"The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever," wrote Soviet space travel pioneer Konstantin Tsiolkovsky in a letter in 1911. Scientists have long written and spoken about a perceived necessity to travel to other planets for the long-term survival of the human species.
While NASA, SpaceX, and other companies have relatively short-term plans to get us to Mars, what of the need to explore beyond our star, the Sun, which is estimated to die out in 7.5 billion years?
Interstellar travel might not happen within our lifetimes, but space agencies and private companies are developing theories and methods to get to other stars. Here are 17 facts about how we might one day travel to other stars.
1. The nearest star to Earth is out of reach using current technologies
On his return from landing on the Moon, Neil Armstrong eloquently described the immense distance from our Moon to the Earth by saying, "it suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn’t feel like a giant. I felt very, very small."
The distance from the Earth to the Moon (383,400km) is only a minuscule fraction of the distance to our Sun, and the distance from the Earth to the Sun (149.81 million km) is a proverbial drop in the ocean when compared with the distance to the closest star to the Sun.
The closest star to our Solar System is Proxima Centauri. It is part of a triple star system called Alpha Centauri and is about 4.24 light-years (or 1.3 parsecs) away from Earth. As NASA explains, that means that Proxima Centauri is 40,208,000,000,000 (4 trillion) km away from Earth.
Our fastest current most reliable and fastest form of space travel is the ion drive, which took the Deep Space 1 mission to Comet Borrelly in 1998. Due to the immense distance from Earth to Proxima Centauri, using the ion drive to travel to our nearest neighboring star would take 18,000 years — approximately 2,700 human generations.
At our current rate of technological innovation setting out on that trip would be futile as we would likely develop a technology that could catch up with, and overtake, the ion drive spacecraft years after it takes off from Earth.
2. Proxima Centauri has a potentially habitable planet in its orbit
In August of 2016 scientists documented a potentially habitable Earth-sized planet orbiting Proxima Centauri, which was subsequently dubbed Proxima b. Proxima b is an exoplanet, meaning the planet falls within the parameters of temperature required for life to develop.
Though this doesn't by any means mean we're going to find life on the planet — its proximity to its sun also means its atmosphere might be exposed to deadly amounts of radiation — the discovery did refresh hopes that we might one day travel to an alien planet orbiting a neighboring star.
Though Proxima Centauri is the closest star, besides the Sun, to Earth, its neighbor Alpha Centauri is much brighter and might also be a goal for missions of the distant future.
3. New methods and theories for interstellar travel are always in development
In his book Magnificent Desolation: The Long Journey Home from the Moon, Apollo 11 astronaut Buzz Aldrin wrote:
"I believe that space travel will one day become as common as airline travel is today. I'm convinced, however, that the true future of space travel does not lie with government agencies — NASA is still obsessed with the idea that the primary purpose of the space program is science — but real progress will come from private companies competing to provide the ultimate adventure ride, and NASA will receive the trickle-down benefits."
Elon Musk's private company, SpaceX, has already reignited the race to get to Mars and beyond with its tried and tested reusable rocket boosters and plans for a historic a manned mission to the ISS with its reusable Crew Dragon capsule in May of this year.
It's not the only company that's looking to take great strides in space travel. Privately funded and volunteer initiatives include the Tau Zero Foundation, the ominously-named Project Icarus and Breakthrough Starshot. All of these are aimed at achieving lift-off for interstellar travel.
4. Private firm Breakthrough Starshot aims to get to Proxima Centauri in our lifetime
Though the ultimate aim is to get humans to other planets and solar systems, one company, Breakthrough Starshot, thinks it can be the first to get an unmanned spacecraft to our nearest neighboring star, Proxima Centauri, using an intriguing method.
The $100 million initiative is privately funded by billionaires Yuri and Julia Milner — the former of Israeli-Russian citizenship — and aims to propel a tiny probe to the star by zapping its extremely lightweight sail using a powerful laser beam shot from Earth.
The company is relying on the miniaturization of future technologies, which would allow a spacecraft so light — weighing less than a gram — that it could be propelled by a laser's impact to eventually accelerate at around one-fifth of the speed of light. At this speed Breakthrough Starshot's spacecraft could reach Proxima Centauri in about 20 years.
In order for this to be achievable, Breakthrough Starshot needs technological advances that would allow a tiny spacecraft to carry thrusters, a power supply, navigation and communication equipment so it can beam back what it sees when it reaches Proxima b.
5. Solar sails could one day take us beyond our stars
In July of last year, the Planetary Society launched and tested a Carl Sagan-inspired solar sail that was successfully shown to be able to change its orbital trajectory using a light sail that converted the energy of photons from sunlight into propulsive energy.
Though the relatively easy and cheap manufacture of solar sails makes them a cost-effective method for space travel, they are unlikely to ever have the propulsive energy needed to carry humans. They also rely on light from stars, meaning that Breakthrough Starshot's laser-based alternative (in point 4) is the more viable option.
In order to gain the speed needed to travel long distances, they would also need time to accelerate. Right now, solar sails are viewed as a more viable method for transporting satellites within our Solar System, rather than humans to distant star systems.
6. The magnetic sail is an alternative to the solar sail
The magnetic sail is a variation on the solar sail that is propelled by solar wind rather than by sunlight. The solar wind is a stream of charged particles that has its own magnetic field. As per New Scientist, a magnetic sail would surround a spacecraft with a magnetic field that repels the field of the solar wind, leading to magnetic propulsion of the spacecraft away from the Sun.
As with solar sails, the magnetic sail, unfortunately, has its limitations as a method for interstellar travel. As a magnetic sail-propelled spacecraft gets farther away from the Sun, the intensity of sunlight and of solar wind would drop dramatically, meaning that they would not be able to pick up the necessary speed to be propelled to another star.
7. Interstellar travel near the speed of light is possible... in theory
The theory of special relativity states that particles of light, photons, travel through a vacuum at a constant speed of 670,616,629 miles per hour. If we could somehow harness a craft that could travel near this speed, interstellar travel would be a completely different proposition to what it is today.
As NASA points out, throughout space there are, in fact, instances of particles, that aren't photons, being accelerated to near the speed of light. From black holes to our near-Earth environment, particles that are being accelerated to incredible speeds — 99.9 percent the speed of light — likely thanks to phenomena such as magnetic reconnection, might point to future research that could help us harness methods for reaching such speeds.
Many theories and hypothetical methods for interstellar travel near the speed of light have already been proposed — several of these are mentioned in the points below.
8. Wormholes might provide a shortcut to other parts of the universe
Aside from predicting the existence of black holes, years before we ever saw one in an image, Einstein's theory of general relativity also allowed for the prediction of the existence of wormholes. This term, "wormhole", which describes tunnel-like shortcuts that traverse space and time, was coined by quantum physicist John Wheeler, who also coined the term “black hole”.
While wormholes are a tantalizing idea for space travel that has lit up the imagination of many a sci-fi enthusiast over the years, the likelihood that we could ever travel through one is incredibly slim. Firstly, we're not even certain wormholes exist; secondly, it is theorized that any type of matter that entered a wormhole would cause it to immediately close up.
Though it might be possible to stabilize the matter surrounding a wormhole and keep it open using a negative energy field called ghost radiation, all theories are very much in the hypothesis stage and most likely won't be tested in any true form for many years to come.
Wormholes are also problematic as the fact that they could transport matter across space would mean they are also a form of time machine, and would, therefore, be a violation of the laws of cause and effect. That hasn't stopped some scientists from devising theories and methods for methods of interstellar travel that utilize wormholes — more on that in section 14.
9. NASA is working on a proposed Em Drive that could allow space travel without the need for fuel
NASA and other organizations are working on a proposed fuel-free engine that might just be impossible. Why? Because the payoff, if they were to succeed, would be so revolutionary it would completely change our capability for interstellar travel and would usher in a new era for humanity.
The 'helical' engine, dubbed the EmDrive, was first proposed by British scientist Roger Shawyer in 2001. Shawyer hypothesized that we could generate thrust by pumping microwaves into a conical chamber. In theory, the microwaves should bounce off the chamber walls exponentially. In doing so, they would create enough propulsion to power a spacecraft without fuel.
If that weren't enough, NASA engineer David Burns, who is part of laboratory tests on the theoretical engine, says that, given the EMDrive needs no fuel, a spacecraft powered by such a device could eventually reach a speed of 99.9 percent the speed of light.
While some researchers claim to have generated thrust during EmDrive experiments, the amount was so low that detractors claim the energy might have really been generated by external factors, such as Earth's seismic vibrations.
10. One of the most obscure theoretical forms of interstellar travel is the dark matter rocket
In a study titled Dark Matter as a Possible New Energy Source for Future Rocket Technology, scientists set out a method for a form of travel that would harness the energy of the universe's mysterious dark matter.
The researchers behind the paper proposed a variation on the EmDrive (see point 9) that would harness the energy of dark matter in order to fuel a rocket. The advantage? Much like the EmDrive it would be an engine that doesn't rely on chemical combustion, meaning it would remove the shackles from our current methods for interstellar travel.
The problem with dark matter rockets? We know next to nothing about dark matter, aside from the fact that it's there. This form of travel relies greatly on future discoveries. It is worth researching though, simply because dark matter is everywhere; if it could be used as a fuel, we'd have an endless supply.
11. Engineers have worked on developing a nuclear fusion reactor for space travel
Fusion rockets are a type of spacecraft that would rely on nuclear fusion reactions to take us to the far reaches of space. The possibility of developing such a rocket was explored in the 1970s by the British Interplanetary Society under its Project Daedalus.
These rockets would rely on the vast amounts of energy released during nuclear fusion. The main method to have been put forth for releasing this energy in rockets is a method called inertial confinement fusion. This method would see high-powered lasers blast a small pellet of fuel to make its outer layers explode. In turn, this would crush the pellet's inner layers and trigger fusion.
Magnetic fields would then be used to direct the energy flow out of the back of the spacecraft in order to propel it forwards. Such a craft could travel the distance to Proxima Centauri in 50 years. The main problem with this method? In spite of decades of work, we are yet to see a working rocket fusion reactor.
12. Nuclear pulse propulsion might be the craziest proposed form of interstellar travel
By far the most reckless, and craziest, form of interstellar travel we've seen proposed is Nuclear pulse propulsion. This method would see a spacecraft propelled by the periodic throwing of a nuclear bomb out of the back of the craft before setting it off at just the right distance.
This method was seriously studied by the US government’s military technology agency DARPA, under the code name Project Orion. A spacecraft using nuclear pulse propulsion would need to be fitted with a giant shock absorber, that would allow for heavy radiation shielding that would protect the passengers.
Though such a spacecraft could theoretically reach speeds of up to 10 percent of the speed of light, the concept was largely dropped after nuclear test bans came into force in the 1960s.
13. The Bussard Ramjet would provide a solution to the problem of heavy fuel
The Bussard ramjet is another solution for one of the limitations of relying on chemical combustion — namely the weight of fuel. With our current best method for interstellar travel, the farther we want to get, the more fuel we need, the heavier the spacecraft, and the slower the acceleration.
The Bussard ramjet, proposed by physicist Robert Bussard in 1960, takes the concept of the fusion rocket (point 11) and gives it a twist; instead of carrying a supply of nuclear fuel, the spacecraft would ionize hydrogen from the surrounding space, and then suck it in using a large “electromagnetic field” scoop (as in the image).
The main problem with this as a method for interstellar travel is that, as levels of hydrogen are so sparse, the scoop might have to be hundreds of kilometers across.
14. NASA is working on developing a real-life warp drive
The Alcubierre drive was first proposed in 1994 by Miguel Alcubierre, a physicist at the University of Wales in Cardiff. The proposed drive would use "exotic matter," which are types of particles that have a negative mass and exert a negative pressure. Rather importantly, "exotic matter" has not yet been discovered, meaning the Alcubierre Drive relies on a future discovery that might never happen.
The particles of "exotic matter" could distort space-time, making space ahead of the spacecraft contract and space behind it to expand. This would mean the craft was inside a "warp bubble" that could theoretically travel faster than light without breaking the laws of relativity.
The main problem? Aside from there being no evidence of "exotic matter" existing, the Alcubierre drive, which is basically a real-life warp drive from Star Trek, would need energy equal to the total energy of the universe to sustain it. Despite this, in 2012, NASA scientist Harold Sonny White and colleagues released a paper, titled Warp Field Mechanics 101, detailing work into the possibility of an Alcubierre drive.
15. Astronauts will likely need traveling ecosystems to survive the trip
For all of the theories of warp drives and EmDrives that could allow for travel at immense speed, the fact is that future astronauts will likely need to be prepared for incredibly long journeys. Even if we could travel at 99.9 percent of the speed of light, it would take us approximately 4 years to get to our nearest star system, Alpha Centauri.
As researcher and professor of experimental architecture Dr. Rachel Armstrong told the BBC, we need to start thinking about the ecosystem that interstellar humanity will occupy out there in between the stars.
“We’re moving from an industrial view of reality to an ecological view of reality,” Armstrong explained. "It’s about the inhabitation of spaces, not just the design of an iconic object."
Rather than the hulking metallic spacecraft of films like Alien and 2001: A Space Odyssey, Armstrong envisions habitats with plenty of space for large biomes full of organic life that can sustain human beings on long interstellar journeys.
16. Cryosleep is also being considered for the incredibly long journeys between stars
Taking yet another leaf out of sci-fi movies and novels, the idea of cryosleep has been seriously considered as a way to allow human beings to travel huge distances without aging and without having to be awake for trips that can last for months.
In 2016, NASA funded research into a type of suspended animation where entire crews are put into cryogenic sleep for the duration of long space missions. The firm behind this, SpaceWorks, is working on developing a method for putting astronauts into a controlled state of advanced hypothermia that would allow them to hibernate during the long journeys through space.
17. Will we ever reach another star? Experts believe we will
“From the outset of human existence we’ve looked up at the stars and projected our hopes and fears, anxieties and dreams there,” researcher Dr. Rachel Armstrong told the BBC. Thanks to the great number of theories, theoretical models, and methods that are being devised today, Armstrong explains, interstellar travel "is no longer just a dream, this is an experiment now."
As Carl Sagan once wrote, "all civilizations become either spacefaring or extinct." That's why interstellar travel is important; whether we reach beyond our solar system a hundred or more than a thousand years from now, the fate of our future civilization ultimately depends on the development of interstellar travel technology that can take us distances that today seem unimaginable, and to places that we can only dream of.
Editor's Note: An earlier version of this article implied that project Breakthrough Starshot was funded by a "Russian billionaire" named Yuri Milner. While Milner does fund the project, he is also an Israeli, which means he is Israeli-Russian. This error has since been corrected to reflect his status of dual-citizenship as an Israeli-Russian. IE regrets this error.