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Interstellar travel: From science fiction to reality

Humans may one day travel beyond the solar system on board rockets powered by nuclear fusion.

An artist's impression of the Voyager 1 spacecraft entering interstellar space. Photo: Nasa

Only two probes have reached interstellar space – the region beyond the solar system – since the start of human space exploration .

It took more than three decades for the first spacecraft, Voyager 1 , to cross the heliopause, a boundary scientists believe is where interstellar space begins, after its launch in 1977.

It was an incredible achievement with invaluable data sent back through a medium not influenced by the Sun.

But with its power supply weakening it is almost impossible Voyager 1 will reach our nearest star, Proxima Centauri, which is 4.2 light years away and would take the probe almost 73,000 years.

Les Johnson, a Nasa scientist and author of several scientific and science fiction books, told The National reaching another star could take 50 to 100 years.

“It is possible we might have the technology to send our first robotic probe to another star within the next 50 to 100 years,” said Mr Johnson, who managed the Interstellar Propulsion Research Project at the US space agency.

“Based on the rate of technology growth, after looking at all the propulsion systems that are based on known physics, I believe these first probes will be propelled to the stars using laser light reflecting from a sail, similar to today’s solar sails but driven by intense laser light instead of sunlight.”

Human travel to interstellar space

It could take significantly longer for a crewed mission to travel to another star, as laser light sails would only work for smaller spacecraft.

Nuclear fusion propulsion, a way of powering a spacecraft using high-energy particles created by fusion reactions, is needed to make human missions to interstellar space possible.

“As for humans, that’s a lot more complicated because it takes a lot of mass to keep a group of humans alive for a decade- to centuries-long space journey and that means a massive ship,” Mr Johnson said.

“For a human crewed ship, we will need fusion propulsion at a minimum and antimatter as the ideal.

“While we know these are physically possible, the technology level needed for interstellar travel seems very far away – perhaps 100 to 200 years in the future.”

While scientists dream of antimatter propulsion, which could enable space travel at 70 per cent of the speed of light, nuclear fusion propulsion appears much closer to reality.

The technology could also help reduce the time it takes to reach Mars, the planet to which most space agencies are trying to send their astronauts.

Laura Forczyk, an author and the founder of space consulting firm Astralytical, said nuclear fusion propulsion has the potential to revolutionise space flight.

"We will not be able to achieve interstellar travel until we engineer a faster and more efficient means of accelerating," said Ms Forczyk.

"We also need to develop long-term, self-sustained robust ecosystems for long-duration voyages and a better means of radiation shielding. We are at least a century away from these advances."

While developing nuclear fusion technology is not easy, with the required temperature to achieve it 10 times hotter than the Sun, a few companies have been trying for many years.

California-based start-up Helicity Space recently received $5 million in seed funding to accelerate its fusion propulsion technology projects.

UK-based start-up Pulsar Fusion is also attempting to develop the technology and has started construction on a large nuclear fusion chamber in England.

When the engine is fired, it would, at least temporarily, be the hottest place in the solar system.

China is also trying to advance the development of nuclear fusion, with reports of a new state-owned company that would help accelerate the production of an "artificial Sun".

Is there an Earth-like planet beyond the solar system?

One of the reasons scientists want to explore interstellar space is to learn more about the universe and possibilities of life beyond Earth.

Nasa has been studying exoplanets, or planets outside of the solar system, for decades by using telescopes.

Mr Johnson said he "doubts we will find Earth 2.0 anywhere close" and that discovering another home-like planet was not thought to be a near-term possibility.

"Over time, we might be able to modify another planet to make it habitable but that will likely take additional centuries or millennia," he said.

"That doesn’t mean we shouldn’t go. The job of science is to learn more about the universe in which we live.

"Studying planets around other stars will help us better understand our own solar system and expand our knowledge of this big universe. That alone, in my opinion, makes the journey one we should make."

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From solar sails to hyperspace: Here’s the future of interstellar travel

Will it ever be possible to zip around the cosmos like Baby Yoda and co in The Mandalorian ? A physicist explains.

Stephen Kelly

Hyperspace travel – the ability for spaceships to travel as fast as the speed of light – is an integral part of the Star Wars universe.

The new series of The Mandalorian , for instance, would be quite a different show if Mando and Grogu had to wait 6,000 years to fly from one planet to the next.

But it does raise the question of just how feasible such technology is. Will we ever attain the ability to travel vast distances across space without having to burn through generations of astronauts? Or are we just marooned in our corner of the Galaxy?

To answer that, Prof Patrick Johnson , author of The Physics Of Star Wars , needs to be clear on what, exactly, hyperspace travel is.

“If we’re talking about the idea that spaceships in Star Wars accelerate forward as fast, or faster than, the speed of light, then that is not plausible,” he says.

“The speed of light is the speed limit of the Universe. Once you travel faster than light, cause and effect begin to break down. You could feasibly view an event happening five light-years away, travel faster than the speed of light towards it and get there before it happens. It’s basically time travel.”

A more realistic option, says Johnson, is that instead of ships in Star Wars accelerating at the speed of light, hyperdrive technology enables them to create a wormhole in order to travel from one point in space to another.

“It would be like folding a piece of paper and punching a hole through it so an ant can get to the other side,” he says.

It would also explain why, in certain hyperspace sequences in Star Wars , the ships look like they’re travelling through a blue tunnel. (Although, of course, there are also other scenes, such as Vice-Admiral Holdo flying a ship at light speed through another ship in The Last Jedi , that lend strength to the accelerating theory).

The problem with creating a wormhole, says Johnson, is that no one knows how to do it; it’s more theoretical than practical. “It would perhaps be easier to find a pre-existing wormhole and just hope it leads to somewhere you want to go,” he says.

“As for creating one, that would take decades’ worth of human energy. And that’s before we figure out how to target a particular location.”

Johnson also insists that any wormhole created would have to be far away from Earth.

“Going back to the paper analogy, it’s not going to end well for the ant if it happens to be in the spot where the pencil bursts through. You would need an empty, unoccupied part of space, along with incredibly complex calculations and some sort of up-to-date star map, to ensure that the planet you’re going to is in the right place.”

At this stage, both options seem as unlikely as each other, which means that we are stuck in our Solar System for the foreseeable future.

But Johnson lives in hope that, even if we can’t accelerate at light speed, we can develop ways to travel long distances across the Galaxy.

“Without hyperdrives, I’d say our best bet for getting far, far away in a reasonable amount of time is solar sail technology,” he says. “These would operate like wind sails, but instead use the light from a star to push the ship forward and essentially give it an unlimited supply of fuel. The acceleration would be small at first, but after 10 years you could get up to around 20 per cent of the speed of light, which is very good.”

Promising – but we wouldn’t book that holiday to Tatooine just yet.

About our expert

Patrick Johnson is an associate teaching professor at Georgetown University’s department of physics, and author of The Physics Of Star Wars . Earning his PhD at Washington University in St Louis, Johnson’s work on quantum mechanics has been published in Physical Review journal.

• Tardigrades could be the first interstellar space travellers
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Interstellar Insights: Relativity Theory Through a Physicist’s Lens

• February 29th, 2024
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Table Of Contents

Interstellar Insights – The film “Interstellar” not only captivated audiences with its compelling narrative and stunning visuals but also served as a striking exploration of the theoretical underpinnings of the universe as depicted by Albert Einstein’s Theory of Relativity. Focusing on the challenges of interstellar travel, the film thoughtfully intertwines science fiction with sophisticated physics, presenting a story that pushes the boundaries of human understanding and cinematic storytelling. Renowned physicist Kip Thorne’s involvement in the film ensured that its portrayal of complex scientific concepts, such as time dilation and gravitational anomalies, adhered as closely as possible to current scientific theories.

While “Interstellar” journeys through the realms of speculative fiction, it does ground its narrative in the real science of space, time, and gravity. An important aspect of the film’s scientific backbone is the Theory of Relativity, which revolutionized how humanity perceives the cosmos. Einstein’s legacy looms large as the characters navigate through wormholes, experience time differently near massive bodies like black holes , and wrestle with the laws of physics that govern their reality. This tale pushes the viewer to consider the true nature of our universe and the current limits of human knowledge and capability in space exploration.

Key Takeaways

• “Interstellar” merges cinematic fiction with real-world physics, particularly Einstein’s Theory of Relativity.
• The science behind the film, rooted in actual theoretical physics, offers a depiction of space-time, black holes, and relativity.
• Portraying the concept of interstellar travel challenges and inspires viewers to think about humanity’s place in the universe.

The Legacy of Albert Einstein

Albert Einstein’s revolutionary discoveries fundamentally reshaped our understanding of the universe . His work on the general theory of relativity provides not just a framework for modern physics but also a lens through which we view space and time.

Foundational Principles

Albert Einstein forever altered the course of physics with his formulation of general relativity in 1915. This theory unveiled gravity as a consequence of the warping of space-time, caused by mass and energy. General relativity has stood the test of time, consistently confirmed by astronomical observations and experiments. Its implications reach far beyond the academic realm, affecting everything from GPS satellite navigation to our understanding of black holes and the expansion of the universe.

Einstein-Rosen Bridge

Among the many intriguing solutions to Einstein’s field equations in general relativity is the concept of the Einstein-Rosen Bridge , often referred to as a ‘wormhole’. Initially postulated in a paper by Einstein and physicist Nathan Rosen, this theoretical tunnel connects disparate points in space-time, positing a shortcut through the universe. While wormholes have captured the public’s imagination and are a staple of science fiction , they remain a speculative yet fascinating consequence of Einstein’s groundbreaking work in the fabric of space-time.

Interstellar Travel in Theory and Fiction

Interstellar travel captivates the imagination, merging the rigor of theoretical physics with the creative liberty of science fiction stories. It stands at the convergence of humanity’s quest for new horizons and the scientific community’s understanding of the universe.

Concepts from Science Fiction

The concept of interstellar travel in science fiction is rooted in the idea of surpassing the vast distances between stars within a human’s lifetime. Sci-Fi often employs advanced technologies and theoretical constructs to make this possible. One prominent example is seen in the movie Interstellar , where characters traverse these immense distances through a wormhole, a theoretical passage through spacetime that can create shortcuts for long journeys.

Examining the Alcubierre Drive

In the realm of theoretical physics, the Alcubierre Drive stands out as a concept that could, theoretically, permit faster-than-light travel, echoing science fiction narratives. Proposed by physicist Miguel Alcubierre in 1994, this speculative technology would involve warping spacetime around a spacecraft. However, the energy requirements and exotic matter needed for an Alcubierre Drive are beyond our current capabilities, reaffirming that interstellar travel remains a vision for the future rather than a current reality.

Exploring the Cosmos

In the vast expanse of the universe, the theoretical possibilities of space travel through wormholes and around black holes captivate physicists and laypersons alike, presenting complex challenges rooted in Einstein’s Theory of Relativity.

Wormholes and Black Holes

Wormholes—hypothetical passages through spacetime—offer a tantalizing means of instantaneously traversing vast distances in the universe. The movie “Interstellar” portrays these phenomena as cosmic shortcuts, which, if traversable, could revolutionize our understanding of travel among the stars. Research into wormholes and their potential for real-world space exploration remains an area of intense scientific interest.

Black holes , on the other hand, are regions of spacetime exhibiting gravitational forces so strong that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it. They play a central role in the film, demonstrating their powerful influence on time dilation, a major prediction of the Theory of Relativity. The depiction of a black hole in “Interstellar” was based on theoretical equations , contributing to a visually accurate portrayal that educated audiences about their appearance and properties.

The Role of Exotic Matter

Exotic matter refers to hypothetical substances that possess unusual properties, such as negative mass or energy density. It’s theorized to be a critical component for creating and maintaining wormholes, as it could theoretically counteract the gravitational pull that would naturally close these spacetime bridges. Interstellar” draws on this concept through its narrative, using exotic matter as a plot device that allows characters to navigate the cosmos . This concept pushes the boundaries of our current understanding and prompts ongoing research within the field of astrophysics.

Each topic discussed here, from the exotic matter required to stabilize wormholes to the extreme gravitational effects of black holes, exists at the edge of our scientific knowledge and the forefront of space exploration discussions. As humanity looks to the stars, these theories lay the groundwork for our cosmic aspirations and the potential for unprecedented exploration of the universe.

The Dynamics of Space and Time

Exploring the intricate ballet of the cosmos necessitates an understanding of the fabric that constitutes the universe: space-time. This fabric bends and stretches, illustrating the profound relationship between space, time, and gravity.

Understanding Space-Time

Space and time are interwoven into a single continuum known as space-time . This four-dimensional fabric serves as the canvas upon which the universe paints its phenomena. One cannot alter one aspect, such as time , without affecting the other, like distance . The Theory of Relativity put forth by Albert Einstein revolutionized the way physicists perceive these entities, proposing that the laws of physics are consistent throughout this continuum.

• Space : Often visualized as a three-dimensional grid extending in all directions.
• Time : Acts as the fourth dimension, a temporal component vital for determining sequences of events.

This framework allows physicists to understand how the universe operates as a cohesive whole, rather than as isolated elements.

Gravity’s Influence on Time

Gravity, a curving of space-time by mass, influences both the passage of time and the trajectory of objects in space. The more massive an object, the deeper the curve and the slower time flows in its vicinity. This is not just conjecture but has been measured—clocks closer to Earth’s surface, where gravity is stronger, literally tick slower than those at higher altitudes.

• Near massive objects : Time dilation occurs due to the stronger gravitational pull.
• Higher altitudes : Weaker gravitational influence results in faster passage of time.

This relationship between gravity and time is not merely abstract but has practical implications for technologies like GPS satellites, which must account for these differences to maintain accuracy. The movie “Interstellar” showcases these concepts, with scenarios where characters experience time at different rates due to the varying forces of gravity they encounter.

The Physics of Mass and Movement

In the realm of physics, mass and movement are fundamental concepts, each governed by their own set of rules. Understanding how mass interacts with forces and the role of speed and light is crucial for grasping the mechanics of the universe.

Matter and Its Properties

Matter is composed of particles with mass and volume. From a physicist’s perspective, any object with mass possesses gravitational pull; the more mass an object has, the stronger its gravitational field. Newton’s Second Law of Motion states that force is equal to mass times acceleration ((F = m \cdot a)), underscoring mass as a pivotal factor in the calculation of movement. In the context of space exploration , the concept of mass becomes even more significant when considering the propulsion of spacecraft and the effects of gravity on interstellar travel.

Speed and Light

Understanding the relationship between speed and light is central to the theory of relativity, which asserts that the laws of physics are the same for all non-accelerating observers. Light, which is comprised of photons, travels at a speed of approximately (3 \times 10^8) meters per second in a vacuum. This universal constant, denoted (c), is a cornerstone in the equations of relativity and has profound implications for space travel . For example, as objects move closer to the speed of light, relativistic effects like time dilation become more pronounced, which has to be considered when planning long-duration space missions .

Observational Astrophysics

In the realm of astrophysics, observational techniques stand as the bedrock for exploring the universe, allowing scientists to gather empirical evidence and test the boundaries of theoretical models like the Theory of Relativity.

Detecting Signals across Space

To grasp the vastness of space, astronomers rely on advanced detection methods. These include radio telescopes and space-based observatories that collect electromagnetic signals. Such tools have been pivotal in confirming the existence of gravitational waves , a prediction of Einstein’s general theory of relativity. NASA’s continued efforts in this field are vital for enhancing our understanding of the cosmos, contributing to discoveries of phenomena such as black holes and neutron stars which emit these ripples in spacetime.

One of the most significant advances in recent years has been the development of techniques to isolate and identify these signals amidst the cosmic noise. By capturing this data, scientists can create detailed maps of stellar objects and gain insights into the life cycle of stars and the fabric of spacetime itself.

Planetary Discoveries

The search for planets outside our solar system has been revolutionized by both ground and space-based telescopes. Discoveries of exoplanets sit at the heart of this pursuit, with missions like NASA’s Kepler telescope having identified thousands of these distant worlds. Using methods like transit photometry, where astronomers observe the dimming of a star as a planet passes in front of it, the specific characteristics of these planets, including their composition and potential habitability, are being unveiled.

Observational astrophysics not only furthers our knowledge about the existence of planets and the potential for life beyond Earth, but it also tests our understanding of planetary formation and evolution. Each new discovery made by scientists works to piece together the complex puzzle of our universe’s story.

The Nature of Scientific Inquiry

In exploring the depths of space through science fiction, both authors and physicists contribute to the scientific inquiry. They expand our understanding of complex theories like relativity and its practical implications.

Methodologies in Modern Physics

In modern physics , methodologies are structured around empirical evidence and mathematical rigor. Experiments in physics, much like those that validate aspects of the Theory of Relativity, require meticulous setup, observations, and validations. This process often involves the following steps:

• Observation : Noting a phenomenon that requires explanation.
• Hypothesis : Proposing a tentative explanation based on prior knowledge.
• Experimentation : Testing the hypothesis through controlled experiments.
• Analysis : Interpreting the data gathered from the experiments.
• Conclusion : Drawing a conclusion that either supports or refutes the hypothesis.

For instance, physicists studying the time dilation effect as depicted in science fiction movies like Interstellar may conduct experiments involving high-speed particles to observe the effect directly. They employ complex mathematical models, often using Einstein’s field equations, to predict outcomes that can be tested.

The Human Element

The human element in scientific inquiry is multifaceted. Physicists are not only driven by curiosity but also by the desire to solve real-world problems, like finding habitable planets as portrayed in Interstellar . The nature of their work demands creativity, a trait often associated with authors , to imagine solutions and new experiments. This element is especially critical when traditional methods fail to provide answers.

Physicists also grapple with philosophical questions about our place in the universe and the ethical implications of their work. Such introspections reflect in science fiction narratives as well, where authors mirror these concerns through compelling stories. Human emotions, ambitions, and fears inform both the creation of speculative worlds and the pursuit of knowledge about our own.

In sum, scientific inquiry is a dynamic interplay of rigorous methodology and human curiosity, both of which are stimulated and reflected in the realms of science fiction and physics.

Interstellar Insights: Frequently Asked Questions

This section addresses some common inquiries about “Interstellar” and its scientific underpinnings, particularly the theory of relativity, providing insight into the movie’s alignment with actual physics.

How accurately does ‘Interstellar’ portray the theory of relativity?

“Interstellar” takes considerable care in its depiction of the theory of relativity, especially in the portrayal of time dilation near massive celestial objects. Caltech physicist Kip Thorne played a significant role in ensuring the film’s scientific veracity, though some dramatization for narrative purposes is present.

Can the representation of black holes in ‘Interstellar’ be reconciled with current physical theories?

The black holes in “Interstellar,” including Gargantua, are visualized with inputs from theoretical physics and are some of the most accurate representations in cinema to date. Their depiction aligns closely with what current physical theories predict about these massive phenomena.

In what ways did physicists contribute to the scientific aspects of ‘Interstellar’?

Physicists, notably Kip Thorne, were integral to “Interstellar,” providing expertise on the scientific aspects of the film. Their contributions range from the depiction of black holes and wormholes to incorporating the effects of relativity throughout the movie.

What are the theoretical implications of wormholes as depicted in ‘Interstellar’?

While wormholes like the one in “Interstellar” are speculative, they are grounded in theoretical physics, based on solutions to Einstein’s field equations. Their cinematic portrayal brings attention to the concept of shortcuts through spacetime, which remains a topic of scientific debate and exploration.

How does ‘Interstellar’ handle the concept of time dilation, and is it consistent with relativity?

The movie presents time dilation in a manner that is broadly consistent with Einstein’s theory of relativity. Characters experience time differently depending on gravitational forces, particularly near the black hole, aligning with the theoretical predictions connected to strong gravity.

What aspects of ‘Interstellar’ have sparked the most discussion within the scientific community?

The film’s use of black holes, wormholes, and time dilation as plot devices has sparked discussions and interest in the scientific community. These conversations revolve around the feasibility of interstellar travel , the nature of time and space, and the visualization of abstract scientific concepts.

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The Legacy of 2001: A Space Odyssey – Insights into Predictions Realized

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These Sci-Fi Visions for Interstellar Travel Just Might Work

In a month or two, NASA will launch its massive Space Launch System rocket from the Kennedy Space Center. While the spacecraft atop it will travel around the moon—the farthest from Earth a crew-capable craft will have ever gone—the rocket will also deploy a bunch of little CubeSats , including one called NEA Scout that will be propelled by a solar sail toward a nearby asteroid.

That project has come to fruition thanks to Les Johnson, head of that mission’s technology team at NASA’s Marshall Space Flight Center in Huntsville, Alabama. It’s a milestone for Johnson, who has been working on solar sails and other advanced propulsion systems for years.

Outside his day job at NASA, Johnson also writes nonfiction and science fiction books for popular audiences, many of which envision future interstellar voyages. His latest, A Traveler’s Guide to the Stars , explores the kinds of propulsion systems that could one day make these deep-space expeditions a reality.

This conversation has been edited for length and clarity.

WIRED: What inspired you to study space propulsion systems?

Johnson: Star Trek , if you go way back. I’ve been a science fiction fan and an advocate for space exploration and space travel since I was in elementary school. I was 7 years old when I watched Neil Armstrong walk on the moon. I was asleep probably, and I was in footie pajamas, and my parents woke me up to come watch this. And later, my older sister allowed me to stay up with her late to watch Star Trek reruns, and Lost in Space , so I was kind of hooked.

I decided at that age that I wanted to study physics and be a scientist. I always had bad vision and had been a scrawny kid, so I knew I wouldn’t be an astronaut—but I wanted to work for NASA. 

One of the first projects I was assigned was to work on something called a space tether. Those are long wires that are deployed on spacecraft, and they can be used for scientific measurements. But there was a secondary effect in test flights: You could actually get propulsion in low Earth orbit using these wires, without electricity orfuel. So I got really excited: “Hey, this is a way to travel through space, at least in Earth orbit, where you may not ever run out of gas.” 

So that’s what got me interested in advanced propulsion. From there it spread out to solar sails, and to nuclear propulsion. As a result of that, I got involved with some groups outside of NASA, people thinking about how we might go to the stars. They’d ask me, “What’s a viable method to go to Proxima Centauri?” So things kind of snowballed from there.

How does a solar sail work?

It’s not the solar wind—that’s an unfortunate naming problem. A solar sail is propelled only by light. Light is made up of photons, and those photons don’t have mass. But they do have momentum, like a molecule of air in the wind. And just like a sailboat on a lake or the ocean, when the wind blows against the sail, some of the momentum of the air particles is absorbed by the sail, which causes it to recoil, which is pushing on the sail. And through the mast, it pulls the boat with it. 

Out in space, as photons of light reflect from the sail, the light gives up a little of its energy and momentum, and that momentum goes into the motion of the sail and it pushes it.

How far from the sun can you go while still getting a significant amount of energy from it?

This is why solar sails are really cool, and this is why I like them for interstellar travel. Let’s go out the Earth’s distance from the sun, 1 AU, 93 million miles. When you unfurl a sail of any size, say it’s 100 square meters, the sunlight falling on it pushes on it. As you move away from the sun, the intensity of sunlight falls off pretty rapidly, and so does the thrust. But if you deploy a sail closer to the sun, the thrust level goes up dramatically. 

If you have a light enough sail, you can get a really big acceleration. If you get well inside the orbit of Mercury and you have a sail that only weighs 1 or 2 grams per square meter—which is about 20 times better than we can do today—and you have a sail that’s like a square kilometer, if you add a laser to boost it, you can get enough thrust to go out of the solar system at a significant fraction of the speed of light, like 10 percent. It’s unbelievable. That’s where you can get a trip that will get you to Alpha Centauri in hundreds of years, as opposed to thousands or tens of thousands with chemical rockets.

When I first saw these numbers, I thought, “That’s great, but we have no material that can stand those loads that’s that lightweight. That material is ‘unobtainium.’” That was pure science fiction. Then in 2004, graphene was found. The discoverers of that got a Nobel Prize for it in 2010. That’s a single layer of carbon. It has all the thermal and mechanical properties you need to build this huge sail; you just have to put something on it to make it reflective, like a layer of aluminum. And suddenly, this looks possible. 

We don’t know how to engineer anything that big yet . But we’ve gone from a material that doesn’t exist to one that does exist in the last two decades. And if you augment that with a high-power laser, like the folks at the Breakthrough Starshot want to do, it’s like a lot more suns falling on it, which means you can accelerate it to much higher speeds, potentially up to 5, 10, 20 percent the speed of light. And all of this without violating the laws of physics. The only laws you’re violating are known engineering. Nobody knows how to build these things, but we will! We’ll figure it out.

How did you get involved with the NEA Scout’s solar sail?

I have been working on solar sails since the early 2000s. It was one technology of many, in a portfolio of advanced propulsion that I was working on at my day job at NASA. It involved electric propulsion, nuclear propulsion, sail propulsion, some chemical work, and solar sails were a part of that. That was about the time little CubeSats were being flown, small, bread-loaf-sized spacecraft that a lot of universities now fly in low Earth orbit. NASA was trying to figure out, “Hey, can we do useful things with these? Does anybody have a payload?” We said, “We have some solar sail hardware. Let’s test a sail deployment in Earth orbit.” 

So in 2010, we flew a 10-square-meter sail called Nanosail-D . And that was successful. Then the Space Launch System was starting to move forward, and someone at NASA said, “This rocket’s going into deep space. It will have extra payload capability, we can take some of these CubeSats.” So I led a team and we wrote the proposal for NEA Scout using a scaled-up version of the Nanosail-D.

Tell me about some speculative propulsions you’ve explored, such as pulsed fusion and antimatter.

Oh, it’s all cool! I could talk for hours! I’ll start with the things I think are possible within the known laws of physics. I don’t want to be arrogant here: Scientists throughout history have made the mistake of saying, “Oh, that’s impossible,” and then 50 years later somebody proves them wrong.

There are a few ways to get to the stars. One is sails—light sails, solar sails. Chemical rockets just don’t have the energy density to do it. Nuclear-thermal rockets basically use a small version of the reactor that produces electrical power in a power station near you. You miniaturize it and put it on a rocket and use fuel, and it’s superheated by the nuclear reactor. That’s an improvement in performance over a chemical rocket, and it’s something I think we ought to be doing for the exploration of our solar system, but it won’t take you to the stars. You can’t carry enough fuel in the mass you have available to make it work.

Its descendant, fusion, which people are working on to try to have a cleaner source of power on Earth, is: Instead of splitting atoms, you’re combining them, like the way the sun produces energy. You’re squeezing hydrogen atoms so tightly until they become helium, and then they give off energy. If you can do that in a controlled reaction, you get a lot more energy out than you put in. You could use that as a propulsion system to build a rocket. It would have to be a really big rocket, because you’d have to carry a lot of fuel: Think of a rocket bigger than the Empire State Building. But it would work. You could get to the nearest few stars, like maybe Proxima Centauri, but not Ross 248, which is 10 light-years away.

One of my favorites after that is antimatter. People hear that and think, “That’s out of Star Trek .” Which it was. But it’s real. In high-energy reactions, like at the CERN collider in Europe and other particle accelerators, when we smash atoms together at high speed, lots of things break apart and fly off. But a curious thing people discovered is that there are things that look like a proton, have the mass of a proton, but have a negative charge. And then they discovered these lighter-weight things that look like electrons, but they have a positive charge. So scientists have taken these antiprotons, combined them with positrons, and made anti-hydrogen. That’s in small quantities, because when these anti-particles encounter their normal matter counterparts, they undergo—in physics terms—annihilation. That mass gets turned into energy. They explode and give off gamma rays, all kinds of secondary particles—it’s a very energetic explosion. A tablespoon of antimatter would basically destroy a city—that’s how much energy is packed into antimatter. 

You could take a lot of this antimatter, store it in a perfect vacuum, and then as you need it for your reaction mass to propel your spaceship, you have a stream of it that goes in and annihilates with normal matter and you use that energy. We don’t know how to do that, but nature says it’s possible. Now, I don’t think I want to build this on Earth, because you’re going to need tons of antimatter. If you lost control of it, that would be a disaster. 

Buried in there is another pretty interesting idea that is not as good as antimatter or fusion, but it’s really close. That’s something called a fission pulse. You may have heard of Project Orion. That was a really cool project in the Cold War, in the late ’50s and into the ’60s, where some scientists including the late Freeman Dyson said, “Maybe instead of using a rocket to put a spacecraft into space, what would happen if we used a series of controlled explosions under a big steel plate?”

It’s like, if you put a rock on top of a firecracker, the rock gets launched, right? Imagine a series of explosions under a steel plate. It’ll start getting off the ground—“Boom, boom, boom!”—to higher and higher speeds as you keep detonating these explosions. You could potentially get this plate or whatever’s on it—a spacecraft—moving to really high speeds. These scientists figured out, if you have a spacecraft the size of an aircraft carrier and you put extremely large plates under it, that are big enough to shield it from the radiation from the bomb going off, and you started exploding atomic bombs every three seconds under it, you could get tremendous speeds and you could use this to send a spacecraft, with a trip time of a few hundred years, to the nearest star. Of course you destroy the ecosystem while you’re launching it. But in theory, yeah, that ought to work!

According to a figure in your book, it looks like it’s hard to strike a balance to achieve both efficiency and thrust—and to also not have something cost a gazillion dollars.

Unfortunately, if we’re talking about building something at the scale to send a reasonably sized spacecraft to the nearest star, it’s going to be—with today’s capabilities—a really expensive endeavor. But over time, the capability evolves.

That curve you’re talking about limits rockets. It applies to any rockets that have fuel on board: chemical rockets, electric rockets, nuclear-thermal, fusion, and even antimatter. You’ve got the mass of your spacecraft, and to get it moving, it requires a certain amount of fuel at a certain thrust level. To keep it going faster, you have to load more fuel on it, which increases the weight, which means you need more fuel to move it initially. Eventually it gets to a point where you get diminishing returns.

That’s why I like sails, where the energy is not on the ship; it comes from somewhere else, so you don’t have to worry about that efficiency curve getting you. That’s a beautiful way to get around that problem.

For very long interstellar trips—things that are farther than the closest star—continuous fusion, antimatter, and sails are the only thing that will let you get there. But the better the thrust performance, the worse the efficiency it has, with every system we’ve looked at.

What motivated you to write this book, A Traveler’s Guide to the Stars ?

I go back to what motivated me to study science: It was our achievements in space, going to the moon. It was the dreamers, science fiction writers, and television shows, and this notion that in this big universe, as we look out and we discover exoplanets and we find that some of these exoplanets live in regions around their star where there might be liquid water, there might be a place where life could go and exist. 

I am a believer that life is good and that it’s a morally good thing to try to preserve and protect and spread life. We as a species, as humans, should strive to use space resources to make life better on Earth and expand our presence in the solar system, and eventually start sending our children to spread life into the rest of the universe, which sure looks like it’s a cold, dead universe. If it is, then let’s go fill it up with people who have hopes, dreams, aspirations, to create art and be human.

How long will it take humanity to design and send a robotic probe to another star system?

Part of that’s going to be a function of how hard we try. If we keep going on the path we’re going—which isn’t a bad path, but it’s taking longer than we thought it would to get the costs of launch down—I think it’ll be 300 years.

But if someone were to come along and say, “Here’s a blank check. Let’s go figure this out,” we could do it probably in less than 100 years. It’s a challenge limited by engineering knowledge, but interest, enthusiasm, and funding could accelerate it. 

Now if it’s the public purse, politicians have to balance that with all the other things: health care, police. I’m just thankful our society places a value on science and exploration at any level. So it’s a balance of priorities.

What might a crewed space journey to another star system look like?

Let’s assume we’re not going to fundamentally change our own biology through genetic engineering, that 100 years from now, people are still people as we’d recognize them today, but maybe living longer, maybe with better health care. I think it would be a voyage of hundreds of years, in a ship where there would be generations that are born and die, before you ever reach the nearest star. It would be a concept like in the movie Passengers , but not with suspended animation, because I’m really skeptical of that. 

Now if we have breakthroughs in medical research that allow us to engineer ourselves to be adapted to spaceflight, perhaps engineer ourselves to be like bears, where we could go into hibernation, and then you combine that with rocket science and propulsion science, a voyage of hundreds of years might still be the case, but wouldn’t necessarily be generations. It might open the possibility of the people who get on the ship being the ones who get off the ship. But that’s two levels of revolutionary breakthroughs.

What are your thoughts about sending robots versus people into space? That seems to be the eternal debate—with the moon, asteroids, and Mars?

It’s going to be both. I think that’s what history has shown. Before we sent people into space, we sent Sputnik and Explorer 1 and other robotic spacecraft. Before we went to the moon, there were the Surveyor missions that we sent, and the Soviets sent spacecraft, and then we sent people. For decades we’ve been sending robotic spacecraft to Mars. I think we will send people to Mars. I’m hoping that will be in my lifetime.

When I look at that debate, I think it’s a false dichotomy. And I’ve got a story in the book: I went to a meeting probably eight to 10 years ago on new strategies for exploring Mars. There was a debate going on there, with panelists on stage, about whether we should send people to Mars. Is it really worth it? There was this reserved chair in the first row that was empty. And then in walks Buzz Aldrin. Buzz, the second man to walk on the moon, makes his entrance, and sits down. And he’s there for like five minutes. He stands up, and raises his hand. He looked at all of us and said, “OK, let’s suppose we had a way to do this tomorrow. How many of you would sign up for a one-way trip to Mars?” I was stunned. I want to go as a tourist, but I want to go back home. But it was over half the people, and a lot of them who raised their hands were those who had been arguing we should only send robots. But as soon as they were given the thought, “Oh, we could send people—then of course I’d go.” That moment crystallized in my head that if the capability exists, we’re going to do both. It will first be the robots, then we’ll send people.

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The U.S.S. Enterprise , depicted here in the 2013 movie Star Trek: Into Darkness , relies on its warp drive to zip across the galaxy.

Inside the Quest for a Real ‘Star Trek’ Warp Drive

It may be a while before starship captains can race across the galaxy, but engineers and physicists have a few ideas for making it so.

Within the Star Trek universe, traveling across the galaxy is a breeze thanks to the famed warp drive . This fictional technology allows humans and other civilizations to zoom between star systems in days rather than centuries.

Such rapid travel times are impossible in the real world, because our best theory for the way the universe works, Einstein’s special relativity , says that nothing moves faster than the speed of light.

While current rocket propulsion systems are bound by this law, plenty of hopeful engineers and physicists are working on concepts that might bring us a step closer to Star Trek ’s vision of racing across the cosmos.

“Currently, even the most advanced ideas behind interstellar travel entail trip times of decades and centuries to even the closest stars, due to the restrictions of special relativity, and our abilities—or lack of—to travel at an appreciable fraction of the speed of light,” says Richard Obousy , director and founder of Icarus Interstellar, a nonprofit dedicated to making progress toward interstellar flight.

“Being able to build starships with the capability to travel faster than the speed of light would open the galaxy for exploration and possible colonization by humans.”

Nuclear Engines

Distances in space are so vast that astronomers usually measure them in light-years, the distance light can travel in a year’s time. A single light-year equals about six trillion miles.

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The closest star to our solar system, Proxima Centauri, is 4.23 light-years away, so even traveling at the speed of light, a one-way voyage there would take 4.23 years. That may seem pokey, but it would be a huge improvement over current technology.

Right now, the fastest spacecraft headed away from Earth is Voyager 1, which is puttering along at about 38,600 miles an hour. At that rate, it would take more than 70,000 years to reach Proxima Centauri.

Still, various teams have proposed ways to at least reach a fraction of light speed and hasten our exploration of interstellar space.

Back in 1958, researchers at San Diego-based defense contractor General Atomics came up with Project Orion , which involved a spacecraft driven essentially by nuclear bombs. A controlled series of nuclear explosions would propel the ship at high speeds, rapidly carrying a hundred tons of cargo and eight astronauts to places like Mars and even the outer solar system.

Faster propulsion technology would allow us to visit our galactic neighbors, like this satellite of the Milky Way known as the Large Magellanic Cloud.

Blueprints were also created showing how to adapt the technology for interstellar travel. However, all experimentation with this so-called nuclear-pulse propulsion came to a halt with the Nuclear Test Ban Treaty of 1963.

Announced earlier this year, the ambitious Breakthrough StarShot initiative represents a less explosive effort to undertake an interstellar mission. Run by a conglomerate of billionaires and big thinkers, including famed physicist Stephen Hawking, the project’s goal is to send a flotilla of postage stamp-size spacecraft to Alpha Centauri, a triple star system that’s 4.3 light-years away. (See “Is the New $100 Million ‘Starshot’ for Real?” )

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The tiny spacecraft would be attached to a thin light sail, a piece of technology that would allow mission managers to propel the probes with lasers shining from Earth’s orbit. The lasers would accelerate the craft to 20 percent the speed of light, and the probes would arrive at their destination in roughly 20 years.

While many of the tiny travelers may never make it to Alpha Centauri, a few of them should survive and may even fly past any planets orbiting the far-off stars , beaming back data about these alien worlds.

“I’m incredibly excited to see private money being used to explore breakthrough ideas that may advance the field of interstellar flight,” Obousy says.

“I hope to see more like this in the future. While there are engineering challenges associated with the Starshot Initiative, none appear insurmountable.”

Warping Reality

Of course, the real breakthrough would be a true warp drive, which requires technology to catch up with our theoretical designs.

In 1994, Trek fans got a glimmer of hope from Mexican theoretical physicist Miguel Alcubierre, who came up with a radical theory of hyper-fast space propulsion that doesn't break Einstein’s special relativity.

Instead of accelerating the spacecraft itself to light speed, why not bend, or warp, the fabric of space and time around the ship itself? Alcubierre presented calculations that produce a bubble in space-time in which one end is expanding and the other is contracting. A spaceship could, in theory, be carried along with the warp bubble and accelerated to velocities up to 10 times the speed of light.

While that sounds simple on paper, to make it work, we may need to harness exotic forms of matter, like antimatter, that for now are poorly understood. In addition, numerous unsolved issues plague the creation and control of a warp bubble, Obousy says.

“One such problem, for example, is the idea of causal disconnection, which implies that any spacecraft sitting within the bubble would not be able to ‘communicate’ with the exterior of the bubble, suggesting that a ship would not be able to ‘turn off’ the bubble once inside of it,” he notes.

As is often the case in space travel, developing true interstellar travel like what we see in Star Trek will require significant changes in the cost and energy requirements.

“Currently, the amount of energy and money required to entertain the notion of manned interstellar travel is measured in large fractions of global output—specifically, tens of trillions of dollars, and energy measured on the scale of what many large countries use annually,” he says.

Still, he adds, “the finest minds of the 15th century could not have predicted the technological wonders of the 21st century. Similarly, who are we to say what technology the humans of the 27th century will have mastered.”

Andrew Fazekas, the Night Sky Guy, is the author of Star Trek: The Official Guide to Our Universe and host of NG Live! " Mankind to Mars " presentations. Follow him on Twitter , Facebook , and his website .

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A Groundbreaking Scientific Discovery Just Gave Humanity the Keys to Interstellar Travel

In a first, this warp drive actually obeys the laws of physics.

If a superluminal—meaning faster than the speed of light—warp drive like Alcubierre’s worked, it would revolutionize humanity’s endeavors across the universe , allowing us, perhaps, to reach Alpha Centauri, our closest star system, in days or weeks even though it’s four light years away.

However, the Alcubierre drive has a glaring problem: the force behind its operation, called “negative energy,” involves exotic particles—hypothetical matter that, as far as we know, doesn’t exist in our universe. Described only in mathematical terms, exotic particles act in unexpected ways, like having negative mass and working in opposition to gravity (in fact, it has “anti-gravity”). For the past 30 years, scientists have been publishing research that chips away at the inherent hurdles to light speed revealed in Alcubierre’s foundational 1994 article published in the peer-reviewed journal Classical and Quantum Gravity .

Now, researchers at the New York City-based think tank Applied Physics believe they’ve found a creative new approach to solving the warp drive’s fundamental roadblock. Along with colleagues from other institutions, the team envisioned a “positive energy” system that doesn’t violate the known laws of physics . It’s a game-changer, say two of the study’s authors: Gianni Martire, CEO of Applied Physics, and Jared Fuchs, Ph.D., a senior scientist there. Their work, also published in Classical and Quantum Gravity in late April, could be the first chapter in the manual for interstellar spaceflight.

POSITIVE ENERGY MAKES all the difference. Imagine you are an astronaut in space, pushing a tennis ball away from you. Instead of moving away, the ball pushes back, to the point that it would “take your hand off” if you applied enough pushing force, Martire tells Popular Mechanics . That’s a sign of negative energy, and, though the Alcubierre drive design requires it, there’s no way to harness it.

Instead, regular old positive energy is more feasible for constructing the “ warp bubble .” As its name suggests, it’s a spherical structure that surrounds and encloses space for a passenger ship using a shell of regular—but incredibly dense—matter. The bubble propels the spaceship using the powerful gravity of the shell, but without causing the passengers to feel any acceleration. “An elevator ride would be more eventful,” Martire says.

That’s because the density of the shell, as well as the pressure it exerts on the interior, is controlled carefully, Fuchs tells Popular Mechanics . Nothing can travel faster than the speed of light, according to the gravity-bound principles of Albert Einstein’s theory of general relativity . So the bubble is designed such that observers within their local spacetime environment—inside the bubble—experience normal movement in time. Simultaneously, the bubble itself compresses the spacetime in front of the ship and expands it behind the ship, ferrying itself and the contained craft incredibly fast. The walls of the bubble generate the necessary momentum, akin to the momentum of balls rolling, Fuchs explains. “It’s the movement of the matter in the walls that actually creates the effect for passengers on the inside.”

Building on its 2021 paper published in Classical and Quantum Gravity —which details the same researchers’ earlier work on physical warp drives—the team was able to model the complexity of the system using its own computational program, Warp Factory. This toolkit for modeling warp drive spacetimes allows researchers to evaluate Einstein’s field equations and compute the energy conditions required for various warp drive geometries. Anyone can download and use it for free . These experiments led to what Fuchs calls a mini model, the first general model of a positive-energy warp drive. Their past work also demonstrated that the amount of energy a warp bubble requires depends on the shape of the bubble; for example, the flatter the bubble in the direction of travel, the less energy it needs.

THIS LATEST ADVANCEMENT suggests fresh possibilities for studying warp travel design, Erik Lentz, Ph.D., tells Popular Mechanics . In his current position as a staff physicist at Pacific Northwest National Laboratory in Richland, Washington, Lentz contributes to research on dark matter detection and quantum information science research. His independent research in warp drive theory also aims to be grounded in conventional physics while reimagining the shape of warped space. The topic needs to overcome many practical hurdles, he says.

Controlling warp bubbles requires a great deal of coordination because they involve enormous amounts of matter and energy to keep the passengers safe and with a similar passage of time as the destination. “We could just as well engineer spacetime where time passes much differently inside [the passenger compartment] than outside. We could miss our appointment at Proxima Centauri if we aren’t careful,” Lentz says. “That is still a risk if we are traveling less than the speed of light.” Communication between people inside the bubble and outside could also become distorted as it passes through the curvature of warped space, he adds.

While Applied Physics’ current solution requires a warp drive that travels below the speed of light, the model still needs to plug in a mass equivalent to about two Jupiters. Otherwise, it will never achieve the gravitational force and momentum high enough to cause a meaningful warp effect. But no one knows what the source of this mass could be—not yet, at least. Some research suggests that if we could somehow harness dark matter , we could use it for light-speed travel, but Fuchs and Martire are doubtful, since it’s currently a big mystery (and an exotic particle).

Despite the many problems scientists still need to solve to build a working warp drive, the Applied Physics team claims its model should eventually get closer to light speed. And even if a feasible model remains below the speed of light, it’s a vast improvement over today’s technology. For example, traveling at even half the speed of light to Alpha Centauri would take nine years. In stark contrast, our fastest spacecraft, Voyager 1—currently traveling at 38,000 miles per hour—would take 75,000 years to reach our closest neighboring star system.

Of course, as you approach the actual speed of light, things get truly weird, according to the principles of Einstein’s special relativity . The mass of an object moving faster and faster would increase infinitely, eventually requiring an infinite amount of energy to maintain its speed.

“That’s the chief limitation and key challenge we have to overcome—how can we have all this matter in our [bubble], but not at such a scale that we can never even put it together?” Martire says. It’s possible the answer lies in condensed matter physics, he adds. This branch of physics deals particularly with the forces between atoms and electrons in matter. It has already proven fundamental to several of our current technologies, such as transistors, solid-state lasers, and magnetic storage media.

The other big issue is that current models allow a stable warp bubble, but only for a constant velocity. Scientists still need to figure out how to design an initial acceleration. On the other end of the journey, how will the ship slow down and stop? “It’s like trying to grasp the automobile for the first time,” Martire says. “We don’t have an engine just yet, but we see the light at the end of the tunnel.” Warp drive technology is at the stage of 1882 car technology, he says: when automobile travel was possible, but it still looked like a hard, hard problem.

The Applied Physics team believes future innovations in warp travel are inevitable. The general positive energy model is a first step. Besides, you don’t need to zoom at light speed to achieve distances that today are just a dream, Martire says. “Humanity is officially, mathematically, on an interstellar track.”

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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Interstellar space: What is it and where does it begin?

Explore the interstellar medium here, with our ultimate guide.

How far away is interstellar space?

Have we traveled to interstellar space, what is interstellar space made of, what temperature is interstellar space, investigating interstellar space, interstellar travel, additional resources.

Interstellar space is the area between the stars, but it is far from empty. It contains vast quantities of neutrinos , charged particles, atoms, molecules, dark matter and photons ranging from the highest-energy radiation to the sluggish light of the cosmic microwave background (CMB) albeit rather sparsely spread out.

According to the National Radio Astronomy Observatory (NRAO), the average distance between stars in the Milky Way galaxy is about 5 light-years , although they are more bunched up near the center of the galaxy than in the outskirts where the sun and Earth are located. 

This means that there is a lot of space between the stars . We collectively call everything that is in this space the "interstellar medium", or ISM for short.

The composition of the ISM is broken down by the scientists at the Infrared Processing and Analysis Center (IPAC) at Caltech: the ISM is mostly composed of atoms of hydrogen (~90%) and helium (~8%), which are the two most common atoms in the universe having been created in the Big Bang , but there are also other trace elements and molecules contributing no more than 2% of the ISM. These elements, heavier than hydrogen and helium, have all originated in the deaths of stars and have been blown into space. The more generations of stars there are, the more enriched in chemical elements the ISM becomes over time. 

The boundary to interstellar space is far away, but perhaps not as far as you might think. In fact, part of our solar system is in interstellar space. 

How can this be? The definition of this interstellar space boundary is the region where the sun 's magnetic bubble weakens and comes to an end. This magnetic bubble — known as the heliosphere — is filled with plasma (ionized gas). The heliosphere is blown by the solar wind that drags magnetic field lines out from the sun.

The plasma in the ISM imparts an inward pressure on the edge of the heliosphere with its own magnetic fields and charged particles, leading to a complex and variable structure at the boundary. The solar wind starts to weaken from between 370–430 miles per hour (600–700 kilometers per hour) as it begins to come up against interstellar space and slows to about 62 mph (100 km/h ). The point at which this occurs is called the Termination Shock. The region in the heliosphere beyond the Termination Shock where the solar wind continues to slow is called the heliosheath, and then the outer boundary of the heliosphere is called the heliopause. It's at the heliopause where the solar wind stops and gives way to interstellar space, approximately 11 billion miles (18 billion km) from the sun. 

Only two active spacecraft have ever crossed the heliopause and entered interstellar space. These are NASA's Voyager 1 and 2 missions. Launched in 1977 they visited the outer planets before continuing on deeper into space. In 2005 JPL's Ed Stone, who was Voyager's project scientist before his retirement in 2022, announced that Voyager 1 had crossed the termination shock at a distance of 94 astronomical units (8.7 billion miles/14 billion km) from the sun.

NASA announced that Voyager 2 had followed suit in August 2007 at a distance of about 83 astronomical units (7.7 billion miles/12.4 billion km). Voyager 1 and Voyager 2 are headed in different directions, and the fact that they crossed the termination shock into the heliosheath at different distances from the sun suggests that the heliosphere is not symmetrical around the solar system, but is squashed. This shape is created by the balance of the strength of the solar wind outwards, and the pressure of the ISM on the heliosphere, particularly in the direction of the sun's motion through space.

Then, NASA announced that on August 25, 2012 Voyager 1 crossed the heliopause and became the first spacecraft to leave the sun's influence and enter interstellar space. It did so at a distance of 121 astronomical units (11 billion miles/18 billion km) from the sun, which is in the 'Scattered Disk' of comets near the edge of our solar system. NASA also revealed that Voyager 2 crossed the heliopause and entered interstellar space on November 5, 2018, at a distance of 121 astronomical units (11.3 billion miles/18.3 billion km). 

But how do you know when you're in interstellar space? The evidence that both spacecraft had crossed the boundary came in the form of changes to the plasma environment around them, in particular measuring a drop in the number of solar wind particles and an increase in the number of galactic cosmic rays from outside the heliosphere.

There is structure to the ISM, even around the solar system. 

The Solar System is currently passing through what astronomers call the Local Interstellar Cloud, which is a wispy cloud of neutral hydrogen gas about 30 to 40 light-years across described in greater detail by Jonathan Slavin of the Harvard–Smithsonian Center for Astrophysics . There are other clouds nearby too, and we and all these clouds exist in the "Local Bubble", which is hundreds of light-years across and blown relatively clear of gas and dust by ancient supernovae explosions from 14 million years ago, according to research led by Harvard's Catherine Zucker. Voyager 2's measurements of the magnetic field in the Local Interstellar Cloud show it to be stronger than expected, but still incredibly weak — millions of times weaker than a fridge magnet — and that it experiences some turbulence, resulting in the magnetic field orientation being tilted by 30 degrees to the plane of the galaxy.

The magnetic field of these interstellar clouds is part of the overall galactic magnetic field; every galaxy has its own intrinsic magnetic field, the origins of which are not well understood.

It's tempting to think of the Local Interstellar Cloud as being like a fog, but it is very diffuse. According to Professor Barbara Ryden of Ohio State University , the ISM's density can get as low as 0.1 atoms per cubic centimeter (although in the bigger nebulae , the density can reach 10,000 atoms per cubic centimeter). Compare this to the density of air on Earth, which is 27 million trillion (10^19) molecules per cubic centimeter. 

Almost all the medium in interstellar space — about 99% — is composed of gas, with just 1% being in the form of dust and ice particles. Even though the dust component is minor, it can have a dramatic effect that astronomers call "reddening", or at its most severe, "dust extinction". 

Dust partially absorbs visible wavelengths of light. So when light from a distant object has to pass through the dusty spiral arms of our galaxy, or through the dust in interstellar space, it becomes dimmer and redder, as described by astronomers at the Swinburne University of Technology. If there's enough dust it can block an object's visible light completely. 

While dust is problematic for visible light observations, infrared light can pass straight through it, which is why astronomers use the likes of the James Webb Space Telescope (JWST) to peer inside nebulas or see faraway galaxies.

Temperatures in the ISM can vary, depending on the local environment. 

There's hot gas present everywhere with temperatures of millions of degrees, but because this gas is so sparse, it would not feel that hot if you were inside it. Hydrogen ionized by the ultraviolet light of nearby hot stars radiates at temperatures of tens of thousands of degrees. At the other end of the scale, clumps of molecular hydrogen gas can barely get above 10 degrees above absolute zero . 

There's more than one way to study interstellar space, but Earth-based observations are difficult to achieve.

The heliosphere's magnetic field helps to protect our solar system from interstellar radiation that barrels towards us in the form of charged particles called "galactic cosmic-rays". However, because the strength of the solar wind, and hence the shape and strength of the heliosphere, waxes and wanes with the sun's 11-year cycle of activity, sometimes this galactic cosmic radiation can infiltrate the solar system. 

Fortunately, Earth is defended by both its own magnetic field and atmosphere, which deflect and block the vast majority of this incoming interstellar radiation. However, this effective planetary defense makes observing galactic cosmic radiation on a consistent basis from Earth unfeasible.

As well as sending spacecraft like the Voyagers into interstellar space, scientists can observe what is happening at the heliopause using spacecraft a little closer to home. This is exactly what NASA's Earth-orbiting Interstellar Boundary Explorer (IBEX) mission does.

IBEX detects so-called "energetic neutral atoms" (ENAs), which the IBEX website at the Southwest Research Institute describes as forming where the charged particles in the solar wind meet neutral, or stable atoms from interstellar space near the heliopause. Enough ENAs flood back towards the inner solar system that IBEX is able to detect up to several dozen per hour, allowing astronomers to study the interaction between the heliosphere and interstellar space based on where the flow of ENAs is strongest and weakest, which corresponds to activity at the heliopause. 

One major discovery has been that of a mysterious ribbon of particles that runs across the sky, in which ENA emissions are two to three times higher than elsewhere. IBEX scientist David McComas of Princeton University described the ribbon in a NASA statement as being "totally unexpected and not anticipated by any theories before we flew the mission." The ribbon remains unexplained.

Despite the vast distances between the stars, future interstellar spacecraft could possibly use the ISM to fuel their colossal voyages.

Traveling fast enough to reach the nearest stars in a few decades would take a vast amount of fuel. One way to possibly generate enough power is via nuclear fusion . However, the amount of fuel required for fusion would greatly increase the launch mass of an interstellar-bound spacecraft, meaning it would need even more fuel to get up to a velocity of 12% the speed of light as proposed by the British Interplanetary Society's Project Daedalus study . 

There is a way that a spacecraft could collect its fuel during its journey. The ionized hydrogen that fills the ISM is a perfect fuel for thermonuclear fusion. A spacecraft could, in principle, generate an immense cone-shaped magnetic field that extends ahead of the spacecraft, scooping up the ionized hydrogen, compressing it and funneling it down the mouth of the magnetic cone into the spacecraft's fusion reactor. Such an engine is called an interstellar ramjet, and is a concept that was invented by the fusion physicist Robert Bussard in 1960 and which is described in more detail by the Institute for Interstellar Studies .

It's not quite as simple as it sounds, though. The magnetic cone needs to be huge: in a higher-density region of the ISM, it needs an area of 3,860 square miles (10,000 square kilometers) emerging from an aperture 62 miles (100 km wide), while in a lower-density region, it would need a magnetic cone 3.8 million square miles (10 million square km) in area protruding from an aperture 186 miles (300 km across). The spacecraft would also have to be traveling fast enough in the first place to be able to rapidly scoop up enough of the diffuse hydrogen to generate a sufficient rate of fusion reactions; however calculations show that if a spacecraft could first accelerate to 2% of the speed of light it would begin to gather enough hydrogen to attain 50% of its potential thrust, and from that point on the ramjet could take over.

Students can learn more with this guide to the ISM from the European Space Agency. Follow the progress of the Voyagers in the final few years of their missions as they continue to explore interstellar space with NASA . Read more about a possible ' Interstellar Probe ' that would succeed the Voyagers and explore the interstellar medium beyond the heliopause on a 50-year mission.

Bibliography

The Road to the Stars by Iain Nicolson (William Morrow and Co., 1978)

Project Daedalus: The Final Report on the BIS Starship Study (Journal of the British Interplanetary Society, 1978)  

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Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

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How do Australian worker commute times compare to Europe and North America?

It is "quite normal" for people to live more than an hour from their workplace. 

Urban sprawl and the search for affordable housing are contributing factors.

What's next?

Experts say governments can ease commute times by bolstering public transport.

The average Australian worker spends more than an hour a day stuck in rush-hour traffic, fighting for a seat on the bus or trying to zone out on the train.

Commute times nationwide are longer than for many European countries — they also surpass other car-dependent countries such as the United States.

But is there a reason we're lagging behind? And what can we do to make our travel time shorter?

Are commuter times likely to get better soon?

It's all relative but commuter times in Australia have been increasing .

For example, workers averaged a weekly commuting time of about 3.7 hours just more than two decades ago. That grew to about 4.5 hours in 2017.

Dr Alan Both from the Australian Urban Observatory said the urban sprawl in some of the larger cities including Melbourne and Sydney was contributing to the increase in commuter travel time.

"It's quite normal for people to be living over an hour away from work," he said.

"Affordability is the main issue with these sorts of things — you can't expect young professionals to be able to afford anywhere near the city, they just simply have to live further out.

"And because of that, they have the experience of longer commutes."

Dr Both explained there were solutions to ease the traffic and add more public transport in newly built suburbs in some of the country's growth areas.

"One of the last things to go in are things like bus stops. So in general, if you're moving to a new area, you are a car user," he said.

"So you have to drive and that increases the traffic on the roads and slows everything down."

He added that due to the heavy reliance on car usage and a lack of public transport options in some of these newly built outer suburbs, delays in getting to work were inevitable.

"New suburbs have really bad congestion. You even see it in the sort of middle- and outer-ring suburbs where just getting onto the highway is a traffic jam," he said.

"So not even actually getting to the highway just, you know, entering and exiting."

RMIT University's Dr Koorosh Gharehbaghi specialises in public transport systems.

He said the average distance people were living away from work was increasing but there were other factors at play.

"In cities that are doing sort of a large upgrade with the Sydney Metro, even Melbourne, for example, the commuter time has increased because they're doing a lot of maintenance works and upgrades," he said.

"But what it comes down to is the actual distance of travel to work."

How does Australia compare to other parts of the world?

Australia does not compare well to international examples in both Europe and North America.

In Europe, more than 60 per cent of workers in the EU travelled less than half an hour from work to home.

But experts including Dr Gharehbaghi have said European examples might not be the best comparison when it comes to Australia.

For example, cities such as Copenhagen were smaller in size and population.

But Dr Gharehbaghi said there were elements like frequency of service that Australia could try to emulate.

"The level of public transport service they have … are not only the latest systems available, but the frequency that they have with their public transport is quite high," he said.

What are some international examples that Australia could look to?

Experts have said Australia should look at cities that have a comparable size with similar levels of urban sprawl of large capitals including Melbourne and Sydney.

"In smaller cities and countries, they have much more [stricter] sort of planning schemes, which means they heavily monitor due to lack of available land," he said.

It's why he thinks some of Australia's largest cities should look to Toronto for models to improve commuter travel.

"What you find with Toronto, is it has a lesser commuter wait time compared to Melbourne," he said.

"Why that is, because they're spending a lot of money on upgrading and updating the systems, more than the state government of Victoria, as well as the federal government."

What can governments do to improve work commute times?

The frequency of public transport services is top of the list for both experts but they say there needs to be more foresight in planning our cities.

"Increasing frequency is one but also in making sure that, again, that the system can be able to handle that higher-intensity service required," Dr Gharehbaghi said.

He believes a bigger problem for increased commuter times, especially in some of the larger capital cities, is unchecked growth in some outer-suburban communities.

"We have urban growth corridors that are designated by the state governments in terms of the population sort of growth for the future," he said.

"But what happens is, when the population of cities are expanding, [they are] not in the particular area that the government had in place in terms of possible growth." 

Dr Both explained there were several things in play, including increased frequency of services and higher density.

But he said there were some quick wins governments could have in improving commuter times.

"One of the big barriers for people using the transit system is inflexibility. You quickly become beholden to the schedule of the public transit," he said.

He added there might be difficulty in increasing services on trains in the short term but said there was an opportunity to strengthen the bus network.

"If you go to the outer suburbs or even middle-ring suburbs, there might be a bus only every 30 minutes," he said.

"If you've just missed that bus, you're waiting there for another 30 minutes."

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If You Test Positive for Covid, Can You Still Travel?

With coronavirus cases on the rise, summer travelers are once again facing difficult questions. Here’s the latest travel guidance from health experts.

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By Shannon Sims

As new coronavirus variants gain traction across the United States, summer travelers are facing a familiar and tiresome question: How will the ever-mutating virus affect travel plans?

In light of updated guidelines from the Centers for Disease Control and Prevention , the answers may be slightly different from those in previous years.

Here’s what to know about traveling this summer if you’re worried about — or think you might have — Covid-19.

What’s going on with Covid?

Recent C.D.C. data show that Covid infections are rising or most likely rising in more than 40 states. Hospitalization rates and deaths, while low compared with the peaks seen in previous years, are also on the rise.

The uptick is tied to a handful of variants — named KP.2, KP.3 and LB.1 — that now account for a majority of new cases .

At the same time, record numbers of people are traveling by car and plane.

I’d planned to travel, but I’m sick with Covid. What should I do?

In short: You should probably delay or cancel your trip.

If you tested positive or are experiencing Covid symptoms, which include fever, chills, fatigue, a cough, a runny nose, body aches and a headache, the C.D.C. recommends that you stay home and keep away from others.

According to its latest guidelines, the agency advises waiting until at least 24 hours after you are fever-free and your overall symptoms are improving before going back to normal activities, including travel.

What are the isolation rules?

New C.D.C. guidelines issued in March made significant changes to the recommended isolation period for people with Covid.

The agency now says that you can resume daily activities if you meet two requirements : You have been fever-free for at least 24 hours (without the use of fever-reducing medications) and your symptoms are improving overall. Previously, the agency recommended isolating for at least five days, plus a period of post-isolation precautions.

Even after your isolation period, you may still be able to spread the virus to others, which is why the C.D.C. encourages you to continue to take precautions for the next five days: Use masks, wash your hands frequently, practice physical distancing, clean your air by opening windows or purifying it, and continue testing yourself before gathering around others.

Are there any lingering testing or vaccine requirements?

Travelers no longer need to show proof of being vaccinated against Covid or take a Covid test to enter the U.S. (This applies to both U.S. citizens and noncitizens.)

The same is true in Europe and most other countries.

How can I prepare before traveling?

First, make sure you stay up-to-date with Covid vaccines .

Next, plan to bring any items that would be helpful should you become sick while traveling.

“Make sure to take a good first aid or medication kit with you,” said Vicki Sowards, the director of nursing resources for Passport Health , which provides travel medical services. Ms. Sowards recommended that your kit include medications that you usually take when you are ill, as well as Covid tests.

You may want to consider packing medications that can help alleviate the symptoms of Covid, like painkillers, cold and flu medicines, and fever reducers. Bringing along some electrolyte tablets (or powdered Gatorade) can also help if you get sick.

Ms. Sowards also suggested speaking with your physician before traveling, particularly if you’re in a vulnerable or high-risk group. Some doctors might prescribe the antiviral Paxlovid as a precautionary measure, she said, to be taken in the event of a Covid infection.

How can I stay safe while traveling?

Wearing a mask on a plane or in crowded areas is still a good idea, said Ms. Sowards. Covid is spread through airborne particles and droplets, “so protecting yourself is paramount, especially if you are immunocompromised or have chronic health conditions.”

If you do get sick, start wearing a mask and using over-the-counter medications such as ibuprofen or acetaminophen for fever or joint aches, Ms. Sowards advised.

Follow New York Times Travel on Instagram and sign up for our weekly Travel Dispatch newsletter to get expert tips on traveling smarter and inspiration for your next vacation. Dreaming up a future getaway or just armchair traveling? Check out our 52 Places to Go in 2024 .