome considers the summation of an individual’s environmental exposures and their interaction with individual factors such as age, sex, genomics, etc. - these interactions are ultimately responsible for risks to the human system. Images used in this figure are courtesy of NASA.

The NASA Human Research Program (HRP) aims to develop and provide the knowledge base, technologies, and countermeasure strategies that will permit safe and successful human spaceflight. With agency resources and planning directed toward extended missions both within low Earth orbit (LEO) and outside LEO (including cis-lunar space, lunar surface operations, a lunar outpost, and exploration of Mars) 3 , HRP research and development efforts are focused on mitigation of over 30 categories of health risks relevant to these missions. The HRP’s current research strategy, portfolio, and evidence base are described in the HRP Integrated Research Plan (IRP) and are available online in the Human Research Roadmap, a managed tool used to convey these plans ( https://humanresearchroadmap.nasa.gov/ ). To determine research priorities, NASA uses an evidence-based risk approach to assess the likelihood and consequence (LxC), which gauges the level of each risk for a set of standard design reference missions (Fig. 2 ) 4 . Risks are assigned a rating for their potential to impact in-mission crew health and performance and for their potential to impact long-term health outcomes and quality of life. “Red” risks are those that are considered the highest priority due to their greatest likelihood of occurrence and their association with the most significant risks to crew health and performance for a given design reference mission (DRM). Risks rated “yellow” are considered medium level risks and are either accepted due to a very low probability of occurrence, require in-mission monitoring to be accepted, or require refinement of standards or mitigation strategies in order to be accepted. Risks rated “green” are considered sufficiently controlled either due to lower likelihood and consequence or because the current knowledge base provides sufficient mitigation strategies to control the risk to an acceptable level for that DRM. Milestones and planned program deliverables intended to move a risk rating to an acceptable, controlled level are detailed in a format known as the path to risk reduction (PRR) and are developed for each of the identified risks. The most recent IRP and PRR documents are useful resources for investigators during the development of relevant research approaches and proposals intended for submission to NASA HRP research announcements ( https://humanresearchroadmap.nasa.gov/Documents/IRP_Rev-Current.pdf ).

figure 2

NASA uses an evidence-based approach to assess likelihood and consequence for each documented human system risk. The matrix used for classifying and prioritizing human system risks has two sets of consequences—the left side shows consequences for in-mission risks while the right side is used to evaluate long-term health consequences (Romero and Francisco) 4 .

This work reviews HRP-defined high priority “red” risks for crew health on exploration missions: (1) space radiation health effects that include cancer, cardiovascular disease, and cognitive decrements (2) Spaceflight-Associated Neuro-ocular Syndrome (3) behavioral health and performance decrements, and (4) inadequate food and nutrition. The approaches used to address these risks are described with the aim of informing potential NASA proposers on the challenges and high priority risks to crew health and performance present in the spaceflight environment. This should serve as a primer to help individual proposers develop projects with high potential for generating both new knowledge and technology to assist with mitigating risks to crew health during exploratory missions.

Space radiation health risks

Outside of the Earth’s protective magnetosphere, crews are exposed to pervasive, low dose-rate galactic cosmic rays (GCR) and to intermittent solar particle events (SPEs) 5 . Exposures from GCR are from high charge (Z) and energy (HZE) ions, high-energy protons, and secondary protons, neutrons, and fragments produced by interactions with spacecraft shielding and human tissues. The main components of an SPE are low-to-medium energy protons. In LEO, the exposures are from GCR modulated by the Earth’s magnetic field and from trapped protons in the South Atlantic Anomaly. The absorbed doses for crews on the International Space Station (ISS) on 6- to 12-month missions range from ~30 to 120 mGy. Outside of LEO, without the protection offered by the Earth’s magnetosphere, absorbed radiation doses will be significantly higher. Estimates for a 1 year stay on the lunar surface range from 100 to 120 mGy, and 300 to 450 mGy for an ~3-year Mars mission (transit and surface stay) 6 . The exact dose a crewmember will receive is highly dependent on exact parameters of a given mission, such as detailed vehicle and habitat designs, and mission location and duration 7 . Time in the solar cycle is also a large factor contributing to crew exposure, with highest GCR exposure occurring during periods of minimum solar activity. The lowest GCR exposures occur during periods of maximum solar activity when the heightened magnetic activity of the Sun diverts some cosmic rays; however, during maximum solar activity, the probability of an SPE is higher 8 , 9 . SPEs, which vary in the magnitude and frequency, will obviously also contribute to total mission doses so it is important to note that total mission exposures are only estimates. Further information on the space radiation environment that astronauts will experience is discussed in Simonsen et al. 5 and Durante and Cucinotta 10 .

An important consideration for risk assessment is that the types of radiation encountered in space are very different from the types of radiation exposure we are familiar with here on Earth. HZE ions, although a small fraction of the overall GCR spectrum compared to protons, are more biologically damaging. They differ from terrestrial forms of radiation, such as X-rays and gamma-rays, in both the amount (dose) of exposure as well as in the patterns of DNA double-strand breaks and oxidative damage that they impart as they traverse through tissue and cells (Fig. 3 ) 5 . The highly energetic HZE particles produce complex DNA lesions with clustered double-stranded and single-stranded DNA breaks that are difficult to repair. This damage leads to distinct cellular behavior and intracellular signaling patterns that may be associated with altered disease outcomes compared to those for terrestrial sources of radiation 11 , 12 , 13 . As an example, persistently high levels of oxidative damage are observed in the intestine from mice examined 1 year after exposure to 56 Fe-ion radiation compared to gamma radiation and unirradiated controls 14 , 15 . The higher levels of residual oxidative damage in HZE ion-irradiated tissue is significant because of the association of oxidative stress and damage with the etiology of many human diseases, including cancer, cardiovascular and late neurodegenerative disorders. These types of alterations are believed to contribute to the higher biological effectiveness of HZE particles 10 , 11 .

figure 3

a HZE ions produce dense ionization along the particle track as they traverse a tissue and impart distinct patterns of DNA damage compared to terrestrial radiation such as X-rays. γH2AX foci (green) illuminate distinct patterns of DNA double-strand breaks in nuclei of human fibroblast cells after exposure to b gamma-rays, with diffuse damage, and c HZE ions with single tracks. Image credits: NASA ( a ) and Cucinotta and Durante 97 ( b and c ).

Within the HRP, the Space Radiation Element (SRE) has developed a research strategy involving both vertical translation and horizontal integration, as well as products focused on mitigating space radiation risks across all phases of a mission. Vertical translation involves the integration of benchtop research with preclinical studies and clinical data. Horizontal integration involves a multidisciplinary approach that includes a range of expertize from physicians to clinicians, epidemiologists to computational modelers 16 . The suite of tools includes computational models of the space radiation environment, mission design tools, models for risk projection, and tools and technologies for accurate simulation of the space radiation environment for radiobiology investigations. Ongoing research is focused on radiation quality, age, sex, and healthy worker effects, medical countermeasures to reduce or eliminate space radiation health risks, understanding the complex nature of individual sensitivity, identification and validation of biomarkers (translational, surrogate, predictive, etc.) and integration of personalized risk assessment and mitigation approaches. Owing to the lack of human data for heavy ion exposure on Earth and the complications of obtaining reliable data for space radiation health effects from flight studies, SRE conducts research at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory. The NSRL is a ground-based analog for space radiation, where a beamline and associated experimental facilities are dedicated to the radiobiology and physics of a range of ions from proton and helium ions to the typical GCR ions such as carbon, silicon, titanium, oxygen, and iron 5 , 17 , 18 .

Radiation carcinogenesis

Central evidence for association between radiation exposure and the development of cancer and other non-cancer health effects comes from epidemiological studies of humans exposed to radiation 19 , 20 , 21 , 22 . Scaling factors are used by NASA and other space agencies in the analysis of cancer (and other risks) to account for differences between terrestrial radiation exposures and cosmic radiation exposures 23 . The risk of radiation carcinogenesis is considered a “red” risk for exploration-class missions due to both the high likelihood of occurrence, as well as the high potential for detrimental impact on both quality of life and disease-free survival post flight. The major cancers of concern are epithelial in origin (particularly cancers of the lung, breast, stomach, colon, and bladder), as well as leukemias ( https://humanresearchroadmap.nasa.gov/Evidence/reports/Cancer.pdf ). Owing to the lack of human epidemiology directly relevant to the types of radiation found in space, current research utilizes a translational approach that incorporates rodent and advanced human cell-based model systems exposed to space radiation simulants along with comparison of molecular pathways across these systems to the human.

A key question that impacts risk assessment and mitigation is how HZE tumors compare to either radiogenic tumors induced by ground-based radiation or spontaneous tumors. As a unifying concept, NASA studies have sought to examine how space radiation exposure modifies the key genetic and epigenetic modifications noted as the hallmarks of cancer (Fig. 4 ) 24 , 25 , 26 , 27 . This approach provides data for development of translational scaling factors (relative biological effectiveness values, quality factors, dose-rate effectiveness factor) to relate the biological effects of space radiation to effects from similar exposures to ground-based gamma- and X-rays and extrapolation of results to large human epidemiology cohorts. It also supports acquisition of mechanistic information required for successful identification and implementation of medical countermeasure strategies to lower this risk to an acceptable posture for space exploration, and it is relevant for the future development of biologically based dose-response models and integrated systems biology approaches 25 . Cancer is a long-term health risk and although it is rated as “red”, most research in this area is currently delayed, as HRP research priorities focus on in-mission risks.

figure 4

Shown are the enabling characteristics and possible mechanisms of radiation damage that lead to these changes observed in all human tumors. (Adapted from Hanahanand Weinberg) 24 .

Risk of cardiovascular disease and other degenerative tissue effects from radiation exposure and secondary spaceflight stressors

A large number of degenerative tissue (non-cancer) adverse health outcomes are associated with terrestrial radiation exposure, including cardiovascular and cerebrovascular diseases, cataracts, digestive and endocrine disorders, immune system decrements, and respiratory dysfunction ( https://humanresearchroadmap.nasa.gov/Evidence/other/Degen.pdf ). For cardiovascular disease (CVD), a majority of the evidence comes from radiotherapy cohorts receiving high-dose mediastinal exposures that are associated with an increased risk for heart attack and stroke 28 . Recent evidence shows risk at lower doses (<0.5 Gy), with an estimated latency of 10 years or more 29 , 30 , 31 . For a Mars mission, preliminary estimates suggest that circulatory disease risk may increase the risk of exposure induced death by ~40% compared to cancer alone 32 . NASA is also concerned about in-flight risks to the cardiovascular system ( https://humanresearchroadmap.nasa.gov/Evidence/other/Arrhythmia.pdf ), when considering the combined effects of radiation exposure and other spaceflight hazards (Fig. 5 ) 33 . The Space Radiation Element is focused on accumulating data specific to the space radiation environment to characterize and quantify the magnitude of the degenerative disease risks. The current efforts are on establishing dose thresholds, understanding the impact of dose-rate and radiation quality effects, uncovering mechanisms and pathways of radiation-associated cardiovascular and cerebrovascular diseases, and subsequent risk modeling for astronauts. Uncovering the mechanistic underpinnings governing disease processes supports the development of specific diagnostic and therapeutic approaches, is a necessary step in the translation of insights from animal models to humans, and is the basis of personalized medicine approaches.

figure 5

In blue are the known risk factors for CVD and in black are the other spaceflight stressors that may also contribute to disease development. Image used in this figure is courtesy of NASA.

This information will provide a means to reduce the uncertainty in current permissible exposure limits (PELs), quantify the impact to disease-free survival years, and determine if additional protection or mitigation strategies are required. The research portfolio includes evaluation of current clinical standard-of-care biomarkers for their relevance as surrogate endpoints for radiation-induced disease outcomes. Studies are also addressing the possible role of chronic inflammation and increased oxidative stress in the etiology of radiation-induced CVD, as well as identification of key events in disease pathways, like endothelial dysfunction, that will guide the most effective medical countermeasures. Products include validated space radiation PELs, models to quantify the risk of CVD for the astronaut cohort, and countermeasures and evidence to inform development of appropriate recommendations to clinical guidelines for diagnosis and mitigation of this risk.

Elucidating the role that radiation plays in degenerative disease risks is problematic because multiple factors, including lifestyle and genetic influences, are believed to play a major role in the etiology of these diseases. This confounds epidemiological analyses, making it difficult to detect significant differences from background disease without a large study population 34 . This issue is especially significant in astronaut cohorts because those studies have small sample sizes 35 . There is also a general lack of experimental data that specifically addresses the role of radiation at low, space-relevant doses 36 . Selection of experimental models needs to be carefully considered and planned to ensure that the cardiovascular disease mechanisms and study endpoints are clinically relevant and translatable to humans 37 , 38 . Combined approaches using data from wildtype and genetically modified animal models with accelerated disease development will likely be necessary to elucidate mechanisms and generate the body of knowledge required for development of accurate permissible exposure limits, risk assessment models, and to develop effective mitigation approaches.

Risk of acute (in-flight) and late CNS effects from space radiation exposure

The possibility of acute (in-flight) and late risks to the central nervous system (CNS) from GCR and SPEs are concerns for human exploration of space ( https://humanresearchroadmap.nasa.gov/Evidence/reports/CNS.pdf ). Acute CNS risks may include altered neurocognitive function, impaired motor function, and neurobehavioral changes, all of which may affect human health and performance during a mission. Late CNS risks may include neurological disorders such as Alzheimer’s disease, dementia, or accelerated aging. Detrimental CNS changes from radiation exposure are observed in humans treated with high doses of gamma-rays or proton beams and are supported by a large body of experimental evidence showing neurocognitive and behavioral effects in animal models exposed to lower doses of HZE ions. Rodent studies conducted with HZE ions at low, mission-relevant doses and time frames show a variety of structural and functional alterations to neurons and neural circuits with associated performance deficits 39 , 40 , 41 , 42 , 43 , 44 . Fig. 6 shows an example of changes in dendritic spine density following HZE ion radiation. However, the significance and relationship of these results to adverse outcomes in astronauts is unclear, as similar decrements are not seen with comparable doses of terrestrial radiation. Therefore, scaling to human epidemiology data, as is done for cancer and cardiovascular disease, is not possible. It is also important to note that to date, no radiation-associated clinically significant operational or long-term deficits have been identified in astronauts receiving similar doses via long-duration ISS missions. It is clear that further development of standardized translational models, research paradigms, and appropriate scaling approaches are required to determine significance in humans 45 , 46 . In addition, elucidation of how space radiation interacts with other mission hazards to impact neurocognitive and behavioral health and performance is critical to defining appropriate PELs and countermeasure strategies. The current research approach is a combined effort of SRE, the human factors and behavioral performance element, and the human health countermeasures element in support of an integrated CBS (CNS/behavioral medicine/sensorimotor) plan ( https://humanresearchroadmap.nasa.gov/Risks/risk.aspx?i=99 ). Further information on this risk area is presented below in the Behavioral Health and Performance section and can also be found at the Human Research Roadmap.

figure 6

Representative digital images of 3D reconstructed dendritic segments (green) containing spines (red) in unirradiated (0 cGy) and irradiated (5 and 30 cGy) mice brains. Multiple comparisons show that total spine numbers (left bar chart) and spine density (right bar chart) are significantly reduced after exposure to 5 or 30 cGy of 16 O particles. Data are expressed as mean ± SEM. * P  < 0.05, ** P  < 0.01 versus control; ANOVA. Adapted from Parihar et al. 39 . Permission to reproduce open-source figure per the Creative Commons Attribution 4.0 International License. https://creativecommons.org/licenses/by/4.0 .

To summarize, the health risks posed by the omnipresent exposure to space radiation are significant and include the “red” risks of cancer, cardiovascular diseases, and cognitive and behavioral decrements. While research on the late health risk of cancer is currently delayed, research on the in-flight effects of radiation on the cardiovascular system and CNS within the context of the space exposome are considered the highest priority and are the focus of investigations. Major knowledge gaps include the effects of radiation quality, dose-rate, and translation from animal models to human systems and evaluation of the requirement for medical countermeasure approaches to reduce the risk.

Spaceflight-Associated Neuro-ocular Syndrome

The Risk of Spaceflight-Associated Neuro-ocular Syndrome (SANS), originally termed the Risk of Vision Impairment Intracranial Pressure (VIIP), was first discovered about 15 years ago. VIIP was the original name used because the syndrome most noticeably affects a crewmember’s eyes and vision, and its signs can appear like those of the terrestrial condition idiopathic intracranial hypertension (IIH; which is due to increased intracranial pressure). Over time, it was realized that the VIIP name required an update. Most notably, SANS is not associated with the classic symptoms of increased intracranial pressure in IIH (e.g., severe headaches, transient vision obscurations, double vision, pulsatile tinnitus), and it has never induced vision changes that meet the definition of vision impairment, as defined by the National Eye Institute. In 2017, VIIP was renamed to SANS, a term that welcomes additional pathogenesis theories and serves as a reminder that this syndrome could affect the CNS well beyond the retina and optic nerve.

SANS presents with an array of signs, as documented in the HRP Evidence Report ( https://humanresearchroadmap.nasa.gov/evidence/reports/SANS.pdf ). Primarily, these include edema (swelling) of the optic disc and retinal nerve fiber layer (RNFL), chorioretinal folds (wrinkles in the retina), globe flattening, and refractive error shifts 47 . Flight duration is thought to play a role in the pathogenesis of SANS, as nearly all cases have been diagnosed during or immediately after long-duration spaceflight (i.e., missions of 30 days duration or longer), although signs have been discovered as early as mission day 10 48 . Because of SANS, ocular data are nominally collected during ISS missions. For most ISS crewmembers, this testing includes optical coherence tomography (OCT), retinal imaging, visual acuity, a vision symptom questionnaire, Amsler grid, and ocular ultrasound (Fig. 7 ).

figure 7

Image courtesy of NASA.

From a short-term perspective (e.g., a 6-month ISS deployment), SANS presents four main risks to crewmembers and their mission: optic disc edema (ODE), chorioretinal folds, shifts in refractive error, and globe flattening 49 . Approximately 69% of the US crewmembers on the ISS experience a > 20 µm increase in peripapillary retinal thickness in at least one eye, indicating the presence of ODE. With significant levels of ODE, a crewmember can experience an enlargement of his/her blind spots and a corresponding loss in visual function. To date, blind spots are uncommon and have not had an impact on mission performance.

If chorioretinal folds are severe enough and located near the fovea (the retina associated with central vision), a crewmember may experience visual distortions or reduced visual acuity that cannot be corrected with glasses or contact lenses, as noted in the SANS Evidence Report. Despite a prevalence of 15–20% in long-duration crewmembers, chorioretinal folds have not yet impacted astronauts’ visual performance during or after a mission. An on-orbit shift in refractive error is due to a shortening of the eye’s axial length (distance between the cornea and the fovea), and it occurs in about 16% of crewmembers during long-duration spaceflight. This risk is mitigated by providing deploying crewmembers with several pairs of “Space Anticipation Glasses” (or contact lenses) of varying power. On-orbit, the crewmember can then select the appropriate lenses to restore best-corrected visual acuity. Approximately 29% of long-duration crewmembers experience a posterior eyeball flattening, which is typically centered around the insertion of the optic nerve into the globe. Globe flattening can induce chorioretinal folds and shifts in refractive error, posing the respective risks described above.

From a longer-term perspective, SANS presents two main risks to crewmembers: ODE and chorioretinal folds. It is unknown if a multi-year spaceflight (e.g., a Mars mission) will be associated with a higher prevalence, duration, and/or severity of ODE compared to what has been experienced onboard the ISS. Since the retina and optic nerve are part of the CNS, if ODE is severe enough, the crewmember risks a permanent loss of optic nerve and RNFL tissue and thus, a permanent loss of visual function. It should be stressed that no SANS-related permanent loss of visual function has yet been discovered in any astronauts.

For choroidal folds, improvement generally occurs post-flight in affected crewmembers; however, significant folds can persist for 10 or more years after long-duration missions. Using MultiColor Imaging and autofluorescence capabilities of the latest OCT device, it was discovered recently that one crewmember’s longstanding (>5 years) post-flight choroidal folds have induced disruption to its overlying retinal pigment epithelium (RPE) 50 . The RPE is a monolayer of pigmented cells located between the vascular-rich choroid and the photoreceptor outer segments. This layer forms the posterior blood-brain barrier for the retina and is essential for maintaining the health of the posterior retina via the transport of nutrients and fluids, among other key functions. If the RPE is damaged, it could potentially lead to a degeneration of the local retina and progress to vision impairment.

Recent long-duration head-down tilt studies have shown potential for recreating SANS signs in terrestrial cohorts 51 . However, SANS is considered a pathology unique to spaceflight. In microgravity, fluid within the body is free to redistribute uniformly. This means that much of the fluid that normally pools in a person’s feet and legs due to gravity can transfer upward towards the head and cause a general congestion of the cerebral venous system. The central pathogenesis theories of SANS are based on these facts, but the actual cause(s) and pathophysiology of SANS are yet unknown 49 . The most publicized theory for SANS has been that cerebral spinal fluid outflow might be impeded, causing an overall increase in intracranial pressure (ICP) 47 , 52 . Other potential mechanisms (see Fig. 8 ) include cerebral venous congestion or altered folate-dependent 1-carbon metabolism via a cascade of mechanisms that may ultimately increase ICP or affect the response of the eye to fluid shifts 53 , 54 . Potential confounding variables for SANS pathogenesis include resistive exercise, high-sodium dietary intake, and high carbon dioxide levels.

figure 8

Image created with BioRender.com.

Discovering patterns and trends in the SANS population has been difficult due to the relatively low number of crewmembers who have completed long-duration spaceflight. This is especially true for female astronauts. However, there is now enough evidence to state—emphatically—that SANS is not a male-only syndrome. OCT has been utilized onboard the ISS since late 2013, and it has revolutionized NASA’s ability to objectively detect and monitor SANS and build a high-resolution database of retinal and optic nerve head images. Through this technology, it has been recently discovered that that a majority of long-duration astronauts (including females) present with some level of ODE and engorgement of the choroidal vasculature 48 , 55 . The trends and patterns of these ocular anatomical changes may hold the key to deciphering the pathophysiology of SANS 48 , 55 .

Beginning in 2009 in response to SANS, all NASA crewmembers receive pre- and post-flight 3 Tesla magnetic resonance imaging of the brain and orbits. Based on these images, there is growing evidence that brain structural changes also occur during long-duration spaceflight. Most notably, a 10.7–14.6% ventricular enlargement (i.e., approximately a 2–3 ml increase) has been detected in astronauts and cosmonauts by multiple investigators 56 , 57 , 58 , 59 . On-orbit and post-flight cognitive testing have not revealed any systemic cognitive decrements associated with these anatomical changes. Moreover, additional research is required to determine if spaceflight-associated brain structural changes are related to ocular structural changes (i.e., SANS) or if the two are initiated by a common cause. Thus, until a relationship is established, SANS will be defined by ocular signs.

Future SANS medical operations, research, and surveillance will focus on: 1) determining the pathogenesis of the syndrome, 2) developing small-footprint diagnostic devices for expeditionary spaceflight, 3) establishing effective countermeasures, 4) monitoring for any long-term health consequences, and 5) discovering what factors make certain individuals more susceptible to developing the syndrome.

In summary, SANS is a top risk and priority to NASA and HRP. The primary SANS-related risk is ODE, due to the possibility of permanent vision impairment; however, choroidal folds also present a short- and long-term risk to astronaut vision. Shifts in refractive error are relatively common in long-duration missions, but crewmembers do not experience a loss of visual acuity if adequate correction is available. SANS affects female astronauts, not just males, although it is not yet known if SANS prevalence is equal between the sexes. There are no terrestrial pathologies identical to SANS, including IIH. Long-duration spaceflight is also associated with brain anatomical changes; however, it is not yet known whether these changes are related to SANS. Finally, the pathogenesis of SANS remains elusive; however, the main theories are related to increased intracranial pressure, ocular venous congestion, and individual anatomical/genetic variability.

Behavioral health and performance

The Risk of Adverse Cognitive and Behavioral Conditions and Psychiatric Disorders (BMed) focuses on characterizing and mitigating potential decrements in performance and psychological health resulting from multiple spaceflight hazards, including isolation and distance from earth. Spaceflight radiation is also recognized as contributing factor, particularly relative to a deep space planetary mission. The potential of additive or synergistic effects on the CNS resulting from simultaneous exposures to radiation, isolation and confinement, and prolonged weightlessness, is also of emerging concern ( https://humanresearchroadmap.nasa.gov/Risks/risk.aspx?i=99 ).

The official risk statement in the BMed Evidence Report notes, “ given the extended duration of future missions and the isolated, confined and extreme environments, there is a possibility that (a) adverse cognitive or behavioral conditions will occur affecting crew health and performance; and (b) mental disorders could develop should adverse behavioral conditions be undetected and unmitigated ” ( https://humanresearchroadmap.nasa.gov/Evidence/reports/BMED.pdf ). Primary outcomes for this risk include decrements in cognitive function, operational performance, and psychological and behavioral states, with the development of psychiatric disorders representing the least likely but one of the most consequential outcomes crew could experience in extended spaceflight. BMed is considered a “red” risk for planetary missions, given the long-duration of isolation, extended confinement, and exposure to additional stressors, including increased radiation exposure. The Human Factors and Behavioral Performance Element within HRP utilizes a research strategy that incorporates flight studies on astronauts, research in astronaut-like individuals and teams in ground analogs, and works with the Space Radiation Element to use animal models supporting research on combined spaceflight stressors.

While astronauts successfully accomplish their mission objectives and report very positive experiences living and working in space, some anecdotal accounts from current and past astronauts suggest that psychological adaptation in the long-duration spaceflight environment can be challenging. However, clinically significant operational decrements have not been documented to date, as noted in the BMed Evidence Report. Discrete events that have been documented include accounts of adverse responses to workload by Shuttle payload specialists, and descriptions of ‘hostile’ and ‘irritable’ crew in the 84-day Skylab 4 mission, as well as symptoms of depression reported on Mir by 2 of the 7 NASA astronauts.

Currently, potential stressors affiliated with missions to the ISS include extended periods of high workload and/or schedule shifting, physiological adaptation including fluid shifts caused by weightlessness and possibly, exposure to other environmental factors such as elevated carbon dioxide (see the BMed Evidence Report). While still physically isolated from home, the presence of the ISS in LEO facilitates a robust ground behavioral health and performance support team who offer services such as bi-weekly private psychological conferences and regular delivery of novel goods and surprises from home in crew care packages. Coupled with the relatively ample volume in the ISS, near-constant real-time communication with Earth, new crewmembers rotating periodically throughout missions, and relatively low levels of radiation exposure, —it is expected that behavioral challenges experienced today do not represent those that future crews will face during exploration missions.

Nevertheless, the few completed behavioral studies on the ISS suggest that subjective perceptions of stress increase over time for some crewmembers, as shown by an in-flight study collecting subjective ratings of well-being and objective measures of fatigue 60 . Notably, it was found that astronaut ratings of sleep quality and sleep duration (also measured through visual analog scales) were found to be inversely related to ratings of stress. Another in-flight investigation seeking to characterize behavioral responses to spaceflight is the “Journals” study by Stuster 61 . This investigation provided a systematic approach to examining a rich set of qualitative data by evaluating astronaut journal entries for temporal patterns of across different behavioral states over the course of a mission (Fig. 9 ). Based on findings, some categories suggest temporal patterns while other categories of outcomes do not suggest a pattern relative to time, which may be due to no temporal relationship between outcomes and time, and/or various contextual factors within missions that negate the presence of such a relationship (e.g., visiting crew). An overall assessment by Stuster of negative comments relative to positive comments over time suggests evidence of a third quarter phenomenon in Adjustment alone, a category which reflects individual morale 61 .

figure 9

Example bar graph showing distribution of journal entries related to general adjustment to the spaceflight enivronment during each quarter of an ISS mission 61 .

Other in-flight investigations support and expand upon contributors to increased stress on-orbit, including studies documenting reductions in sleep duration 62 , 63 and evaluation of crew responses to habitability and human factors during spaceflight 64 . While no studies have assessed potentially relevant mechanisms for behavioral or other reported symptoms, a recently completed investigation suggests neurostructural changes may be occurring in the spaceflight environment 56 . Magnetic resonance imaging scans were conducted on astronauts pre- and post-flight on both long-duration missions to the ISS or short-duration Shuttle missions. Assessments from a subgroup of participants ( n  = 12) showed a slight upward shift of the brain after all long-duration flights but not after short-duration flights ( n  = 6), and they also showed narrowing of cerebral spinal fluid spaces at the vertex after all long-duration flights ( n  = 6) and in 1 of 6 crew after short-duration flights. A retrospective analysis of free water volume in the frontal, temporal, and occipital lobes before versus after spaceflight suggests alterations in free water distribution 65 . Whether there is a functionally relevant outcome as a result of such changes remains to be determined. Hence, while certain aspects of the spaceflight environment have been shown to increase some behavioral responses (e.g., reduced sleep owing to workload), the direct role of spaceflight-specific factors (such as fluid shifts and weightlessness) on behavioral outcomes or functional performance has not yet been established.

Future long-duration missions will pose threats to behavioral health and performance, such as extreme confinement in a small volume and communication delays, that are distinct from what is currently experienced on missions to the ISS. Analog research is concurrently underway to help further characterize the likelihood and consequence of an adverse behavioral outcome, and the effectiveness of potential countermeasures. Ground analogs, such as the Human Exploration Research Analog (HERA) at NASA Johnson Space Center, provide a test bed where controlled studies of small teams for periods up to 45 days, can be implemented (Fig. 10 ). HERA can be used to provide scenarios and environments analogous to space (e.g., isolation and confinement, communication delays, space food, and daily tasks and schedules) to investigate their effects on behavioral health, human factors, exploration medical capabilities, and communication and autonomy. Research in locations such as Antarctica also offer a unique opportunity to conduct research in less controlled but higher fidelity conditions. In general, these studies show an increased risk in deleterious effects such as decreased mood and increased stress, and in some instances, psychiatric outcomes (see the BMed Evidence Report).

figure 10

HERA is used to simulate environments and mission scenarios analogous to spaceflight to investigate a variety of behavioral and human factors issues. Images courtesy of NASA.

In 2014, Basner and colleagues 62 completed an assessment of crew health and performance in a 520-day mission at an isolation chamber in Moscow at the Institute for Biomedical Problems (IBMP). During this simulated mission to Mars, the crew of six completed behavioral questionnaires and additional testing weekly. One of six (20%) crew reported depressive symptoms based on the Beck Depression Inventory in 93% of mission weeks, which reached mild-to-moderate levels in >10% of mission weeks. Additional indications of changes in mood were observed via the Profile of Mood States. Additionally, two crewmembers who had the highest ratings of stress and physical exhaustion accounted for 85% of the perceived conflicts, and other crew demonstrated dysregulation in their circadian entrainment and sleep difficulties. Two of the six crewmembers reported no adverse behavioral symptoms during the missions 62 . Building on this work, the NASA HRP and the IBMP have ongoing studies in the SIRIUS project, a series of long-duration ground-analog missions for understanding the effects of isolation and confinement on human health and performance ( http://www.nasa.gov/analogs/nek/about ).

Finally, more recent research in the HERA analog at Johnson Space Center is underway to assess not only individual, psychiatric outcomes but also changes in team dynamics and team performance over time (Fig. 10 ). A recent publication reported that conceptual team performance (e.g., creativity) seems to decrease over time, while performance requiring cognitive function and coordinated action improved 66 . While results from additional team studies in HERA are currently under review, the Teams Risk Evidence Report ( https://humanresearchroadmap.nasa.gov/Evidence/reports/Team.pdf ) provides a thorough overview of the evidence surrounding team level outcomes.

In summary, evidence from spaceflight and spaceflight analogs suggests that the BMed Risk poses a high likelihood and high consequence risk for exploration. Given the possible synergistic effects of prolonged isolation and confinement, radiation exposure, and prolonged weightlessness, mitigating such enhanced risks faced by future crews are of highest priority to the NASA HRP.

Inadequate food and nutrition

Historically, nutrition has driven the success—and often the failure—of terrestrial exploration missions. For space explorers, nutrition provides indispensable sustenance, provides potential countermeasures to some of the negative effects of space travel on human physiology, and also presents a multifaceted risk to the health and safety of astronauts ( https://humanresearchroadmap.nasa.gov/Evidence/other/Nutrition-20150105.pdf ).

At a minimum, the need to prevent nutrient deficiencies is absolute. This was proven on voyages during the Age of Sail, where scurvy—caused by vitamin C deficiency— killed more sailors than all other causes of death. On a closed (or even semi-closed) food system, the risk of nutrient deficiency is increased. On ISS missions, arriving vehicles typically bring some fresh fruits and/or vegetables to the crew. While limited in volume and shelf-life, these likely provide a valuable source of nutrients and phytochemicals every month or two. One underlying concern is that availability of these foods may be mitigating nutrition issues of the nominal food system, and without this external source of nutrients on exploration-class missions, those issues will be more likely to surface.

As a cross-cutting science, nutrition interfaces with many, if not all, physiological systems, along with many of the elements associated with space exploration, including the spacecraft environment (Fig. 11 ). Thus, beyond the basics of preventing deficiency of specific nutrients, at best, nutrition can serve as a countermeasure to mitigate risks to other systems. Conversely, at worst, diet and nutrition can exacerbate risks to other physiological systems and crew health. For example, many of the diseases of concern as related to space exploration are nutritionally modifiable on Earth, including cancer, cardiovascular disease, osteoporosis, sarcopenia, and cataracts.

figure 11

Many of the physiological systems and performance characteristics that are touched by nutrition are shown in white text, while the unique elements of spacecraft and space exploration are shown in red text.

The NASA Nutritional Biochemistry Laboratory approaches astronaut health with both operational and research efforts. These efforts aim to keep current crews healthy while working to understand and define optimal nutrition for future crews, to maximize performance and overall health while minimizing damaging effects of spaceflight exposure.

A Clinical Nutrition Assessment is conducted for ISS astronauts dating back to ISS Expedition 1 67 , 68 , which includes pre- and post-flight biochemical analyses conducted on blood and urine samples, along with in-flight monitoring of dietary intake and body mass. The biochemical assessments include a wide swath of nutritional indicators such as vitamins, minerals, proteins, hematology, bone markers, antioxidant markers, general chemistry, and renal stone risk. These data are reported to the flight surgeon soon after collection for use in the clinical care of the astronaut. Initial findings from the Clinical Nutritional Assessment protocol identified evidence of vitamin D deficiency, altered folate status, loss of body mass, increased kidney stone risk, and more 69 , 70 . These initial findings led to several research efforts (described below), including the Nutritional Status Assessment flight project, and research in the Antarctic on vitamin D supplementation 71 , 72 .

In addition to in-flight dietary intake monitoring, research to understand the impact and involvement of nutrition with other spaceflight risks such as bone loss and visual impairments, and interaction with exercise and spacecraft environment, are performed by the Nutrition Team using both flight and ground-analog research efforts. Tracking body mass is a very basic but nonetheless indispensable element of crew health 73 . Loss of body mass during spaceflight and in ground analogs of spaceflight is associated with exacerbated bone and muscle loss, cardiovascular degradation, increased oxidative stress, and more 70 , 73 , 74 . Historically, it was often assumed that some degree of body mass loss was to be expected, and that this was a typical part of adaptation to microgravity. Fluid loss is often assumed to be a key factor, but research has documented this to be a relatively small contributor, of approximately 1% of weight loss being fluid 74 , 75 . While on average, crewmembers on ISS missions have lost body mass over the course of flight, not all do 74 . Importantly, those that did not lose body mass managed to maintain bone mineral density (discussed below) 76 .

Bone loss has long been a concern for space travelers 77 , 78 , 79 , 80 , 81 . It has been shown that an increase in bone resorption was the likely culprit and that bone formation was largely unchanged in microgravity or ground analogs 77 , 78 , 79 . The search for a means to counteract this bone loss, and this hyper-resorptive state specifically, has been extensive. The potential for nutrition to mitigate this bone loss was identified early but studies of increasing intakes of calcium, or fluoride, or phosphate, were unsuccessful 74 , 77 , 79 , 82 , 83 , 84 .

Exercise provides a multisystem countermeasure, and heavy resistive exercise specifically provides for loading of bone to help mitigate weightlessness-induced bone loss.

In evaluating the data from astronauts using the first “interim” resistive exercise device (iRED) on ISS compared to a later, “advanced” resistive exercise device (ARED) (Fig. 12 ), it was quickly realized that exercise was not the only difference in these two groups of astronauts. ARED crews had better dietary intakes (as evidenced by maintenance of body mass) and better vitamin D status as a result of increased dose of supplementation and awareness of the importance of these supplements starting in 2006 76 . Bone mineral density was protected in these astronauts 76 , proving that diet and exercise are a powerful countermeasure combination. Follow-on evaluations showed similar results and further that the effects of microgravity exposure on bone health in men and women were similar 85 despite differences in pre-flight bone mass.

figure 12

Sunita Williams exercising on the iRED ( a ), and on a later mission, Sandy Magnus exercises on the much improved ARED device ( b ). Images courtesy of NASA.

From a purely nutrition perspective, ISS and associated ground analog research has identified several specific dietary effects on bone health. Fish intake, likely secondary to omega-3 fatty acid intake, is beneficial for bone health 86 . Conversely, high intakes of dietary protein 87 , 88 , iron 89 and sodium 90 are detrimental to bone. The mechanism of the effect of protein and sodium on bone are likely similar, with both contributing to the acidogenic potential of the diet, leading to bone dissolution 91 , 92 . This effect was recently documented in a diet and bone health study on ISS, where the acidogenic potential of the diet correlated with post-flight bone losses 93 . The data from terrestrial research, along with the more limited spaceflight research, clearly identifies nutrition as important in maintenance of bone health and in the mitigation of bone loss. While initial evaluations of dietary quality and health are underway at NASA, much work remains to document the full potential of nutrition to mitigate bone loss and other disease processes in space travelers.

Another health risk with nutrition underpinnings is SANS, which was described earlier. When this issue first arose, an examination of data from the aforementioned ISS Nutrition project was conducted. This analysis revealed that affected crewmembers had significantly higher circulating concentrations of homocysteine and other one-carbon pathway metabolites when compared to non-cases and that these differences existed before flight 53 . Many potential confounding factors were ruled out, including: sex, kidney function, vitamin status, and coffee consumption, among others. After identifying differences in one-carbon biochemistry, the next logical step was to examine the genetics—single-nucleotide polymorphisms (SNPs)—involved in this pathway as possible causes of the biochemical differences, but perhaps also their association with the astronaut ocular pathologies. An initial study examined a small set of SNPs—five to be exact—and when the data were statistically modeled, it was found that B-vitamin status and genetics were significant predictors of many of the observed ophthalmic outcomes in astronauts 94 . Interestingly, the same SNPs identified in astronauts to be associated with ophthalmic changes after flight were associated with greater changes in total retina thickness after a strict head-down tilt with 0.5% CO 2 bed rest study 54 . A follow-on study is underway to evaluate a much broader look at one-carbon pathway and associated SNPs, potentially to help better characterize this relationship.

A hypothesis was developed to plausibly link these genetics and biochemical differences with these ophthalmic outcomes, as there is no existing literature regarding such a relationship. This multi-hit hypothesis posits that one-carbon pathway genetics is an indispensable factor, and that the combination with one or more other factors (e.g., fluid shifts, carbon dioxide, radiation, endocrine effects) lead to these pathologies. This has been detailed in a hypothesis paper 95 and in a recent review 96 . In brief, the hypothesis is that genetics and B-vitamin status contribute to endothelial dysfunction, as folate (and other B-vitamins) play critical roles in nitric oxide synthesis and endothelial function. A disruption in nitric oxide synthesis can also lead to an activation of matrix metalloproteinase activation, increasing the turnover and breakdown of structural elements of the sclera, altering retinal elasticity and increasing susceptibility to fluid shifts to induce ophthalmic pathologies like optic disc edema and choroidal folds 54 . This is likely exacerbated cerebrally due to limitations of transport of B-vitamins across the blood-brain barrier. In or around the orbit, endothelial dysfunction, oxidative stress, and potentially individual anatomical differences contribute to leaky blood vessels, and subsequent edema. This can impinge on cerebrospinal fluid drainage from the head, increasing those fluid pressures, which can impinge upon the optic nerve and eye itself, yielding the aforementioned ophthalmic pathologies. These are hypotheses proposed as starting points for further research. Given the irrefutable biochemical and genetic findings to date, this research should be a high priority to either prove or dismiss these as contributing factors in SANS to mitigate that “red” risk.

Another intriguing element from this research is that there is a clinical population that has many of the same characteristics of affected astronauts (or characteristics that they are purported to have), and that is women with polycystic ovary syndrome (PCOS) 95 , 96 . Women with PCOS have higher circulating homocysteine concentrations (as do their siblings and fathers), and also have cardiovascular pathology, including endothelial dysfunction. Studies are underway between NASA and physicians at the Mayo Clinic in Minnesota to evaluate this further. If validated, women with PCOS might represent an analog population for astronaut ocular issues, and research to counteract this could benefit both populations 87 . This research may lead to the identification of one-carbon pathway genetic influences on cardiovascular function in astronauts (and women with PCOS). This information will not be used in any sort of selection process, for several reasons, but as a means to identify countermeasures. Given the effects are intertwined with vitamin status, and likely represent higher individual vitamin requirements, targeted B-vitamin supplementation is the most obvious, and lowest risk, countermeasure that needs to be tested. There is tremendous potential for nutrition research to solve one of the key risks to human health on space exploration missions.

To summarize, nutrition is a cross-cutting field that has influence on virtually every system in the body. While we need to understand nutrition to avoid frank deficiencies, we need to understand how optimizing nutrition might also help mitigate other spaceflight-induced human health risks. Examples of this are myriad, ranging from effects of dietary intake on cognition, performance, and morale, inadequate intake on cardiovascular performance, excess nutrient intakes, leading to excess storage and increased oxidative stress, nutrient insufficiencies, leading to bone loss, insufficient fruit and vegetable intake on bone health, radiation protection, and cardiovascular health, to name a just few. Throughout history, nutrition has served, or failed, many a journey to explore. We need to dare to use and expand our twenty-first century knowledge of nutrition, uniting medical and scientific teams, to enable future exploration beyond LEO, while simultaneously benefitting humanity.

The NASA Human Research Program is focused on developing the tools and technologies needed to control the high priority “red” risks to an acceptable level—a great challenge as the risks do not exist in the vacuum of space as standalone entities. They are inherently interconnected and represent the intersection points where the five hazards of spaceflight overlap, and nature meets nurture. This is the space exposome: the total sum of spaceflight and lifetime exposures and how they relate to individual genetics and determine the whole-body outcome. The space exposome will be an important unifying concept as the hazards and risks of spaceflight are evaluated in a systems biology framework to fully uncover the emergent effects of the extraterrestrial experience on the human body. This framework will provide a path forward for mitigating detrimental health and performance outcomes that may stand in the way of successful, long-duration space travel, especially as NASA plans for a return to the Moon, to stay, and beyond to Mars.

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Acknowledgements

This review was supported in part by a grant to Dr. Patel from the Translational Research Institute for Space Health (TRISH) from the Baylor College of Medicine (The Red Risk School). It was also supported by funding through NASA Human Health and Performance Contract #NNJ15HK11B (Z.S.P., S.R.Z., J.L.H.) and NASA directly (T.J.B., W.J.T., A.M.W., S.M.S., J.L.H.).

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Zarana S. Patel

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Zarana S. Patel, William J. Tarver, Alexandra M. Whitmire, Sara R. Zwart & Scott M. Smith

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Tyson J. Brunstetter

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Sara R. Zwart

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Drs. Z.S.P. and J.L.H. compiled and edited the overall manuscript and drafted the radiation risk overviews. Drs. T.J.B. and W.J.T. drafted the SANS risk overview, Dr. A.M.W. drafted the behavioral health risks overview, and Drs. S.R.Z. and S.M.S. drafted the nutrition risk overview. Data are available upon request.

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Correspondence to Zarana S. Patel .

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Patel, Z.S., Brunstetter, T.J., Tarver, W.J. et al. Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars. npj Microgravity 6 , 33 (2020). https://doi.org/10.1038/s41526-020-00124-6

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5 dangers of space travel

Space Travel: Danger at Every Phase (Infographic)

Infographic: Some of the most harrowing space disasters that have occurred.

Enormous energy. Tremendous acceleration. Lethal environments. Every stage of a space flight is filled with risks. Here are some of the ways that things can go wrong:

Mission Phase 1: Pre-launch Preparations

APOLLO 1, U.S., 1967

Astronauts Gus Grissom, Ed White and Roger Chaffee are sealed into their command module for a routine ground test. When a fire suddenly erupts in the cockpit, the men are unable to open the complex escape hatch in time.

LUNAR LANDING TRAINER VEHICLE CRASH, U.S., 1968

Astronaut Neil Armstrong, rehearsing for his historic moon mission, must eject to safety when his training vehicle malfunctions.

NEDELIN DISASTER, U.S.S.R., 1960 

With 30 minutes remaining before a test launch of Russia’s new R-16 missile, hundreds of technicians and military officers work near the launch pad. Suddenly a rocket engine ignites prematurely. The missile explodes.

DEATHS: 126 estimated

Mission Phase 2: Liftoff and Ascent

SPACE SHUTTLE CHALLENGER, U.S., 1986 

Cold weather causes rubber seals in the booster rockets to become leaky. As the shuttle ascends, escaping flames lick across the huge external fuel tank. When the tank explodes, the orbiter disintegrates. After falling for nearly 3 minutes, the crew cabin crashes into the ocean.

Mission Phase 3: The Vacuum of Space

APOLLO 13, U.S., 1970 

A damaged oxygen tank explodes, crippling the spacecraft. Quick action by the astronauts and by Mission Control allows the use of the moon lander as a lifeboat. Astronauts James Lovell, Fred Haise and Jack Swigert return home safely.

A 3-foot flame breaks out from an oxygen generator, burning for about 14 minutes. Mir’s three crewmen are cut off from one of their two Soyuz escape vehicles. The crew use extinguishers to control the fire. They wear oxygen masks to prevent suffocation in the thick smoke.

VOSKHOD 2, U.S.S.R., 1965

After his historic first-ever spacewalk, cosmonaut Alexei Leonov attempts to return to his capsule. He discovers that his spacesuit has unexpectedly ballooned out, preventing him from entering the hatch. His heart racing, Leonov reduces pressure in his suit until he is able to enter the ship.

SOYUZ 11, U.S.S.R., 1971 

Cosmonauts Georgi Dobrovolski, Viktor Patsayev and Vladislav Volkov undock their Soyuz craft from the Salyut 1 space station. Sections of their vehicle, not needed for the return home, are blasted away by explosive bolts. The shock jams open a valve, allowing all the breathable air to escape into space. Automatic systems return Soyuz 11 to Earth. When rescuers open the hatch, they discover that the crew has suffocated.

GEMINI 8, 1966

A stuck maneuvering thruster sends the capsule rotating at a dizzying rate. Astronauts Neil Armstrong and David Scott are seconds away from blacking out when they manage to shut down the malfunctioning rockets and stop the spin using engines needed for the descent to Earth.

Mission Phase 4: Re-entry and Landing

SPACE SHUTTLE COLUMBIA, 2003

At launch, the shuttle’s heat shield tiles are damaged by falling debris. The damage is not considered serious and the 16-day scientific mission proceeds as planned. When the crew of seven attempts to return home, the heat of re-entry burns through the damaged heat shield. The vehicle is torn apart.

SOYUZ 1, U.S.S.R., 1967

The first flight of the new Soyuz spacecraft did not go well. Several serious failures forced an early end to the mission. Cosmonaut Vladimir Komarov attempted a risky manual re-entry. A malfunctioning parachute does not slow the vehicle, and Komarov crashes to the ground at about 90 mph (140 km/h).

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In Search of Space

How safe is space travel? An in-depth look at the risks and how they’re mitigated

space travel in the future

Space tourism is seen as a glamorous and exciting adventure by many, but what is often forgotten are the significant risks involved in undertaking a journey to outer space. From the dangers of radiation exposure to the potential for collisions with space debris, there are many hazards that can occur during a space mission.  Space tourism may seem like the final frontier, but it’s actually been around for decades. With advances in technology, more and more people are looking to explore outer space. However, with this new frontier comes new risks. How safe is space travel today? And space travel in the future will be more safely? Is it worth the risk? 

The risks associated with spaceflight are numerous and include everything from accidents during launch to collisions with space debris.

In this blog post, we’re going to take a look at the risks involved in space travel and how they’re mitigated. We’ll also discuss the current state of oversight and regulation in the space industry and why it’s important for ensuring safety. 

How Dangerous are Rocket Flights?

One of the biggest dangers associated with space tourism is the risk of accidents during launch. Rockets are incredibly powerful and can cause a lot of damage if something goes wrong. In fact, there have been several major accidents over the years that resulted in the loss of life or significant damage.

The most famous crash occurred on January 27, 1986, when the Space Shuttle Challenger exploded 73 seconds after takeoff, killing all seven crew members on board. The disaster was caused by a faulty O-ring that failed to seal one of the shuttle’s solid rocket boosters, causing hot gas to escape and ignite the external tank.

Other notable accidents include:

  • The Apollo 13 mission, which suffered an explosion en route to the moon in 1970
  • The Soyuz-11 flight, which crashed on its return to Earth in 1971
  • Kalpana Chawla, who was killed in the Columbia disaster while on her second spaceflight in 2003
  • The Soyuz-TMA-14M spacecraft lost pressure during reentry and had to make an emergency landing in Kazakhstan

These disasters illustrate just how dangerous rocket flights can be. Luckily, more recent missions have seen much higher levels of safety with no major accidents since 1986’s Challenger explosion. 

This is largely due to the increased use of automation and computer systems for controlling spacecrafts during lift-off. Most modern rockets are able to launch themselves without any human intervention at all from liftoff through landing back onto Earth’s surface.

However, even though there haven’t been many serious accidents recently – it doesn’t mean space tourism is completely safe! There’s still a lot that could go wrong with these types of missions, including engine failure or loss of communication links between the ground and the spacecraft. But in the same way that there are fewer accidents today than in years ago, with the advancement of technology, space travel in the future will be safer and safer. But let’s continue to analyze the current scenario of space tourism.

What is the Risk of Space Debris?

Space debris can be a major hazard for satellites and astronauts. However, there are many different types of space junk floating around Earth’s orbit. Some examples include:

  • Meteoroids (small pieces of rock or metal that come from outer space) 
  • Asteroids (larger than meteoroids but still small enough not to be considered planets) 
  • Comets (large comets with tails made up of mostly water ice) 
  • Meteorites (pieces of meteoroids that have fallen onto Earth) 
  • Satellites and their parts like solar panels or antennae. This includes both operational satellites as well as defunct ones.

Space junk poses a serious risk for astronauts and spacecraft alike, particularly at low altitudes where it can be difficult to avoid collisions with larger objects, such as orbital debris from previous missions. The International Space Station has had several close calls over the years where pieces of space junk came within meters away before being detected by radar systems on board ISS’s Russian modules!

Accountable to no one?

The space industry is largely unregulated, and there are many safety concerns that have yet to be addressed. Not only space travel in the future, but even now, in our days, needs to have a serious and rigid regulatory system, especially in relation to safety.

For example, the United States has no laws on how much radiation astronauts can receive while in orbit or what kind of training they should receive before going into space. These issues are left up to individual companies who may not prioritize worker health over profit margins. 

There’s also little oversight regarding commercial space travel overall. More than important subject to be re-evaluated for space travel in the future. It’s basically an untapped market right now with no regulations in place at all! This means anyone can set up shop without having their plans properly vetted by experts first – posing a potential threat both financially (as consumers might lose money) but also physically/psychologically as well since people could suffer from illness due to exposure while on board the spacecraft.

The lack of regulation is particularly concerning when it comes to reusable rockets, which use a lot more fuel than traditional launch vehicles but can reduce costs by up to 50%. The increased fuel consumption could lead to explosions or other failures during the flight that may not be detected before lift-off if proper safety measures aren’t taken beforehand. 

Space tourism has always been a risky business – even today! There are many factors at play, and it’s impossible for us humans (or any species) ever truly know what will happen in space until we actually get there ourselves. As technology advances, though, so too does our understanding about how best to mitigate these risks and ensure maximum safety for all those involved, whether they’re on the ground or in orbit.

Current Oversight and Regulation

The space industry is currently overseen by a variety of different agencies, and it can be difficult to keep track of who’s responsible for what.

Some of the key players include:

  • The Federal Aviation Administration (FAA) in the United States is responsible for regulating all aspects of civil aviation, including commercial space travel
  • NASA, as an agency within the Department of Defense, is primarily responsible for civilian and military space exploration
  • Roscosmos is Russia’s federal space agency and oversees all Russian space programs
  • SpaceX is a private American aerospace manufacturer and space transport services company
  • Virgin Galactics is a British commercial spaceflight company founded by Sir Richard Branson

Each agency has its own specific regulations in place, and there’s some overlap as well, particularly when it comes to safety requirements for spacecraft and satellites.

There are also international agreements between countries that have been signed (like the 1967 Outer Space Treaty), which set out guidelines for how we should behave in space – but these treaties are often vague and open to interpretation, leading to further confusion.

So who’s responsible if something goes wrong? As you can see, it’s not always clear! This can lead to delays or even cancellations of launches when different agencies can’t agree on who should be footing the bill. It also makes it difficult to track down what went wrong.

While all these agencies have their own specific areas of expertise, there is often overlap and confusion about who’s in charge when it comes to safety. For example, the FAA has been known to defer regulation of commercial space launches to NASA , which can create a conflict since NASA is also responsible for conducting its own research and development programs.

There are also many different international organizations involved in space travel, such as the United Nations Office for Outer Space Affairs (UNOOSA) and the International Telecommunications Union (ITU). These groups work together to develop treaties and regulations governing outer space activities. However, given that no one agency is really in charge of ensuring overall safety across all industries.

There are also other international organizations like ESA (European Space Agency) that work together on various missions but don’t have any regulatory power themselves. As mentioned earlier, there are many concerns that have yet to be addressed when it comes to space tourism.

So, is space travel really safe? It depends on who you ask! There are many risks and hazards associated with both manned and unmanned missions into outer space, but thanks to advances in technology and safety protocols, the chances of something going wrong have been greatly reduced over the years. And we hope that space travel in the future will be regulated and supervised with the aim of increasing security. With that said, it’s important to keep a close eye on the industry as it continues to grow and evolve – we don’t want another Challenger disaster happening anytime soon.

Suborbital Rocket: How does a suborbital rocket land when it returns after space tourism?

The unfortunate crash of russia’s mir space station: going back to the pages of astronomical history, with product you purchase, subscribe to our mailing list to get the new updates.

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Is It Safe for Humans To Go Up to Space? ISS Experiments Reveal Risks for Future Space Flights

By Osaka Metropolitan University November 19, 2022

Astronaut in Outer Space

Astronauts may be exposed to high energy charged particles from galactic cosmic rays and solar particle events, as well as secondary protons and neutrons after they leave Earth’s protective atmosphere. Because biomolecules, cells, and tissues have different ionization patterns than terrestrial radiation, the associated biological consequences are poorly understood, and the degree of danger involved is subject to enormous uncertainty.

The study in mouse cells analyzed the impact of space radiation and will help scientists better estimate the safety and dangers of space travel.

An international team of scientists conducted a long-term experiment onboard the International Space Station to investigate the impact of space radiation on mouse embryonic stem cells. Their research will help scientists make a more accurate assessment of the risks and safety of space radiation for future human space flights.

The team recently published their findings in the journal Heliyon .

The researchers conducted a direct quantitative evaluation of the biological impact of space radiation in their study by transporting frozen mouse embryonic stem cells from Earth to the International Space Station , subjecting them to space radiation for four years, and quantifying the biological effect by evaluating chromosome aberrations. The results of their experiment demonstrate, for the first time, that the biological impact of space radiation is closely in line with prior predictions derived from the physical measurement of space radiation.

Outline of the “Stem Cells” Space Experiment

Frozen mouse embryonic stem cells were launched from the ground to the International Space Station, stored for a long period of time, recovered on the ground, and examined for chromosome aberrations. Credit: Takashi Morita, OMU

Now that regular people can travel in space, the likelihood of lengthy human missions to distant planets like the Moon and Mars is growing. However, space radiation continues to be a barrier to human exploration. In-depth research has been done by scientists to measure the physical doses of space radiation and better understand how it affects the human body. However, since most previous studies were done on the ground rather than in space, the findings were subject to uncertainty, given that space radiation consists of many different types of particles with varying energies and astronauts are continually irradiated at low dosage rates. On Earth, the space environment cannot be precisely reproduced.

“Our study aims to address the shortcomings of previous ground-based experiments by performing a direct quantitative measurement of the biological effect of space radiation on the International Space Station and comparing this real biological effect with physical estimates in the ground-based experiments,” said Takashi Morita, a professor at the Graduate School of Medicine, Osaka Metropolitan University. “The findings contribute to reducing uncertainties in risk assessments of human space flights.”

The team prepared about 1,500 cryotubes containing highly radio-sensitized mouse embryonic stem cells and sent them to space. Their study was complex in its scope, with seven years of work before launch, four years of work after launch, and five years for analysis. “It was difficult to prepare the experiment and to interpret the results, but we successfully obtained quantitative results related to space radiation, meeting our original objective,” said Professor Morita.

Looking ahead, the researchers hope to take their studies a step further. “For future work, we are considering using human embryonic stem cells rather than mouse embryonic stem cells given that the human cells are much better suited for human risk assessment, and it is easier to analyze chromosome aberrations,” said Professor Morita.

Future studies might also include launching individual mice or other experimental animals to analyze their chromosome aberrations in space. “Such experiments in deep space can further contribute to reducing uncertainties in risk assessments of prolonged human journeys and stays in space,” concluded Professor Morita.

Reference: “Comparison of biological measurement and physical estimates of space radiation in the International Space Station” by Kayo Yoshida, Megumi Hada, Akane Kizu, Kohei Kitada, Kiyomi Eguchi-Kasai, Toshiaki Kokubo, Takeshi Teramura, Sachiko Yano, Hiromi Hashizume Suzuki, Hitomi Watanabe, Gen Kondoh, Aiko Nagamatsu, Premkumar Saganti, Francis A. Cucinotta and Takashi Morita, 17 August 2022, Heliyon . DOI: 10.1016/j.heliyon.2022.e10266

The study was funded by the Japan Aerospace Exploration Agency, the Japan Space Forum, and the Ministry of Education, Culture, Sports, Science, and Technology of Japan. 

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Human Health during Space Travel: State-of-the-Art Review

Chayakrit krittanawong.

1 Department of Medicine and Center for Space Medicine, Section of Cardiology, Baylor College of Medicine, Houston, TX 77030, USA

2 Translational Research Institute for Space Health, Houston, TX 77030, USA

3 Department of Cardiovascular Diseases, New York University School of Medicine, New York, NY 10016, USA

Nitin Kumar Singh

4 Biotechnology and Planetary Protection Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Richard A. Scheuring

5 Flight Medicine, NASA Johnson Space Center, Houston, TX 77058, USA

Emmanuel Urquieta

6 Department of Emergency Medicine and Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Eric M. Bershad

7 Department of Neurology, Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Timothy R. Macaulay

8 KBR, Houston, TX 77002, USA

Scott Kaplin

9 Department of Dermatology, Baylor College of Medicine, Houston, TX 77030, USA

Stephen F. Kry

10 Department of Radiation Physics, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

Thais Russomano

11 InnovaSpace, London SE28 0LZ, UK

Marc Shepanek

12 Office of the Chief Health and Medical Officer, NASA, Washington, DC 20546, USA

Raymond P. Stowe

13 Microgen Laboratories, La Marque, TX 77568, USA

Andrew W. Kirkpatrick

14 Department of Surgery and Critical Care Medicine, University of Calgary, Calgary, AB T2N 1N4, Canada

Timothy J. Broderick

15 Florida Institute for Human and Machine Cognition, Pensacola, FL 32502, USA

Jean D. Sibonga

16 Division of Biomedical Research and Environmental Sciences, NASA Lyndon B. Johnson Space Center, Houston, TX 77058, USA

Andrew G. Lee

17 Department of Ophthalmology, University of Texas Medical Branch School of Medicine, Galveston, TX 77555, USA

18 Department of Ophthalmology, Blanton Eye Institute, Houston Methodist Hospital, Houston, TX 77030, USA

19 Department of Ophthalmology, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

20 Department of Ophthalmology, Texas A and M College of Medicine, College Station, TX 77807, USA

21 Department of Ophthalmology, University of Iowa Hospitals and Clinics, Iowa City, IA 52242, USA

22 Departments of Ophthalmology, Neurology, and Neurosurgery, Weill Cornell Medicine, New York, NY 10021, USA

Brian E. Crucian

23 National Aeronautics and Space Administration (NASA) Johnson Space Center, Human Health and Performance Directorate, Houston, TX 77058, USA

Associated Data

The field of human space travel is in the midst of a dramatic revolution. Upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX, Virgin Galactic) have already started the process of preparing for long-distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s. With the emergence of space tourism, space travel has materialized as a potential new, exciting frontier of business, hospitality, medicine, and technology in the coming years. However, current evidence regarding human health in space is very limited, particularly pertaining to short-term and long-term space travel. This review synthesizes developments across the continuum of space health including prior studies and unpublished data from NASA related to each individual organ system, and medical screening prior to space travel. We categorized the extraterrestrial environment into exogenous (e.g., space radiation and microgravity) and endogenous processes (e.g., alteration of humans’ natural circadian rhythm and mental health due to confinement, isolation, immobilization, and lack of social interaction) and their various effects on human health. The aim of this review is to explore the potential health challenges associated with space travel and how they may be overcome in order to enable new paradigms for space health, as well as the use of emerging Artificial Intelligence based (AI) technology to propel future space health research.

1. Introduction

Until now space missions have generally been either of short distance (Low Earth Orbit—LEO) or short duration (Apollo Lunar Missions). However, upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX) have already started the process of preparing for long distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s [ 1 ].

Within the extraterrestrial environment, a multitude of exogenous and endogenous processes could potentially impact human health in several ways. Examples of exogenous processes include exposure to space radiation and microgravity while in orbit. Space radiation poses a risk to human health via a number of potential mechanisms (e.g., alterations of gut microbiome biosynthesis, accelerated atherosclerosis, bone remodeling, and hematopoietic effects) and prolonged microgravity exposure presents additional potential health risks (e.g., viral reactivation, space motion sickness, muscle/bone atrophy, and orthostatic intolerance) [ 2 , 3 , 4 , 5 , 6 , 7 ]. Examples of endogenous processes potentially impacted by space travel include alteration of humans’ natural circadian rhythm (e.g., sleep disturbances) and mental health disturbances (e.g., depression, anxiety) due to confinement, isolation, immobilization, and lack of social interaction [ 8 , 9 , 10 ]. Finally, the risk of unknown exposures, such as yet undiscovered pathogens, remain persistent threats to consider. Thus, prior to the emergence of long distance, long duration space travel it is critical to anticipate the impact of these varied environmental factors and identify potential mitigating strategies. Here, we review the available medical literature on human experiments conducted during space travel and summarize our current knowledge on the effects of living in space for both short and long durations of time. We also discuss the potential countermeasures currently employed during interstellar travel, as well as future directions for medical research in space.

1.1. Medical Screening and Certification Prior to Space Travel

When considering preflight medical screening and certification, the requirements and recommendations vary based on the duration of space travel. Suborbital spaceflight, part of the new era of space travel, has participants launching to the edge of space (defined as the Karman line, 100 Km above mean sea level) for brief 3–5 min microgravity exposures. Orbital spaceflight, defined as microgravity exposure for up to 30 days, involves healthy individuals with preflight medical screening. In addition to a physical examination and metabolic screening, preflight medical screening assessing aerobic capacity (VO 2max ), and muscle strength and function may be sufficient to ensure proper conditioning prior to mission launch [ 11 , 12 , 13 , 14 ]. Age-appropriate health screening tests (e.g., colonoscopy, serum prostate specific antigen in men, and mammography in women) are generally recommended for astronauts in the same fashion as their counterparts on Earth. In individuals with cardiovascular risk factors or with specific medical conditions, additional screening may be required [ 15 ]. The goal of these preflight screening measures is to ensure that medical conditions that may result in sudden incapacitation are identified and either disqualified or treated before the mission begins. In addition to the medical screening described above, short-duration space travelers are also required to undergo acceleration training, hypobaric and hypoxia exposure training, and hypercapnia awareness procedures as part of the preflight training phase.

In preparation for long-duration space travel, astronauts generally undergo a general physical examination, as well as imaging and laboratory studies at the time of initial selection. These screening tests would then be repeated annually, as well as upon assignment to an International Space Station (ISS) mission. ISS crew members are medically certified for long-duration spaceflight missions through individual agency medical boards (e.g., NASA Aerospace Medical Board) and international medical review boards (e.g., Multilateral Space Medicine Board) [ 16 , 17 ]. In order for an individual to become certified for long-duration space travel, an individual must be at the lowest possible risk for the occurrence of medical events during the preflight, infight, and postflight periods. Following spaceflight, it is recommended that returning astronauts undergo occupational surveillance for the remainder of their lifetime for the detection of health issues related to space travel (e.g., NASA’s Lifetime Surveillance of Astronaut Health program) [ 18 ]. Table 1 summarizes the preflight, inflight, and postflight screening recommendations for each organ system. Further research utilizing data from either long-term space missions or simulated environments is required in order to develop an adequate preflight scoring system capable of predicting inflight and postflight health outcomes in space travelers based on various risk factors.

Summarizes the pre-flight, in-flight and post-flight screening in each system.

Below we discuss potential Space Hazards for each organ system along with possible countermeasures ( Table 2 ). Table 3 lists prospective opportunities for artificial intelligence (AI) implementation.

Summary of Space Hazards to each organ system and potential countermeasures.

Potential AI applications in space health.

1.2. Effects on the Cardiovascular System

During short-duration spaceflight, microgravity alters cardiovascular physiology by reducing circulatory blood volume, diastolic blood pressure, left ventricular mass, and cardiac contractility [ 42 , 123 ]. Several studies have demonstrated that peak exercise performance is reduced both inflight and immediately after short-duration spaceflight due primarily to a reduction in maximal cardiac output and O 2 delivery [ 124 , 125 ]. Prolonged exposure to microgravity does cause unloading of the cardiovascular system (e.g., removal of expected loading effects from Earth’s gravity when upright during the day), resulting in cardiac atrophy. These changes may be an example of adaptive physiologic changes (“physiologic atrophy”) that returns to baseline after returning from spaceflight. This process may be similar to the adaptive physiologic changes to the cardiovascular system seen during athletic training (“physiologic hypertrophy”). Thus far, there is no evidence that the observed short-term cardiac atrophy could permanently impair systolic function. However, this physiologic adaptation to microgravity in space could lead to orthostatic hypotension/intolerance upon returning to Earth’s gravity due to changes in the comparative position of peripheral resistance and sympathetic nerve activity [ 41 , 126 , 127 ]. Figure 1 demonstrates potential effects of the space environment on each organ system.

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Potential effects of the space environment on each organ system.

Another potential effect of microgravity exposure is that an alteration of hydrostatic forces in the vertical gravitational (Gz) axis could lead to the formation of internal jugular vein thromboses [ 28 , 29 ]. Anticoagulation would not be an ideal choice for prevention as astronauts have an increased risk of suffering traumatic injury during spaceflight, thus potentially inflating the risk of developing an intracerebral hemorrhage or subdural hematoma. In addition, if a traumatic accident were to occur during spaceflight, the previously discussed cardiovascular adaptations could impair the body’s ability to tolerate blood loss and shock [ 45 , 46 , 47 ].

During long-duration spaceflight, one recent study demonstrated that astronauts did not experience orthostatic hypotension/intolerance during routine activities or after landing following 6 months in space [ 128 ]. It is worth noting that all of these astronauts performed aggressive exercise countermeasures while in flight [ 128 ]. Another study of healthy astronauts after 6 months of space travel showed that the space environment caused transient changes in left atrial structure/electrophysiology, increasing the risk of developing atrial fibrillation (AF) [ 129 ]. However, there was no definitive evidence of increased incidence of supraventricular arrhythmias and no identified episodes of AF [ 129 ]. Evaluation with echocardiography or cardiac MRI may be considered following long-duration spaceflight in certain cases.

Prior human studies with supplemental data obtained from animal studies, have shown that healthy individuals with prolonged exposure to ionizing radiation may be at increased risk for the development of accelerated atherosclerosis secondary to radiation-induced endothelial damage and a subsequent pro-inflammatory response [ 3 , 4 , 57 , 58 , 59 , 60 , 123 ]. One study utilizing human 3D micro-vessel models showed that ionizing radiation inhibits angiogenesis via mechanisms dependent on the linear energy transfer (LET) of charged particles [ 130 ], which could eventually lead to cardiac dysfunction [ 131 , 132 ]. In fact, specific characteristics of the radiation encountered in space may be an important factor to understanding its effects. For example, studies of pediatric patients undergoing radiotherapy have shown an increase in cardiac-related morbidity/mortality due to radiation exposure, but not until radiation doses exceeded 10 Gy [ 133 ]. At lower dose levels the risk is less clear: while a study of atomic bomb survivors with more than 50 years of followup demonstrated elevated cardiovascular risks at doses < 2 Gy [ 134 ]. A recent randomized clinical trial with a 20-year follow-up showed no increase in cardiac mortality in irradiated breast cancer patients with a median dose of 3.0 Gy (1.1–8.1 Gy) [ 135 ]. The uncertainty in cardiovascular effects of ionizing radiation, are accentuated in a space environment as the type and quality of radiation likely play an important role as well.

Further research is required to understand the radiation dosage, duration, and quality necessary for cardiovascular effects to manifest, as well as develop preventive strategies for AF and internal jugular vein thrombosis during space travel.

1.3. Effects on the Gastrointestinal System

During short-duration spaceflight, the presence of gastrointestinal symptoms (e.g., diarrhea, vomiting, and inflammation of the gastrointestinal tract) are common due to microgravity exposure [ 35 , 136 , 137 ]. Still unknown however is whether acute, surgical conditions such as cholecystitis and appendicitis occur more frequently due to microgravity-induced stone formation or alterations in human physiology/anatomy, and immunosuppression [ 40 ]. Controlling for traditional risk factors associated with the development of these conditions (e.g., adequate hydration, maintenance of a normal BMI, dietary fat avoidance, etc.) may help mitigate the risk.

During long-duration spaceflight, it is possible that prolonged radiation exposure could lead to radiation-induced gastrointestinal cancer. Gamma radiation exposure is a known risk factor for colorectal cancer via an absence of DNA methylation [ 138 ]. NASA has recently developed a space radiation simulator, named the “GCR Simulator”, which allows for the more accurate radiobiologic research into the development and mitigation of radiation-induced malignancies [ 139 ]. Preflight colorectal cancer screening via colonoscopy or inflight screening via gut microbiome monitoring may be beneficial, but further research is required to demonstrate their clinical utility. Several studies, including the NASA Twins study have shown that microgravity could lead to alterations in an individual’s gut microbial community (i.e., gut dysbiosis) [ 2 , 140 , 141 , 142 ]. While changes to an individual’s gut microbiome can cause inflammation of the gastrointestinal tract [ 143 , 144 ], it remains unclear whether the specific alterations observed during spaceflight pose a risk to astronaut health. In fact, increased gut colonization by certain bacterial species is even associated with a beneficial effect on the gastrointestinal tract [ 2 , 140 ]. ( Table S1 ) Certain limitations of these studies, such as variations in genomic profile, diet, and a lack of adjusted confounders (e.g., the microbial content of samples) should be considered. Another potential consequence of prolonged microgravity exposure is the possibility of increased fatty-acid processing [ 145 ], leading to the development of non-alcoholic fatty liver disease (NAFLD) and hepatic fibrosis [ 146 , 147 ].

Further research is required to better understand gut microbial dynamics during space travel, as well as spaceflight-associated risk factors for the development of NAFLD, cholecystitis, and appendicitis.

1.4. Effects on the Immune System

During spaceflight, exposure to microgravity could potentially induce modifications in the cellular function of the human immune system. For example, it has been hypothesized that microgravity exposure could lead to an increase in the production of inflammatory cytokines [ 148 ] and stress hormones [ 149 , 150 ], alterations in the function of certain cell lines (NK cells [ 151 , 152 ], B cells [ 153 ], monocytes [ 154 ], neutrophils [ 154 ], T cells [ 5 , 155 ]), and impairments of leukocyte distribution [ 156 ] and proliferation [ 155 , 157 , 158 ]. The resultant immune system dysfunction could lead to the reactivation of latent viruses such as Epstein-Bar Virus (EBV), Varicella-Zoster Virus (VZV), and Cytomegalovirus (CMV) [ 31 , 32 ]. Persistent low-grade pro-inflammatory responses microgravity could lead to space fever. [ 159 ] Studies are currently underway to evaluate countermeasures to improve immune function and reduce reactivation of latent herpesviruses [ 33 , 160 , 161 , 162 ]. Microgravity exposure could also lead to the development of autoantibodies, predisposing astronauts to various autoimmune conditions [ 136 , 163 ]. ( Table S2 ) Most importantly, studies have shown that bacteria encountered within the space environment appear to be more resistant to antibiotics and more harmful in general compared to bacteria encountered on Earth [ 164 , 165 ]. This is in addition to the threat of novel bacteria species (e.g., Methylobacterium ajmalii sp. Nov. [ 76 ]) that we have not yet discovered.

Upon returning from the space environment astronauts remain in an immunocompromised state, which has been particularly problematic in the era of the COVID-19 pandemic. Recently, NASA has recommended postflight quarantine and immune status monitoring (i.e., immune-boosting protocol) to mitigate the risk of infection [ 77 ]. This is similar to the Apollo and NASA Health Stabilization Programs that helped establish the preflight protocol (pre-mission quarantine) currently used for this purpose.

Further research is required to understand the mechanisms of antibiotic resistance and the modifications in inflammatory cytokine dynamics, in order to develop immune boosters and surrogate immune biomarkers.

1.5. Effects on the Hematologic System

During short-duration spaceflight, the plasma volume and total blood volume de-crease within the first hours and remain reduced throughout the inflight period, a finding previously identified as space anemia [ 166 ]. Space anemia during spaceflight is perhaps due to a normal physiologic adaptation of newly released blood cells and iron metabolism to microgravity [ 167 ].

During long-duration spaceflight, microgravity exposure could potentially induce hemoglobin degradation, leading to hemolytic anemia. In a recent study of 14 astronauts who were on 6-month missions onboard the ISS, a 54% increase in hemolysis was ob-served after landing one year later [ 50 ]. In another small study, nearly half of astronauts (48%) landing after long duration missions were anemic and hemoglobin levels were characterized as having a dose–response relationship with microgravity exposure [ 51 ]. An additional study collected whole blood sample from astronauts during and after up to 6 months of orbital spaceflight [ 168 ]. Upon analysis, once the astronauts returned to Earth RBC and hemoglobin levels were significantly elevated. It is worth noting that these studies analyzed blood samples from astronauts collected after spaceflight, which may be influenced by various factors (e.g., the stress of landing and re-adaptation to conditions on Earth). In addition, these studies may be confounded by other extraterrestrial environmental factors such as fluid shifts, dehydration, and alteration of the circadian cycle.

Further research is urgently needed to understand plasma volume physiology dur-ing spaceflight and delineate the etiology and degree of hemolysis with longer space exposure, such as 1-year ISS or Mars exploration missions.

1.6. Oncologic Effects

Even during short-duration spaceflight, the stochastic nature of cancer development makes it possible that space radiation exposure could cause cancer via epigenomic modifications [ 63 ]. Currently, our epidemiological understanding of radiation-induced cancer risk is based primarily on atomic bomb survivors and accidental radiation exposures, which both show a clear association between radiation exposure and cancer risk [ 169 , 170 ]. However, these studies are hard to generalize to spaceflight as the patient populations vary significantly (generally healthy astronauts vs. atomic bomb survivors [NCRP 126]) [ 171 ]. Moreover, the radiation encountered in space is notably different than that associated with atomic bomb exposure. Most terrestrial exposures are based on low LET radiation (e.g., atomic bomb survivors received <1% dose from high LET neutrons) [ 172 ], whereas space radiation is comprised of higher LET ions (solar energetic particles and galactic cosmic rays) [ 173 , 174 ].

During long-duration spaceflight, our current understanding of cancer risk is also largely unknown. Our current epidemiologic understanding of long-duration radiation exposure and cancer risk is primarily based on the study of chronic occupational exposures and medically exposed individuals, supplemented with data obtained from animal studies, which are again based overwhelmingly on low LET radiation [ 169 , 170 , 175 , 176 ]. In animal studies, exposure to ionizing radiation (up to 13.5 months) has been associated with an increased risk of developing a variety of cancers [ 162 , 177 , 178 , 179 , 180 ]. Ionizing radiation exposure may cause DNA methylation patterns similar to the specific patterns observed in human adenocarcinomas and squamous cell carcinomas [ 63 ]; however, this response is not yet certain [ 181 , 182 ].

For the purposes of risk prediction, the elevated biological potency of heavy ions is modeled through concepts such as the radiation weighting factor, with NASA recently releasing unique quality factors ( Q NASA ) focused on high density tracks [ 183 ]. Although these predictive models can only estimate the impact of radiation exposure, extrapolation of current terrestrial-based data suggest that this risk could be at least substantial for astronauts. NASA, for example, has updated crew permissible career exposure limits to 0.6Sv, independent of age and sex. This degree of exposure results in a 2–3% mean increased risk of death from radiation carcinogenesis (NCRP 2021) [ 184 ]. This limit would be reached between 200 and 400 days of space travel (depending on degree of radiation shielding) [ 48 ].

Further research is urgently needed to understand the true risk of space radiation exposure. This is especially important for individuals with certain genotype-phenotype profiles (e.g., BRCA1 or DNA methylation signatures) who may be more sensitive to the effects of radiation exposure. Most importantly, the utilization of genotype-phenotype profiles of astronauts or space travelers is valuable not just for pre-flight screening, but also during in-flight travel, especially for long-duration flights to deeper space. An individual’s genetic makeup will in-variably change during spaceflight due to the shifting epigenetic microenvironment. Future crewed-missions to deep space will have to adapt to these anticipated changes, be-come aware of impending red-flag situations, and determine whether any meaningful shift or change to ones’ genetic makeup is possible. For example, personalized radiation shields could potentially be tailored to an individuals’ genotype-phenotype profile, individualized pulmonary capillary wedge pressure under microgravity may be different due to transient changes in left atrial structure, or preflight analysis of the globin gene for the prediction of space anemia [ 50 , 129 , 185 ]. This research should be designed to identify the radiation type, dose, quality, frequency, and duration of exposure required for cancer development.

1.7. Effects on the Neurologic System

During the initial days of spaceflight, space motion sickness (SMS) is the most commonly encountered neurologic condition. Microgravity exposure during spaceflight commonly leads to alterations in spatial orientation and gaze stabilization (e.g., shape recognition [ 186 ], depth perception and distance [ 187 , 188 ]). Postflight, impairments in object localization during pitch and roll head movements [ 189 , 190 ] and fine motor control (e.g., force modulation [ 191 ], keyed pegboard completion time [ 192 ], and bimanual coordination [ 193 ]) are common. Anecdotally, astronauts also reported alterations in smell and taste sensations during their missions [ 27 , 194 , 195 ]. The observed impairment in olfactory function is perhaps due to elevated intracranial pressure (ICP) with increased cerebrospinal fluid outflow along the cribriform plate pathways [ 196 ]. However, to date, there have been no studies directly measuring ICP during spaceflight.

Upon returning from spaceflight, studies have observed that astronauts experience decrements in postural and locomotor control that can increase fall risk [ 197 ]. These decrements have been observed in both standard sensorimotor testing and functional tasks. While recovery of sensorimotor function occurs rapidly following short-duration spaceflight (within the first several days after return) [ 192 , 198 ], recovery after long-duration spaceflight often takes several weeks. Similar to SMS, post-flight motion sickness (PFMS) is very common and occurs soon after g-transition [ 30 ]. Deficits in dexterity, dual-tasking, and vehicle operation [ 199 ] are also commonly observed immediately after spaceflight. Therefore, short-duration astronauts are recommended to not drive automobiles for several days, and only after a sensorimotor evaluation (similar to a field sobriety test).

Similarly to the effects seen following short-duration spaceflight, those returning from long-duration spaceflight can also experience deficits in dexterity, dual-tasking, and vehicle operation. Long-duration astronauts are recommended to not drive automobiles for several weeks, and also require a sensorimotor evaluation. While central nervous system (CNS) changes [ 53 ] associated with long-duration spaceflight are commonly observed, the resulting effects of these changes both during and immediately after spaceflight remain unclear [ 199 ]. Observed CNS changes include structural and functional alterations (e.g., upward shift of the brain within the skull [ 54 ], disrupted white matter structural connectivity [ 55 ], increased fluid volumes [ 56 ], and increased cerebral vasoconstriction [ 200 ]), as well as modifications to adaptive plasticity [ 53 ]. Adaptive reorganization is primarily observed in the sensory systems. For example, changes in functional connectivity during plantar stimulation have been observed within sensorimotor, visual, somatosensory, and vestibular networks after spaceflight [ 201 ]. In addition, functional responses to vestibular stimulation were altered after spaceflight―reducing the typical deactivation of somatosensory and visual cortices [ 202 ]. These studies provide evidence for sensory reweighting among visual, vestibular, and somatosensory inputs.

Further research is required to fully understand the observed CNS changes. In addition, integrated countermeasures are needed for the acute effects of g-transitions on sensorimotor and vestibular function.

1.7.1. Effects on the Neuro-Ocular System

Prolonged exposure to ionizing radiation is well known to produce secondary cataracts [ 61 , 62 ]. Most importantly, Spaceflight Associated Neuro-Ocular Syndrome (SANS) is a unique constellation of clinical and imaging findings which occur to astronauts both during and after spaceflight, and is characterized by: hyperopic refractive changes (axial hyperopia), optic disc edema, posterior globe flattening, choroidal folds, and cotton wool spots [ 43 ]. Ophthalmologic screening for SANS, including both clinical and imaging assessments is recommended. ( Figure 2 ) Although the precise etiology and mechanism for SANS remain ill-defined, some proposed risk factors for the development of SANS include microgravity related cephalad fluid shifts [ 203 ], rigorous resistive exercise [ 204 ], increased body weight [ 205 ], and disturbances to one carbon metabolic pathways [ 206 ]. Many scientists believe that the cephalad fluid shift secondary to microgravity exposure is the major pathophysiological driver of SANS [ 203 ]. Although inflight lumbar puncture has not been attempted, several mildly elevated ICPs have been recorded in astronauts with SANS manifestations upon returning to Earth [ 43 ]. Moreover, changes to the pressure gradient between the intraocular pressure (IOP) and ICP (the translaminar gradient) have been proposed as a pathogenic mechanism for SANS [ 207 ]. The translaminar gradient may explain the structural changes seen in the posterior globe such as globe flattening and choroidal folds [ 207 ]. Alternatively, the microgravity induced cephalad fluid shift may impair venous or cerebrospinal drainage from the cranial cavity and/or the eye/orbit (e.g., choroid or optic nerve sheath). Impairment of the glymphatic system has also been proposed as a contributing mechanism to SANS, but this remains unproven [ 208 , 209 ]. Although permanent visual loss has not been observed in astronauts with SANS, some structural changes (e.g., posterior globe flattening) may persist and have been documented to remain for up to 7 years of long-term follow-up [ 210 ]. Further research is required to better understand the mechanism of SANS, and to develop effective countermeasures prior to longer duration space missions.

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Ophthalmologic screening for SANS.

1.7.2. Effects on the Neuro-Behavioral System

The combination of mission-associated stressors with the underlying confinement and social isolation of space travel has the potential to lead to cognitive deficits and the development of psychiatric disorders [ 211 ]. Examples of previously identified cognitive deficits associated with spaceflight include impaired concentration, short-term memory loss, and an inability to multi-task. These findings are most evident during G-transitions, and are likely due to interactions between vestibular and cognitive function [ 212 , 213 ]. Sopite syndrome, a neurologic component of motion sickness, may account for some cognitive slowing. The term “space fog”, has been used to describe the generalized lack of focus, altered perception of time, and cognitive impairments associated with spaceflight, which can occur throughout the mission. This may be related to chronic sleep deprivation as deficiencies (including decreased sleep duration and quality of sleep) are prevalent despite the frequent use of sleep medications [ 71 ]. These results highlight the broad impact of space travel on cognitive and behavioral health, and support the need for integrated countermeasures for long-duration explorative missions.

1.8. Effects on the Musculoskeletal System

During short-duration spaceflight, low back pain and disk herniation are common due to the presence of microgravity. While the pathogenesis of space-related low back pain and disk herniation is complex, the etiology is likely multifactorial in nature (e.g., microgravity induced hydration and swelling of the vertebral disk, muscle atrophy of the neck and lower back) [ 19 , 214 , 215 ]. Additionally, various joint injuries (e.g., space-suited shoulder injuries) can also occur in space due to the presence of microgravity [ 16 , 216 , 217 , 218 ]. Interestingly, one study showed that performing specific exercises could potentially promote automatic and tonic activation of lumbar multifidus and transversus abdominis as well as prevent normal lumbopelvic positioning against gravity following bed rest as a simulation of space flight [ 219 ], and the European Space Agency suggested that exercise program could relieve low back pain during spaceflight [ 220 ]. Further longitudinal studies are required to develop specialized exercise protocols during space travel.

During long-duration spaceflight, the presence of microgravity could cause an alteration in collagen fiber orientation within tendons, reduce articular cartilage and meniscal glycosaminoglycan content, and impair the wound healing process [ 22 , 23 , 24 , 221 ]. These findings seen in animal studies suggest that mechanical loading is required in order for these processes to occur in a physiologic manner. It is theorized that there is a mandatory threshold of skeletal loading necessary to direct balanced bone formation and resorption during healthy bone remodeling [ 222 , 223 ]. Despite the current countermeasure programs, the issue of skeletal integrity is still not solved [ 224 , 225 , 226 ].

Space radiation could also impact bone remodeling, though the net effect differs based on the amount of radiation involved [ 6 ]. In summary, high doses of space radiation lead to bone destruction with increased bone resorption and reduced bone formation, while low doses of space radiation actually have a positive impact with increased mineralization and reduced bone resorption. Most importantly, space radiation, particularly solar particle events in the case of a flare, may induce acute radiation effects, leading to hematopoietic syndrome [ 7 ]. This risk is highest for longer duration missions, but can be substantially minimized with current spacecraft shielding options.

Longitudinal studies are required to develop special exercise protocols and further assess the aforementioned risk of space radiation on the development of musculoskeletal malignancies.

1.9. Effects on the Pulmonary System

During short-duration spaceflight, a host of changes to normal, physiologic pulmonary function have been observed [ 73 , 227 ]. Studies during parabolic flight have shown that the diaphragm and abdomen are displaced cranially due to microgravity, which is accompanied by an increase in the diameter of the lower rib cage with outward movement. Due to the observed changes to the shape of the chest wall, diaphragm, and abdomen, alterations to the pressure-volume curve resulted in a net reduction in lung volumes [ 228 ]. In five subjects who underwent 25 s of microgravity exposure during parabolic flight, functional residual capacity (FRC) and vital capacity (VC) were found to be reduced [ 229 ]. During the Spacelab Life Sciences-1 mission, microgravity exposure resulted in 10%, 15%, 10–20%, and 18% reductions in VC, FRC, expiratory reserve volume (ERV), and residual volume (RV), respectively, compared to values seen in Earth’s gravity [ 227 ]. The observed physiologic change in FRC is primarily due to the cranial shift of the diaphragm and abdominal contents described previously, and secondarily to an increase in intra-thoracic blood volume and more uniform alveolar expansion [ 227 ].

One surrogate measure for the inhomogeneity of pulmonary perfusion can be assessed through changes in cardiogenic oscillations of CO 2 (oscillations in exhaled gas composition due to differential flows from different lung regions with differing gas composition). Following exposure to microgravity, the size of cardiogenic oscillations were significantly reduced to 60% in comparison to the preflight standing values [ 230 , 231 ]. Possible causes of the observed inhomogeneity of ventilation include regional differences in lung compliance, airway resistance, and variations in motion of the chest wall and diaphragm. Access to arterial blood gas analysis would allow for enhanced physiologic evaluations, as well as improved management of clinical emergencies (e.g., pulmonary embolism) occurring during space travel. However, there is currently no suitable method for assessing arterial blood in space. The earlobe arterialized blood technique for collecting blood gas has been proposed, but evidence is limited [ 232 ]. Further research is required in this area to establish an effective means for sampling arterial blood during spaceflight.

In comparison to the changes seen during short-duration spaceflight, studies conducted during long-duration spaceflight showed that the heterogeneity of ventilation/perfusion (V/Q) was largely unchanged, with preserved gas exchange, VC, and respiratory muscle strength [ 73 , 233 , 234 ]. This resulted in overall normal lung function. This is supported by long-duration studies (up to 6 months) in microgravity which demonstrated that the function of the normal human lungs is largely unchanged following the removal of gravity [ 233 , 234 ]. It is worth noting that there were some small changes which were observed (e.g., an increase in ERV in the standing posture) following long-duration spaceflight, which can perhaps be attributed to a reduction in circulating blood volume [ 233 , 234 ]. However, while microgravity can causes temporary changes in lung function, these changes were reversible upon return to Earth’s gravity (even after 6 months of exposure to microgravity). Based on the currently available data, the overall effect of acute and sustained exposure to microgravity does not appear to cause any deleterious effects to gas exchange in the lungs. However, the biggest challenge for long-duration spaceflight is perhaps extraterrestrial dust exposure. Further research is required to identify the long term consequences of extraterrestrial dust exposure and develop potential countermeasures (e.g., specialized face masks) [ 73 ].

1.10. Effects on the Dermatologic System

During short-duration space travel, skin conditions such as contact dermatitis, skin sensitivity, biosensor electrolyte paste reactions, and thinning skin are common [ 44 , 235 ]. However, these conditions are generally mild and unlikely to significantly impact astronaut safety or prevent completion of space missions [ 44 ].

The greatest dermatologic concern for long-duration space travelers is the theoretical increased risk of developing skin cancer due to space radiation exposure. This hypothesis is supported by one study which found the rate of basal cell carcinoma, melanoma, and squamous cell carcinoma of the skin to be higher among astronauts compared to a matched cohort [ 236 ]. While the three-fold increase in prevalence was significant, there were a number of confounders (e.g., the duration of prolonged UV exposure on Earth for training or recreation, prior use of sunscreen protection, genetic predisposition, and variations in immune system function) that must also be taken into account. A potential management strategy for dealing with various skin cancers during space travel involves telediagnostic and telesurgical procedures. Further research is needed to improve the telediagnosis and management of dermatological conditions (e.g., adjustment for a lag in communication time) during spaceflight.

1.11. Diagnostic Imaging Modalities in Space

In addition to routine physical examination, various medical imaging modalities may be required to monitor and diagnose medical conditions during long-duration space travel. To date, ultrasound imaging acquired on space stations has proven to be helpful in diagnosing a wide array of medical conditions, including venous thrombosis, renal and biliary stones, and decompression sickness [ 29 , 237 , 238 , 239 , 240 , 241 , 242 ]. Moreover, the Focused Assessment with Sonography for Trauma (FAST), utilized by physicians to rapidly evaluate trauma patients, may be employed during space missions to rule out life-threatening intra-abdominal, intra-thoracic, or intra-ocular pathology [ 243 ]. Remote telementored ultrasound (aka tele-ultrasound) has been previously investigated during the NASA Extreme Environment Missions Operations (NEEMO) expeditions [ 244 ]. Today, the Butterfly iQ portable ultrasound probe can be linked directly to a smartphone through cloud computing, allowing physicians/specialists to promptly analyze remote ultrasound images [ 245 ].

Currently, alternative imaging modalities such as X-ray, CT, PET and MRI scan are unable to be used in space due to substantial limitations (e.g., limited space for large imaging structures, difficulties in interpretation due to microgravity). However, it is possible that the future development of a photocathode-based X-ray source may one day make this a possibility [ 101 , 246 ]. If X-ray imaging was possible, certain caveats would need to be taken into account for accurate interpretation. For example, pleural effusions, air-fluid levels, and pulmonary cephalization commonly seen on terrestrial imaging, would need to be interpreted in an entirely different way due to the effect of microgravity [ 247 ]. While this adjustment might be challenging, the altered principles of weightless physiology may provide some advantages as well. For example, one study found that intra-abdominal fluid was better able to be detected in space than in the terrestrial environment due to gravitational alterations in fluid dynamics [ 248 ]. Further research is required to identify and optimize inflight imaging modalities for the detection and treatment of various medical conditions.

1.12. Medical and Surgical Procedures in Space

Despite the presence of microgravity, both basic life support and advanced cardiac life support are feasible during space travel with some modifications [ 249 , 250 ]. For example, the recent guidelines for CPR in microgravity recommend specialized techniques for delivering chest compressions [ 251 ]. The use of mechanical ventilators, and moderate sedation or general anesthesia in microgravity are also possible but the evidence is extremely limited [ 252 , 253 ]. In addition, there are several procedures such as endotracheal intubation, percutaneous tracheostomy, diagnostic peritoneal lavage, chest tube insertion, and advanced vascular access which have only been studied through artificial stimulation [ 254 , 255 ].

Once traditionally “surgical” conditions are appropriately diagnosed, the next step is to determine whether these conditions should be managed medically, percutaneously, or surgically (laparoscopic vs. open procedures) [ 47 , 256 ]. For example, acute appendicitis or cholecystitis that would historically be managed surgically in terrestrial hospitals, could instead be managed with antibiotics rather than surgery. While the use of antibiotics for these conditions is usually effective on Earth, there remain concerns due to space-induced immune alterations, increased pathogenicity and virulence of microorganisms, and limited resources to “rescue” cases of antibiotic failure [ 39 ]. In cases of antibiotic failure, one potential minimally invasive option could be ultrasound-guided percutaneous drainage, which has previously been demonstrated to be possible and effective in microgravity [ 257 ]. Another potential approach is to focus on the early diagnosis and minimally invasive treatment of appropriate conditions, rather than treating late stage disease. In addition to expediting the patient’s post-operative recovery, minimally invasive surgery in space has the added benefit of protecting the cabin environment and the remainder of the crew [ 258 , 259 ].

As in all aspects of healthcare delivery in space, the presence of microgravity can complicate even the most basic of procedures. However, based on collective experience to date, if the patient, operators, and all required equipment are restrained, the flow of surgical procedures remains relatively unchanged compared to the traditional, terrestrial experience [ 260 ]. A recent animal study confirmed that it was possible to perform minor surgical procedures (e.g., vessel and wound closures) in microgravity [ 261 ]. Similar study during parabolic flight has further confirmed that emergent surgery for the purpose of “damage control” in catastrophic scenarios can be conducted in microgravity [ 262 ]. As discussed previously, telesurgery may be feasible if the surgery can be performed with an acceptably brief time lag (<200 ms) and if the patient is within a low Earth orbit [ 263 , 264 ]. However, further research and technological advancements are required for this to come to fruition.

1.13. Lifestyle Management in Space

Based on microgravity simulation studies, NASA has proposed several potential biomedical countermeasures in space [ 33 , 160 , 161 ]. Mandatory exercise protocols in space are crucial and can be used to maintain physical fitness and counteract the effects of microgravity. While these protocols may be beneficial, exercise alone may not be enough to prevent certain effects of microgravity (e.g., an increase in arterial thickness/stiffness) [ 20 , 265 , 266 , 267 ]. For example, a recent study found that resistive exercise alone could not suppress the increase in bone resorption that occurs in space [ 20 ]. Hence, a combination of resistance training and an antiresorptive medication (e.g., bisphosphonate) appears to be optimal for promotion of bone health [ 20 , 21 ]. Further research is needed to identify the optimal exercise regimen including recommended exercises, duration, and frequency.

In addition to exercise, dietary modification may be another potential area for optimization. The use of a diet based on caloric restriction (CR) in space remains up for debate. Based on data from terrestrial studies, caloric restriction may be useful for improving vascular health; however, this benefit may be offset by the associated muscle atrophy and osteoporosis [ 268 , 269 ]. Given that NASA encourages astronauts to consume adequate energy to maintain body mass, there has been an attempt to mimic the positive effects of CR on vascular health while providing appropriate nutrition. Further research is needed this area to identify the ideal space diet.

Based on current guidelines, only vitamin D supplementation during space travel is recommended. Supplementation of A, B6, B12, C, E, K, Biotin, folic acid are not generally recommended at this time due to insufficient evidence [ 64 ] ( Table S3 ). The use of traditional prescription medications may not function as intended on Earth. Therefore, alternative methods such as synthetic biologic agents or probiotics may be considered [ 35 , 38 ]. However, evidence in this area is extremely limited, and it is possible that the synthetic agents or probiotics may themselves be altered due to microgravity and radiation exposure. Further research is needed to investigate the relationship between these supplements and potential health benefits in space.

Currently, most countermeasures are directed towards cardiovascular system and musculoskeletal pathologies but there is little data against issues like immune and sleep deprivation, SANS, skin, etc. Artificial Gravity (AG) has been postulated as adequate multi-system countermeasure especially the chronic exposure in a large radius systems. Previously, the main barrier is the huge increase in costs [ 270 , 271 , 272 ]. However, there are various studies that show the opposite and also the recent decrease in launch cost makes the budget issue nearly irrelevant especially when a huge effort is paid to counteract the lack of gravity. The use of AG especially long-radius chronic AG is feasible. Further studies are needed to determine the utilization of AG in long-duration space travel.

1.14. Future Directions for Precision Space Health with AI

In this new era of space travel and exploration, ‘future’ tools and novel applications are needed in order to prepare deep space missions, particularly pertaining to strategies for mitigating extraterrestrial environmental factors, including both exogenous and en-dogenous processes. Such ‘future’ tools could help assist and ensure a safe travel to deep space, and more importantly, help bring space travelers and astronauts back to Earth. These tools and methods may initially be ‘remotely’ controlled, or have its data sent back to Earth for analysis. Primarily efforts should be focused on analyzing data in situ, and on site during the mission itself, both for the purpose of efficiency, and for the progressive purpose of slowly weaning off a dependency on Earth.

AI is an emerging tool in the big data era and AI is considered a critical aspect of ‘fu-ture’ tools within the healthcare and life science fields. A combination of AI and big data can be used for the purposes of decision making, data analysis and outcome prediction. Just recently, there have been encourage in advancements in AI and space technologies. To date, AI has been employed by astronauts for the purpose of space exploration; however, we may just be scratching the surface of AI’s potential. In the area of medical research, AI technology can be leveraged for the enhancement of telehealth delivery, improvement of predictive accuracy and mitigation of health risks, and performance of diagnostic and interventional tasks [ 273 ]. The AI model can then be trained and have its inference leveraged through cloud computing or Edge TPU or NVIDIA Jetson Nano located on space stations. ( Table 3 and Table S4 ) Figure 3 demonstrates potential AI applications in space.

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Potential AI applications in space.

As described previously, the capability to provide telemedicine beyond LEO is primarily limited by the inability to effectively communicate between space and Earth in real-time [ 274 ]. However, AI integration may be able to bridge the gap and advance communication capabilities within the space environment [ 275 , 276 ]. One study demonstrated a potential mechanism for AI incorporation in which an AI-generated predictive algorithm displayed the projected motion of surgical tools to adjust for excess communication lag-time [ 277 ]. This discovery could potentially enable AI-enhanced robotics to complete repetitive, procedural tasks in space without human inputs (e.g., vascular access) [ 278 ]. Today, procedures performed with robotic assistance are not yet fully autonomous (they still require at least one human expert). It is possible with future iterations that an AI integration could be created with the ability to fully replicate the necessary human steps to make terrestrial procedures (e.g., percutaneous coronary intervention, incision and drainage [ 103 ], telecholecystectomy [ 105 , 106 ], etc.) feasible in space [ 275 , 279 ]. The seventh NEEMO mission previously demonstrated that robotic surgery controlled by a remote physician is feasible within the environment of a submarine, but it remains to be seen whether this can be expanded to the space environment [ 280 ].

On space stations, Edge TPU-accelerated AI inference could be used to generate accurate risk prediction models based on data obtained from simulated environments (e.g., NASA AI Risk Prediction Challenge) [ 281 ]. For example, AI could potentially utilize data (e.g., -omics) obtained from research conducted both on Earth and in simulated environments (e.g., NASA GCR Simulator) to predict an astronaut’s risk of developing cancer due to high-LET radiation exposure (cytogenetic damage, mitochondrial dysregulation, epigenetic alterations, etc.) [ 63 , 78 , 79 , 282 , 283 , 284 ].

Another potential area for AI application is through integration with wearable technology to assist in the monitoring and treatment of a variety of medical conditions. For example, within the field of cardiovascular medicine, wearable sensor technology has the capability to detect numerous biosignals including an individual’s cardiac output, blood pressure, and heart rate [ 285 ]. AI-based interpretation of this data can facilitate prompt diagnosis and treatment of congestive heart failure and arrhythmias [ 285 ]. In addition, several wearable devices in various stages of development are being created for the detection and treatment of a wide array of medical conditions (obstructive sleep apnea, deep vein thrombosis, SMS, etc.) [ 285 , 286 , 287 , 288 ].

As discussed previously, the confinement and social isolation associated with prolonged space travel can have a profound impact on an astronaut’s mental health [ 8 , 10 , 67 ]. AI-enhanced facial and voice recognition technology can be implemented to detect the early signs of depression or anxiety better than standardized screening questionnaires (e.g., PHQ-9, GAD-7) [ 68 , 69 ]. Therefore, telepsychology or telepsychiatry can be used pre-emptively for the diagnosis of mental illness [ 68 , 69 , 289 ].

2. Conclusions

Over the next decade, NASA, Russia, Europe, Canada, Japan, China, and a host of commercial space companies will continue to push the boundaries of space travel. Space exploration carries with it a great deal of risk from both known (e.g., ionizing radiation, microgravity) and unknown risk factors. Thus, there is an urgent need for expanded research to determine the true extent of the current limitations of long-term space travel and to develop potential applications and countermeasures for deep space exploration and colonization. Researchers must leverage emerging technology, such as AI, to advance our diagnostic capability and provide high-quality medical care within the space environment.

Acknowledgments

The authors would like to thank Tyson Brunstetter, (NASA Johnson Space Center, Houston, TX) for his suggestions and comments on this article as well as providing the update NASA’s SANS Evidence Report, Ajitkumar P Mulavara, (Neurosciences Laboratory, KBRwyle, Houston, TX), Jonathan Clark, (Neurology & Space Medicine, Center for Space Medicine, Houston, TX), Scott M. Smith, (Nutritional Biochemistry, Biomedical Research and Environmental Sciences Division, Human Health and Performance Directorate, NASA Johnson Space Center, Houston, TX) for his suggestions and providing the update NASA’s Nutrition Report, G. Kim Prisk, (Department of Medicine, Division of Physiology, University of California, San Diego, La Jolla, CA), Lisa C. Simonsen (NASA Langley Research Center, Hampton, VA), Siddharth Rajput, (Royal Australasian College of Surgeons, Australia and Aerospace Medical Association and Space Surgery Association, USA), David S. Martin, MS, (KBR, Houston, TX), ‪David W. Kaczka, (Department of Anesthesia, University of Iowa Carver College of Medicine, Iowa City, Iowa), Benjamin D. Levine (Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital, Dallas, University of Texas Southwestern Medical Center), Afshin Beheshti (NASA Ames Research Center), Christopher Wilson (NASA Goddard Space Flight Center), Michael Lowry (NASA Ames Research Center), Graham Mackintosh (NASA Advanced Supercomputing Division), and staff from NASA Goddard Space Flight Center for their suggestions. In addition, the authors would like to thank the anonymous reviewers for their careful reading of our manuscript, constructive criticism, and insightful comments and suggestions.

Abbreviations

Supplementary materials.

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cells12010040/s1 , Table S1: title Summary of gut microbial alteration during spaceflight; Table S2: title Summary of immune/cytokine changes during spaceflight; Table S3: title Summary of diet recommendation during spaceflight; Table S4: title Summary of AI technology and potential applications in space.

Funding Statement

This research received no external funding.

Conflicts of Interest

Krittanawong discloses the following relationships-Member of the American College of Cardiology Solution Set Oversight Committee, the American Heart Association Committee of the Council on Genomic and Precision Medicine, the American College of Cardiology/American Heart Association (ACC/AHA) Joint Committee on Clinical Data Standards (Joint Committee), and the American College of Cardiology/American Heart Association (ACC/AHA) Task Force on Performance Measures, The Lancet Digital Health (Advisory Board), European Heart Journal Digital Health (Editorial board), Journal of the American Heart Association (Editorial board), JACC: Asia (Section Editor), and The Journal of Scientific Innovation in Medicine (Associate Editor). Other authors have no disclosure.

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

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Viterbi Conversations in Ethics

5 dangers of space travel

Profitable Risk: The Dangers of Consumer Spaceflight and Space Tourism

Society is rapidly approaching an era in which ordinary civilians can purchase tickets to become passengers on space vehicles. Companies worldwide are deep in the development of infrastructure and technology to provide spaceflight for amusement and transportation. These endeavors deviate fundamentally from traditional spaceflight and raise questions about the ethical implications of commercial spaceflight with civilian passengers.

Of the 562 humans who have participated in spaceflight, almost everyone has been on a government mission for research. Merriam Webster defines astronaut as “a person who travels beyond Earth’s atmosphere,” and when most of us picture an astronaut we think of Neil Armstrong walking on the moon, or scientists performing research aboard the International Space Station (ISS). However, this mental definition of astronaut will become increasingly challenged as the space industry privatizes, a process that began in 1990. That year, Congress passed a law requiring NASA to purchase launch services from commercial providers when possible. In 2010, NASA put an end to the Space Shuttle program altogether and started offering grants to private companies capable of shuttling NASA astronauts into orbit. This combination of events opened the floodgates for commercial space companies to begin developing new technology.

The wave of new companies entering the spaceflight industry in the 1990s and 2000s made leaps in space technology development, and their successes have encouraged them to pursue new opportunities beyond the bounds of conventional research-based space travel. This is where the concept of space tourism begins. Until now, all spaceflight has been some sort of “mission,” typically carried out by a government program to conduct scientific research. Instead, commercial space companies are now looking to expand into the private sector by offering recreational space flights to civilian consumers. To date, there have been a small number of sub-orbital space flights conducted with civilians on board, but none at the scale of what these companies hope to achieve. Flying paying customers into space is an entirely new form of human-space interaction, and it is essential that we carefully consider the new implications this has on the space industry.

In the past, all space vehicle crews have been thoroughly screened, highly trained, and extremely prepared for their space missions. These astronauts are a select few of the 100,000+ applicants NASA receives for their space program. These astronauts and the organization behind them are motivated by science, innovation, and technological advancement. But the concept of paid space tourism shifts that motivation from these rather altruistic ideals to simple financial gain. This is where ethical concerns are born. While NASA will only select the most qualified individuals, a private company is more likely to take an ill-equipped passenger to space just because they are willing to pay tens of millions of dollars. With money as the primary motivating factor instead of scientific discovery, a private company is less likely to do their full due diligence in verifying the qualifications of prospective civilian spaceflight participants.

The idea that these spaceflight passengers will be less equipped to deal with being on a flight is not the only concern. Another is that the flights themselves are highly dangerous. Humans have been blasting off from the surface of Earth for less than 58 years, a short time for a new technology. Space flight, regardless of “scientific certainty,” is anything but certain. Few are likely to forget the tragedies of the Space Shuttles Challenger and Columbia just 17 years apart. Like the revolutionary Titanic, or the cutting-edge Hindenburg, the promises of radical new travel technologies have often proved perilous to their unfortunate early passengers. Even disregarding the possibility of catastrophic failure and loss of life, when launching and touching down, a spacecraft experiences intense vibration and acceleration which may cause injury or illness in some less healthy individuals. And while in orbit, microgravity and radiation exposure pose a threat with unknown long-term effects.

NASA astronauts understand the risk they are taking when they launch themselves into space. They take that risk willingly and with informed consent. But a commercial passenger simply buying a ticket into space is unlikely to have the same understanding of those risks. Further, a private company does not have any real motivation to tell them. Although companies are required by United States law to inform spaceflight participants that the US government does not certify that spaceflight and space vehicles are safe for humans, there are no clear legal requirements for what companies must tell passengers beyond this. Companies are required to provide a list of the risks involved in writing, but this list is likely to appear as just one page in a stack of waivers most passengers may sign without reading.

It would seem logical to have strong legal guidelines governing private companies bringing civilian passengers into space, but unfortunately this is not the case. There is some law regarding human spaceflight from the US, but this applies only to space crew and is surprisingly hands-off regarding civilian non-crew members and space tourists. All space crew members must have a Class II Airman Medical Certification, which restricts individuals with medical conditions like diabetes from participating in space flight [1]. In addition, all space crew must undergo a medical screening every 12 months by a licensed physician board-certified in aerospace medicine [2]. Although these basic guidelines seem entirely sensible for anyone participating in space flight, they are not required for any civilian passenger. There are currently no legal criteria for civilian spacefarers. The FAA has released a report entitled “FAA Recommended Practices” regarding commercial spaceflight, but, as the title states, these are recommendations, not requirements. In a paper discussing space flight safety, Dr. Sara Langston says “Recommended practices do not constitute legal obligation, and unlike for crewmembers there are no professional board requirements or specific expertise legally required for the medical practitioners certifying [spaceflight participants] for spaceflight” [3].

Nothing is legally stopping a company from putting a 65-year-old man with a high risk for heart attacks on a rocket and launching him into space. This illuminates a glaring issue with the burgeoning private space industry: the lack of regulation. To effectively hold companies to responsible standards and ethical practices, it is crucial that a legitimate governing body be created to regulate commercial space flight. Specifically, regarding civilian spaceflight participants, there must be clear laws put in place to protect space travelers. Ethically mandated regulations should align with the practices of spaceflight companies, but unfortunately, this will not always be the case. Although most companies might act responsibly, extensive measures are necessary to mitigate the risk of spaceflight to consumers. Without oversight, companies do not have the incentive to go to the same lengths NASA does when sending astronauts into orbit. The FAA “Recommended Practices” makes some bare-minimum and rather mediocre suggestions for safety, but without a much more robust set of requirements, would-be commercial space passengers face dangers they are likely unaware of and certainly unprepared for.

There are currently only a handful of companies with publicly announced plans to offer passenger tickets for space flight, including Boeing, Blue Origin, SpaceX, XCOR, and Virgin Galactic. These companies plan to offer an array of different space trips, including sub-orbital flight (simply up and down to and from the same places), transcontinental suborbital flight for rapid transportation, and even interplanetary space travel. These flights are all still theoretical, but in 2018 SpaceX announced concrete plans to launch seven civilians on a return mission around the moon in 2023. Between the varying mission designs and the companies developing them, it will become increasingly difficult to assess the risk of each without an established authority overseeing all forms of civilian space flight. The expansion of this industry necessitates the implementation of both a robust ethical code and firmer laws and regulations.

However, as we move into an era where private companies aim to launch several times a day, it may be that the risk involved in civilian spaceflight is effectively minimized by future innovations and development. If the industry can move forward practically, sustainably, and ethically, the possibilities for human advancement are thrilling. Dozens of successful commercial flights could result in public opinion embracing the idea of commercial space travel for civilians. This could be fantastic for the industry; increased attention, interest, and investment could catapult space technology forward to reach incredible new heights. However, if risk is handled poorly by hasty companies trying to make quick revenue, the consequences could be tragic for both passengers and the environment. Therefore, it is imperative that a governing body be put in place to deal with the inherent risks of space flight. With effective regulations in place, the space industry will be able to flourish and carry humankind closer to the stars than ever before.

By Brandon Dillon, Viterbi School of Engineering, University of Southern California

About the Author

At the time of writing this paper, Brandon Dillon was a junior at USC studying Astronautical Engineering.

[1] “14 CFR Subpart C – Second-Class Airman Medical Certificate.”  Legal Information Institute . [Online]. Available:  www.law.cornell.edu/cfr/text/14/part-67/subpart-C

[2] “14 CFR § 460.5 – Crew Qualifications and Training.”  Legal Information Institute . [Online]. Available: www.law.cornell.edu/cfr/text/14/460.5.

[3] S. M. Langston, “Commercial space travel understanding the legal, ethical and medical implications for commercial spaceflight participants and crew,” 2017 8th International Conference on Recent Advances in Space Technologies (RAST), Istanbul, 2017, pp. 489-494, doi: 10.1109/RAST.2017.8002956.

[4] A. Mann. “Space Tourism to Accelerate Climate Change.”  Nature News , 22 Oct. 2010. [Online]. Available: www.nature.com/news/2010/101022/full/news.2010.558.html#B1.

[5] M. Ross, M. Mills, & D.  Toohey. “Potential climate impact of black carbon emitted by rockets” in  Geophysical Research Letters, vol. 57, no. 24, 2010. [Online]. Available: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010GL044548?scrollTo=references

Related Links

https://www.wired.co.uk/article/spacex-blue-origin-space-tourism

https://www.vox.com/2015/2/6/18073658/private-space-flight#E6903638

http://ethics.calpoly.edu/nanoethics/paper042406.html

ScienceDaily

Do astronauts experience 'space headaches'?

Space travel and zero gravity can take a toll on the body. A new study has found that astronauts with no prior history of headaches may experience migraine and tension-type headaches during long-haul space flight, which includes more than 10 days in space. The study was published in the March 13, 2024, online issue of Neurology ® , the medical journal of the American Academy of Neurology.

"Changes in gravity caused by space flight affect the function of many parts of the body, including the brain," said study author W. P. J. van Oosterhout, MD, PhD, of Leiden University Medical Center in the Netherlands. "The vestibular system, which affects balance and posture, has to adapt to the conflict between the signals it is expecting to receive and the actual signals it receives in the absence of normal gravity. This can lead to space motion sickness in the first week, of which headache is the most frequently reported symptom. Our study shows that headaches also occur later in space flight and could be related to an increase in pressure within the skull."

The study involved 24 astronauts from the European Space Agency, the U.S. National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency. They were assigned to International Space Station expeditions for up to 26 weeks from November 2011 to June 2018.

Prior to the study, nine astronauts reported never having any headaches and three had a headache that interfered with daily activities in the last year. None of them had a history of recurrent headaches or had ever been diagnosed with migraine.

Of the total participants, 22 astronauts experienced one or more episode of headache during a total of 3,596 days in space for all participants.

Astronauts completed health screenings and a questionnaire about their headache history before the flight. During space flight, astronauts filled out a daily questionnaire for the first seven days and a weekly questionnaire each following week throughout their stay in the space station.

The astronauts reported 378 headaches in flight.

Researchers found that 92% of astronauts experienced headaches during flight compared to just 38% of them experiencing headaches prior to flight.

Of the total headaches, 170, or 90%, were tension-type headache and 19, or 10%, were migraine.

Researchers also found that headaches were of a higher intensity and more likely to be migraine-like during the first week of space flight. During this time, 21 astronauts had one or more headaches for a total of 51 headaches. Of the 51 headaches, 39 were considered tension-type headaches and 12 were migraine-like or probable migraine.

In the three months after return to Earth, none of the astronauts reported any headaches.

"Further research is needed to unravel the underlying causes of space headache and explore how such discoveries may provide insights into headaches occurring on Earth," said Van Oosterhout. "Also, more effective therapies need to be developed to combat space headaches as for many astronauts this a major problem during space flights."

This research does not prove that going into space causes headaches; it only shows an association.

A limitation of the study was that astronauts reported their own symptoms, so they may not have remembered all the information accurately.

The study was supported by the Netherlands Organization for Scientific Research.

  • Headache Research
  • Cold and Flu
  • Diseases and Conditions
  • Disorders and Syndromes
  • Spirituality
  • Space Station
  • Space Probes
  • Tension headache
  • Cluster headache
  • Space elevator
  • International Space Station
  • Lunar space elevator
  • Outer space

Story Source:

Materials provided by American Academy of Neurology . Note: Content may be edited for style and length.

Journal Reference :

  • Willebrordus P.J. van Oosterhout, Matthijs J.L. Perenboom, Gisela M. Terwindt, Michel D. Ferrari, Alla A. Vein. Frequency and Clinical Features of Space Headache Experienced by Astronauts During Long-Haul Space Flights . Neurology , 2024; 102 (7) DOI: 10.1212/WNL.0000000000209224

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Do astronauts experience 'space headaches'?

by American Academy of Neurology

International Space Station

Space travel and zero gravity can take a toll on the body. A new study has found that astronauts with no prior history of headaches may experience migraine and tension-type headaches during long-haul space flight, which includes more than 10 days in space. The study was published in Neurology .

"Changes in gravity caused by space flight affect the function of many parts of the body, including the brain," said study author W. P. J. van Oosterhout, MD, Ph.D., of Leiden University Medical Center in the Netherlands.

"The vestibular system , which affects balance and posture, has to adapt to the conflict between the signals it is expecting to receive and the actual signals it receives in the absence of normal gravity. This can lead to space motion sickness in the first week, of which headache is the most frequently reported symptom. Our study shows that headaches also occur later in space flight and could be related to an increase in pressure within the skull."

The study involved 24 astronauts from the European Space Agency, the U.S. National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency. They were assigned to International Space Station expeditions for up to 26 weeks from November 2011 to June 2018.

Prior to the study, nine astronauts reported never having any headaches and three had a headache that interfered with daily activities in the last year. None of them had a history of recurrent headaches or had ever been diagnosed with migraine.

Of the total participants, 22 astronauts experienced one or more episodes of headache during a total of 3,596 days in space for all participants.

Astronauts completed health screenings and a questionnaire about their headache history before the flight. During space flight, astronauts filled out a daily questionnaire for the first seven days and a weekly questionnaire each following week throughout their stay in the space station .

The astronauts reported 378 headaches in flight. Researchers found that 92% of astronauts experienced headaches during flight compared to just 38% of them experiencing headaches prior to flight.

Of the total headaches, 170, or 90%, were tension-type headache and 19, or 10%, were migraine.

Researchers also found that headaches were of a higher intensity and more likely to be migraine-like during the first week of space flight. During this time, 21 astronauts had one or more headaches for a total of 51 headaches. Of the 51 headaches, 39 were considered tension-type headaches and 12 were migraine-like or probable migraine.

In the three months after return to Earth, none of the astronauts reported any headaches.

"Further research is needed to unravel the underlying causes of space headache and explore how such discoveries may provide insights into headaches occurring on Earth," said Van Oosterhout. "Also, more effective therapies need to be developed to combat space headaches as for many astronauts this a major problem during space flights."

This research does not prove that going into space causes headaches; it only shows an association.

A limitation of the study was that astronauts reported their own symptoms, so they may not have remembered all the information accurately.

Journal information: Neurology

Provided by American Academy of Neurology

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5 dangers of space travel

UK and US lack regulation to protect space tourists from cosmic ray dangers

Damage to dna, mutations, uncontrolled cell division and malignancy. is space tourism worth the risk.

International regulations governing space flight lack rules to protect space tourism passengers from the ill-effects of cosmic radiation, according to researchers.…

Over the last decade, entrepreneurial billionaires such as Jeff Bezos and Richard Branson have launched commercial space flights targeting tourists that are prepared to part with huge sums of money for a few minutes of low orbit travel.

However, scientists point to the the dangers of sudden changes in space weather — the radiation present in space, such as solar wind of heavy ions and atomic nuclei. These changes include ground level enhancement (GLE), when the charged particles from the Sun have enough energy measurable at the Earth's surface. They also include solar particle events (SPEs) when particles emitted by Sun — mostly protons — are accelerated by the Sun's atmosphere or space by a coronal mass ejection shock.

Either can have “significant health implications for crew and passengers,” the research published by journal Space Policy says.

“Exposure to elevated levels of ionising radiation, such as those possible during GLE or SPE events, has been noted by the UK Health Protection Agency, to potentially ‘cause damage to DNA, lead to mutations, uncontrolled cell division and lead to malignancy’,” the paper from the University of Surrey says.

“The space tourism industry is currently not fully aware of the radiation risks and is instead relying on incomplete ‘informed’ consent basis for non-crew space flight participants. Further, the current regulation for the industry places the risk burden firmly on the space tourist,” adds the research paper led by Chris Rees, postgraduate research student.

The study reckons that existing regulation applies to space flight crews rather than paying passengers. “For the potential space tourist, regulation is still catching up with this new industry. Further input into potential future regulation will be required going forward, noting that current fit-for-purpose legislation for radiation protection and associated risk assessment for space tourism does not exist,” the paper says.

On both sides of the Atlantic a patchwork of laws fails to provide protection for space tourists, the researchers argue.

For example, UK Air Navigation Order (ANO) rules for cosmic radiation impose limits on air crew exposure over a year. “The ANO forms the basis of regulation in the UK for very high altitude flights and could potentially be applied to future space tourism,” the paper points out.

However, the ANO does not set out requirements for real-time radiation monitoring on commercial aircraft or spacecraft.

Space tourism is expected to take off. Research found the market worth $848 million in 2023 and could grow to $27.9 billion by 2032.

Virgin Galactic took its first paying passengers in June last year, taking off from Spaceport America in New Mexico.

In May 2021, Jeff Bezos's Blue Origin space tourism venture set its price for just such an experience at $1.4m (£1m) or best offer. It made its first flight in July that year. ®

UK and US lack regulation to protect space tourists from cosmic ray dangers

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Why flying is still safe despite high-profile problems

Juliana Kim headshot

Juliana Kim

5 dangers of space travel

Two United Airlines Boeing 737s are parked at the gate at the Fort Lauderdale-Hollywood International Airport in Fort Lauderdale, Fla., on July 7, 2022. Wilfredo Lee/AP hide caption

Two United Airlines Boeing 737s are parked at the gate at the Fort Lauderdale-Hollywood International Airport in Fort Lauderdale, Fla., on July 7, 2022.

Despite a string of high-profile flight mishaps on United Airlines flights in recent days, commercial air travel is still very safe, experts say.

"Planes that we fly on today, they're safer than they've ever been," says Anthony Brickhouse, a professor of aerospace safety at Embry-Riddle Aeronautical University in Daytona Beach, Fla.

This past week, United Airlines was under the spotlight for a series of emergencies: Last Monday, the engine of one of its planes ingested bubble wrap and caught fire in mid-air. On Thursday, a tire of an aircraft fell off and smashed into a parked car shortly after takeoff. On Friday, one of its planes experienced a hydraulics issue.

No one was injured in any of the United incidents. The airline said in a statement that safety is its top priority and it will investigate each event.

The back-to-back emergencies come two months after a door plug tore off and left a gaping hole during an Alaska Airlines flight. No one aboard was seriously injured.

That plane, along with the jetliners involved in Monday and Thursday's United incidents, was made by Boeing. Meanwhile, the plane involved in Friday's emergency was made by Airbus.

Boeing is withholding key details about door plug on Alaska 737 Max 9 jet, NTSB says

Boeing is withholding key details about door plug on Alaska 737 Max 9 jet, NTSB says

While those recent events paint an unsettling image of the skies, aviation safety experts say it's not the whole picture, and it's not enough to write off Boeing, United or flying in general.

When asked about the recent concerns over flying, Transportation Secretary Pete Buttigieg said federal data shows flight safety has been improving over the years. "American aviation is the safest means of travel in the world," he said at a press conference Monday.

5 dangers of space travel

A damaged car is seen in an airport employee parking lot after tire debris from a Boeing 777 landed on it at San Francisco International Airport on Thursday. Haven Daley/AP hide caption

Here's what to know.

Globally, commercial plane accidents and deaths are rare and declining

According to the International Air Transport Association (IATA), a trade association for airlines, the number of accidents involving commercial planes has been going down over the years.

In 2023, the commercial aviation industry saw 30 total accidents, one of which was fatal — 72 people were killed in Nepal in what investigators said was pilot error . A year prior, there were five deadly accidents with 158 fatalities. In 2013, there were 11 fatal accidents causing 638 fatalities, according to the IATA.

To put it differently, the risk of boarding a fatal flight has been getting lower. Arnold Barnett, a statistician at the Massachusetts Institute of Technology who has studied airline safety , tells NPR that from 2018 to 2022, the chances of a passenger being killed on a flight anywhere in the world was 1 in 13.4 million. Between 1968 to 1977, the chance was 1 in 350,000.

The FAA gives Boeing 90 days to fix quality control issues. Critics say they run deep

The FAA gives Boeing 90 days to fix quality control issues. Critics say they run deep

"Worldwide flying is extremely safe, but in the United States, it's extraordinarily so," Barnett said.

In the U.S., there has not been a fatal plane crash involving a major American airline since February 2009, though there have been a handful of fatalities since then.

Brickhouse, who has studied aviation safety for over 25 years, often tells people that the biggest risk of any air journey tends to be driving to the airport.

More than 40,000 people are killed on U.S. roads each year.

"Aviation remains the safest mode of transportation," he says.

Planes are designed to keep working even if something goes wrong

Despite a missing tire, an inflamed engine, and a hydraulics issue, all three United Airlines planes were still able to safely land last week.

"Not to minimize these recent events because they each were serious, but aircraft are designed with what we call redundancy," Brickhouse says. "So if one system fails, there's a backup."

According to United, its aircraft are built to land safely with missing or damaged tires. The airline added that the plane with the hydraulics issue had two additional hydraulic systems in place for the same reason.

Flight attendants don't earn their hourly pay until aircraft doors close. Here's why

Flight attendants don't earn their hourly pay until aircraft doors close. Here's why

Pilots and crew are well-trained for emergencies.

Just like planes are built to function when something goes awry, flight crews are also extensively trained to know what to do in emergencies.

"There's a big misconception with flight attendants, that they're there to serve us drinks and help us find our seats," Brickhouse says. "The primary role of flight attendants is safety."

He adds that pilots and flight attendants practice both inside and outside the classroom on how to evacuate in water or when the plane is filled with smoke, and even what to do when a laptop catches on fire.

There are steps passengers can take to increase their own safety

Have you ever worn a polyester shirt or heels to a flight? According to Brickhouse, that's exactly what not to wear in case of an emergency.

He says natural fibers such as cotton and comfortable, closed-toe footwear provide an extra layer of protection and coverage in extreme scenarios, like a fire. He also recommends passengers eat a meal before they fly, as it helps with "survivability" if there's an extended time without food available.

Here's how to tell if your next flight is on a Boeing 737 Max 9

Here's how to tell if your next flight is on a Boeing 737 Max 9

On the flight, Brickhouse recommends making sure to listen to the safety briefings and take note of where the emergency exits are. "That only takes a few minutes and then you can get on to what you need to do," he says.

NPR's Joel Rose contributed reporting.

Middle East latest: Former IDF commander tells Israelis to overthrow government - after top US leader says Netanyahu must go

The US senate majority leader launches a major attack on Benjamin Netanyahu, saying the Israeli leader must go. The UN, meanwhile, hits out at Israel after a strike on a food distribution centres - and the EU's aid chief says parts of Gaza are already suffering famine.

Thursday 14 March 2024 21:16, UK

  • Israel-Hamas war

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  • Schumer calls for Netanyahu to go in 'significant development'
  • 'Take to the streets': Former IDF commander calls for Israelis to overthrow Netanyahu government
  • Explained: What did Chuck Schumer say about Netanyahu?
  • UN denounces deadly Israeli strike on UN food centre in Gaza
  • Israeli tank strike killed 'clearly identifiable' reporter, UN report finds
  • Pockets of famine already present in Gaza, EU aid chief says
  • Dominic Waghorn: Failures of Israel and international community mean more innocents will starve to death
  • Live reporting by Brad Young  and (earlier) Samuel Osborne

Hamas says it has put forward a "comprehensive vision" of a ceasefire deal to mediators.

The "comprehensive" proposal was based on the provision of aid, return of displaced Palestinians, withdrawal of Israeli forces, and a "prisoner exchange", Hamas said.

It did not elaborate on any of the proposals, only adding that the "vision" was proposed following negotiations with Egypt and Qatar.

Talks have failed to find a breakthrough for weeks, with the US saying there was a deal on the table and it was up to Hamas to accept it.

Satellite photos show progress in the construction of a jetty in Gaza which will receive maritime aid deliveries.

World Central Kitchen, the charity founded by celebrity chef Jose Andres, is preparing the port to receive much-needed food aid from two ships.

The satellite photos show how construction has progressed between 11-13 March.

Mr Andres tweeted to say the construction was nearing 60m and shared footage of a bulldozer reversing over a hardpacked rubble jetty on the Gaza shore.

Of the two ships heading to Cyprus from Gaza, the first is towing a barge carrying almost 200 tonnes of food aid and is nearing the besieged Palestinian enclave in a pilot trial for maritime deliveries.

A second ship is being loaded with food aid for Gaza in Cyprus, a charity arranging the mission has said.

World Central Kitchen said it was loading the vessel at Larnaca port with 300 tonnes of food aid - including legumes, canned tuna, vegetables, rice and flour.

It did not say when the vessel would set sail. 

A call by US Senate majority leader Chuck Schumer for fresh elections in Israel has prompted a torrent of reactions.

His criticism of the Israeli prime minister on the floor of the Senate was perhaps the most scathing of any American politician of his ranking since the war began.

While it does not represent White House policy - the Biden administration has distanced itself, saying Congress is an independent body - the comments have prompted heavy criticism from Mr Netanyahu's Likud party.

What did Chuck Schumer say?

Mr Schumer spoke for 30 minutes on what he called the four obstacles to peace: Hamas and any Palestinians who support it; radical right-wing Israelis; Palestinian leader Mahmoud Abbas; and Benjamin Netanyahu.

He first called for Mr Abbas to step down for his evasion of elections, failure to empower future leadership, and his embrace of antisemitism.

But it was Mr Schumer's criticism of Benjamin Netanyahu that has drawn the most attention.

"As the first Jewish majority leader of the US Senate and the highest-ranking Jewish elected official ever, I also feel very keenly my responsibility as... a guardian of the people of Israel," he began.

"To achieve that lasting peace that we so long for, Israel must make some significant course corrections."

Netanyahu 'stifling' Israeli people

"The Netanyahu coalition no longer fits the needs of Israel after 7 October. The world has changed — radically — since then, and the Israeli people are being stifled right now by a governing vision that is stuck in the past."

'Too willing to tolerate the civilian toll in Gaza'

"Netanyahu lost his way by allowing his political survival to take precedence over the best interests of Israel. He has put himself in coalition with far-right extremists... and as a result has been too willing to tolerate the civilian toll in Gaza, which is pushing support for Israel worldwide to historic lows. Israel cannot survive if it becomes a pariah."

The day after

"He has shown zero interest in doing the courageous and visionary work required to pave the way for peace even before this present conflict," he said.

Mr Schumer said the Israeli prime minister's rejection of Palestinian statehood was a "grave mistake" for the region and the world, and that a "single state controlled by Israel... guarantees certain war forever".

"The only just solution to this predicament is one in which each people can flourish in their own state, side by side."

Call for elections

"As a democracy, Israel has the right to choose its own leaders, and we should let the chips fall where they may.

"But the important thing is that Israelis are given a choice. There needs to be a fresh debate about the future of Israel after 7 October. In my opinion, that is best accomplished by holding an election."

Suggestion US should use leverage with Israel to bring peace 

Sky News US correspondent Mark Stone said one section of the speech may be considered key - in which Mr Schumer suggested the US may opt to make at least parts of its support for Israel conditional upon the country's government changing its approach to Gaza.

"If Prime Minister Netanyahu's current coalition remains in power after the war begins to wind down, and continues to pursue dangerous and inflammatory policies that test existing US standards for assistance, then the United States will have no choice but to play a more active role in shaping Israeli policy by using our leverage to change the present course."

He added: "If extremists continue to unduly influence Israeli policy, then the administration should use tools at its disposal to make sure our support for Israel is aligned with our broader goal of achieving long-term peace and stability in the region."

The comments follow suggestions from some commentators who argue that Israel's reliance on US support is such that Washington would likely have the leverage to force the country to end the offensive in Gaza if it chose to.

Joe Biden committed a "strategic mistake" by "bear-hugging" Benjamin Netanyahu, an ally of the US president has said.

Ro Khanna, a congressional Democrat and Biden campaign surrogate, accused the Israeli prime minister of conducting "a callous war" in Gaza in defiance of the United States.

It comes as US Senate majority leader and fellow Democrat Chuck Schumer launched a blistering attack on Mr Netanyahu, saying he must go.

In remarks on the One Decision podcast reported by The Guardian , Mr Khanna called Mr Netanyahu "insufferably arrogant" for acting as if he is "somehow an equal" to Mr Biden.

He said Mr Biden needed to set out "clear consequences for Netanyahu" if Israel does not change course.

He added: "He needs to say, 'I'm for Israel, but I’m not for this extreme rightwing government.' And that means if [Mr Netanyahu] defies the United States, not allowing aid, or going into Rafah, [then] no more weapons transfers… unconditionally."

Mr Netanyahu has said he intends to launch an offensive in the southern city of Rafah.

Mr Khanna continued: "It means not protecting [Mr Netanyahu] from the entire international community at the United Nations, it means recognising a Palestinian state. And those are the things I think some of the Arab American community want."

A former deputy chief of staff of the Israeli Defence Forces has backed Chuck Schumer's scathing criticism of Benjamin Netanyahu's government - and gone a step further.

Yair Golan, who also served in the Israeli parliament until 2022, called for "democratic forces" in Israel to take to the streets and "overthrow the worst government in our history".

If you're just joining us, Mr Schumer, the US Senate leader, called for new Israeli elections and said the prime minister's toleration of civilian death had pushed "support for Israel worldwide to historic lows".

"The sharp, painful and unprecedented words of the Senate majority leader, Chuck Schumer, reflect the feelings of the Israeli public," said Mr Golan, who announced last month that he would take part in the Israeli Labor Party primary elections in May.

"The Israeli government and its leaders are leading us to political isolation, an unprecedented break in relations with our strategic ally, and a complete failure in the war. 

"All democratic forces in Israel must unite, take to the streets and. It is in our soul."

Israeli finance minister Bezalel Smotrich has also weighed in, rejecting the call for fresh elections.

"We expect the largest democracy in the world to respect Israeli democracy," he said of Mr Schumer's comments.

The European Union's top humanitarian aid official says Israel has not provided any evidence to back its accusations against staff from the UN Palestinian refugee agency, UNRWA.

Israel alleged 12 members of the agency, which provides aid and services, were involved in Hamas's 7 October attacks, prompting donors to pause UNRWA funding.

Janez Lenarci, head of humanitarian aid and crisis management at the European Commission, said he nor anyone else at the commission, nor any other donors, had been presented with evidence by Israel.

"Even if those allegations, at the end of the day, prove to be true, that doesn't mean that UNRWA is the perpetrator," he said.

In that case, Mr Lenarcic said individual accountability would be in order rather than summary justice - and the "irreplaceable" agency would be asked to clean up and carry on.

"UNRWA has of course a critical role to play here because it has unmatched infrastructure, warehouses, shelters, logistical capacities."

He said the agency had reacted to the allegations "properly, immediately, effectively" with an investigation and review.

Benjamin Netanyahu's Likud party has rejected a call for the prime minister to go by one of America's most senior politicians.

US Senate majority leader Chuck Schumer said the Israeli prime minister had "lost his way" and called for new elections, adding the Israeli people were "stifled right now by a governing vision that is stuck in the past".

In response to the Democrat's speech, the Likud Party said Israel was "not a banana republic".

"Contrary to Schumer's words, the Israeli public supports a total victory over Hamas, rejects any international dictates to establish a Palestinian terrorist state, and opposes the return of the Palestinian Authority to Gaza," the Likud statement said.

"Senator Schumer is expected to respect Israel's elected government and not undermine it. This is always true, and even more so in wartime."

By Mark Stone, US correspondent

The first reaction has come in on the comments made by the majority leader of the US Senate, Chuck Schumer (see 15.06pm post).

Israel's ambassador to the US, Michael Herzog (who is the Israeli president's brother), tweeted: 

Blunt language, given it came from a diplomat.

The White House has commented too.

The spokesman of the National Security Council, Admiral John Kirby, said the White House had been given advance notice Mr Schumer would make the comments.

"He did give our team advance notice... we did have advance notice that he was going to deliver those remarks," Admiral Kirby said, adding: "This wasn't about approval, or disapproval or editing in any way.

"We fully respect his right to make those remarks and to decide for himself what he's going to say on the Senate floor. He obviously feels strongly about this. We understand and respect that.

"We're going to stay focused on making sure that Israel has what it needs to defend itself while doing everything that they can to avoid civilian casualties."

The White House has called for a swift investigation into an Israeli airstrike on a UN food distribution facility in Gaza.

Israel said the strike killed a Hamas commander, while the Hamas-run Gaza health ministry said it killed four more people including a UN worker and wounded another 22.

The UN has also denounced the strike (see 9.03am post).

White House national security spokesperson John Kirby told reporters the US is very concerned about the strike and called for a swift investigation by the Israelis into exactly what happened.

Chuck Schumer's strong criticism of Benjamin Netanyahu and his call for the Israeli prime minister to go is a "significant development" in the Israel-Hamas conflict, a former US ambassador has said.

The Senate majority leader said Mr Netanyahu has "lost his way" in the war in Gaza and said "Israel cannot survive if it becomes a pariah" (see 3.06pm post)

Patrick Gaspard, who served as US ambassador to South Africa under Barack Obama, tweeted to say the speech was a "major shift that should lead to immediate review of unconditional weapons support that pours from US to Netanyahu govt every 36 hours."

Alistair Bunkall, Sky News' Middle East correspondent, said: "Although Chuck Schumer is not a member of the White House administration, he is a senior politician, but I suppose he's gone a lot further than the White House has done.

"But the idea that US politicians, certainly on the Democratic Party side of things, are unhappy with Netanyahu, is something that has been growing in recent weeks."

However, Mark Stone, Sky News' US correspondent, said that even if Mr Netanyahu were to move on "there's no suggestion that the way the war is being prosecuted would change much."

He said the IDF was "the most popular institution in Israel" and that other leaders "while perhaps slightly more moderate than Netanyahu has been, would broadly prosecute the same war."

He added: "What they may differ on is the way after, and they may find that they are less beholden to the far right of Israel's government as well."

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5 dangers of space travel

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    Every stage of a space flight is filled with risks. Here are some of the ways that things can go wrong: Mission Phase 1: Pre-launch Preparations. APOLLO 1, U.S., 1967. Astronauts Gus Grissom, Ed ...

  12. Top 5 risks of space exploration Part 1: Gravity

    These include: gravity, radiation, isolation and confinement, hostile living environment, and. distance from Earth. In the upcoming series we will present each of these risks and investigate the work NASA is doing to keep our astronauts safe on the ISS and our venture beyond Low Earth Orbit.

  13. Five Hazards of Human Spaceflight

    Exploration to the Moon and Mars will expose astronauts to five known hazards of spaceflight: Space Radiation. Isolation and Confinement. Distance From Earth. Gravity Fields. Hostile Closed Environments. Watch a playlist of videos to explain each hazard and to learn what NASA's Human Research Program is doing to protect humans in space.

  14. Traveling to Mars? Top 6 health challenges

    Image via NASA. 3. Weaker muscles. There is no gravity on the International Space Station (ISS), and Mars only has about a third of Earth's gravity. This plays havoc with the human body, Jurblum ...

  15. Space Radiation is Risky Business for the Human Body

    Space radiation can lead to other effects. Radiation can alter the cardiovascular system, damaging the heart, harden and narrow arteries, and/or eliminate some of the cells in linings of the blood vessels, leading to cardiovascular disease. Radiation exposure can hinder neurogenesis, the process of generating new cells in the brain.

  16. How safe is space travel? An in-depth look at the ...

    Space tourism is seen as a glamorous and exciting adventure by many, but what is often forgotten are the significant risks involved in undertaking a journey to outer space. From the dangers of radiation exposure to the potential for collisions with space debris, there are many hazards that can occur during a space mission.

  17. Is It Safe for Humans To Go Up to Space? ISS Experiments ...

    The study in mouse cells analyzed the impact of space radiation and will help scientists better estimate the safety and dangers of space travel. An international team of scientists conducted a long-term experiment onboard the International Space Station to investigate the impact of space radiation on mouse embryonic stem cells. Their research ...

  18. Human Health during Space Travel: State-of-the-Art Review

    Suborbital spaceflight, part of the new era of space travel, has participants launching to the edge of space (defined as the Karman line, 100 Km above mean sea level) for brief 3-5 min microgravity exposures. Orbital spaceflight, defined as microgravity exposure for up to 30 days, involves healthy individuals with preflight medical screening.

  19. Profitable Risk: The Dangers of Consumer Spaceflight and Space Tourism

    The wave of new companies entering the spaceflight industry in the 1990s and 2000s made leaps in space technology development, and their successes have encouraged them to pursue new opportunities beyond the bounds of conventional research-based space travel. This is where the concept of space tourism begins. Until now, all spaceflight has been ...

  20. Do astronauts experience 'space headaches'?

    Space travel and zero gravity can take a toll on the body. A new study has found that astronauts with no prior history of headaches may experience migraine and tension-type headaches during long ...

  21. Do astronauts experience 'space headaches'?

    Space travel and zero gravity can take a toll on the body. A new study has found that astronauts with no prior history of headaches may experience migraine and tension-type headaches during long ...

  22. Understanding the Psychological Hazards of Spaceflight on the Space

    In addition to research into the physical health of astronauts, scientists also use the International Space Station and low-Earth orbit to investigate the psychological and behavioral impacts of long-duration missions in space. Isolation and distance from Earth present unique challenges when choosing a crew for missions to the Moon and Mars.

  23. UK and US lack regulation to protect space tourists from cosmic ray dangers

    Damage to DNA, mutations, uncontrolled cell division and malignancy. Is space tourism worth the risk? International regulations governing space flight lack rules to protect space tourism ...

  24. Why flying is still safe despite high-profile problems : NPR

    Wilfredo Lee/AP. Despite a string of high-profile flight mishaps on United Airlines flights in recent days, commercial air travel is still very safe, experts say. "Planes that we fly on today ...

  25. Space Radiation Risks

    The biological effects of space radiation, including acute radiation risks (ARS), are a significant concern for human spaceflight. The primary data that are currently available are derived from analyses of medical patients and persons accidentally exposed to high doses of radiation. High doses of radiation can induce profound radiation sickness ...

  26. Middle East latest: UN hits out at Israel after strike on food

    Israel's military strike on a United Nations food distribution centre in Gaza is "another symbol of the fact there is less and less respect for humanitarian work" and the protection the UN flag ...

  27. The Human Body in Space

    The Key: The current strategy to reduce the health risks of space radiation exposure is to implement shielding, radiation monitoring, and specific operational procedures. Compared to typical six-month space station missions, later Moon and Mars missions will be much longer on average. Consequently, the total amount of radiation experienced and associated health risks may increase.