Five Real-Life Methods of Interstellar Travel

The answer to the question, as is always the case with a good question, is: “It depends.” It depends on a lot of factors that have to do with the nature of space travel beyond the protective envelope of Earth, which shields us from a lot of harmful radiation, debris and dust, and so on. But technologically speaking, if you’re patient enough, then yes, we could. Actually, the military has been speculating about interstellar travel for years.

I don’t mean UFOs or anything like that (I’m not a believer, and if the military is sitting on such things, I certainly don’t know about them). Instead, I’m talking about a few really cool if completely speculative and probably impossible schemes cooked up by analysts since the 1970s.

The Problem: Too Far, Too Slow, Too Old

Basically by definition, anything that’s going to travel to another star, even a close one, has to do be able to perform one of several special tricks. Even the closest stars are light-years away — so, at least many decades away, if not millennia, with any conceivable method of propulsion (barring some magical discovery of warp drive or similar sci-fi fantasies). 

Which leads to the first question: fuel. Fuel is such a problem that it already seriously interferes with our space missions just to planets within our own star system. All of the probes we plan to operate a long time now generally rely on solar power to operate their instruments — and solar power won’t help you much past the middle of the solar system. On its way to Mercury, the Messenger spacecraft is following a meandering seven-year flight path in order to improve its fuel efficiency. Astronomers and mathematicians have worked out a series of corridors called the Interplanetary Transport Network, which represent the paths spacecraft can follow to get the maximum benefit of gravitational slingshot maneuvers and therefore move around the solar system with extremely low fuel requirements.

Once we’re beyond Neptune, things get infinitely tougher. The Voyager probes are still operational, thanks to their radioactive generators, but they won’t be able to do any fancy maneuvering any more — they would need engine fuel for that. The fastest spacecraft humans have yet built, the New Horizons probe currently en route to Pluto, is zooming along at 16.5 km/s (or about 10 miles per second). Voyager is now faster, thanks to its slingshot maneuvers at Saturn and Jupiter. However, at that speed, neither of those speed demons would reach Proxima Centauri (the nearest star) for at least another 70 000 years.

So we’re going to have to be creative. And since the 1970s, people have tried to be.

#1: A Standard Space Probe with an Ion Drive and/or Gravity Slingshots
Plausibility: High
Travel Time: 19,000 – 80,000 years

We couldn’t use the extremely efficient ion drive to reach Pluto with New Horizons, because it couldn’t get that ship fast enough, quick enough, but it has been successfully used on most of the comet and asteroid encounter craft. According to Ian O’Neill, we could use pretty much the same technology to send a space probe to Centauri, too. And we could further accelerate it using gravity assists, as was done with Voyager on its way into the outer solar system.

Since all of this relies on already-in-service technology, obviously it’s the most plausible option. The problem is, we’ve only had agriculture for ten thousand years. By the time our super-probe gets where it’s going, either it’s going to look like the equivalent of Stone Age technology to us — or we’ll have gone extinct anyways.

#2: Fission Power; Or, What to Do With All Our Nukes
Plausibility: Modest
Travel Time: 85 years

In the late 1950s, the U.S. government initiated Project Orion, the attempt to use atomic bomb detonations as a propulsion mechanism for spacecraft. Essentially, the Orion spacecraft would work the same way desperate space travellers occasionally do in sci fi movies when their starship has broken down: shoot an extremely powerful explosive behind the ship, wait for it to blow up, and then ride the shockwave. While most schemes for long-distance travel require small, lightweight probes, Orion would actually need an enormous, heavily built ship in order to absorb the shock of the nuclear detonation. Initially, the American government believed it would use the design to send a ship into space by bouncing it upwards in a series of very small atomic explosions, whichWikipedia imaginatively likens to “an atomic pogo stick.”

There are obvious applications to elsewhere, however. Stack up enough of these nukes, and the ship would be capable of extremely fast missions, like one-year round trips to Pluto. A sufficiently sturdy ship could be accelerated to 5% of the speed of light, and, according to O’Neill, reach Centauri in less than a century.

#3: A Solar Sail
Plausibility: Low
Travel Time: 40 years

According to American physicist Robert L. Forward, one alternative is to forget about engines entirely, and just do our exploring the old-fashioned way – with a sailboat. Of course, there’s a catch: Starwisp, as he calls it, would have to be extremely small, because it would never be able to get very much thrust. Starwisp is based on the existing technology of the solar sail. Unfortunately, solar wind wouldn’t be enough for Starwisp. So, Forward suggests we build a high-powered microwave transmitter to give it its initial boost. That could work, provided that the entire apparatus – space probe and solar probe together — weighs less than three pounds. This little package could be accelerated to 10% of the speed of light, and reach the Centauri system in just forty years.

Once Starwisp reached its cruising speed, we could simply turn off the microwave beam — because it would pass beyond the range at which we could give it any more meaningful acceleration. However, as it approached the target, Forward says we would turn it on again, and Starwisp could use the very weak beam to charge its batteries, turn on its sensors, and transmit some data back to Earth. Of course, there would be no way to stop Starwisp, so it would just zip through Centauri and head back into deep space again. But the design is simple enough and cheap enough, says Forward, that we could mass-produce them, and have a regular stream of them sending back data to us.

Sounds simple enough. Of course, at just three pounds total weight, there is no way we could build into Starwisp any sort of protective material to shield it from stellar radiation, or to prevent the delicate sail from getting bent out of shape by microscopic space dust. Very likely it would just be a broken hunk of debris by the time it reached another star.

#4: Fusion Power
Plausibility: Very Low
Travel Time: 40-100 years

Two competing plans for a fusion-powered interstellar spacecraft have been drawn up, the American Project Longshot and the British Project Daedalus. Longshot would be an unmanned probe with a fission reactor, used to power a complex array of lasers which would in turn power a fusion engine. It would be built in orbit, probably by astronauts living in the International Space Station, and then sent on its way. Once it reached the target, it would jettison its engine section, enter orbit, and begin transmitting data full-time back to Earth with its remaining power. But Longshot is slow: its fusion reactor couldn’t push it to 5% of the speed of light, and it would probably take over a century to get anywhere meaningful.

A more technically challenging alternative is the British Daedalus spacecraft. Daedalus would weigh far more than the few-hundred-ton Longshot: it would be a 54,000-ton monster (almost 90% of that weight would be the fuel tanks). This spacecraft could theoretically reach 12% of the speed of light, making the trip to the Centauri system in just 33 years. There’s no way it could ever slow down again, of course, so the Brits suggested that Daedalus actually be built as a carrier spacecraft for a large assortment of smaller probes, which would be launched close to the end of the mission. The Daedalus plan also called for robotic self-repair technologies as well as for special explosive vehicles which would be used to break up large objects in its path at a safe distance of several hundred kilometres.

One of the chief problems with this more fanciful strategy is that it relies on helium-3 for power (so far this is the only way we can make a functional fusion power system). We don’t actually have any helium-3 available on Earth, at least not in anything like sufficient quantities for an interstellar engine (Longshot needed 265 tons). Most proposals for acquiring helium-3 is a power source call for mining the Moon, the surface of which supposedly has been laced with thraces of the element by the solar wind. The Daedalus team was more optimistic, however: they suggested that we bypass the moon and just collect the stuff in Jupiter’s upper atmosphere using specially designed balloons.

#5: Antimatter
Plausibility: Ludicrous
Travel Time: Less than 10 years?

Back to America, this time for Project Valkyrie, the most off-the-wall yet (don’t worry, the truly off-the-wall contributions from the theoretical physicists get an article all their own, and involve things like black holes). Designed by Charles Pellegrino and Jim Powell, Valkyrie would be small like Longshot, rather than enormous like Daedalus, and would be modular, with its various essential components (fuel tanks, shielding, equipment, the crew habitat, etc.) towed along by the giant engine module. (This part actually isn’t essential; Pellegrino and Powell suggested the design because it would be orders of magnitude lighter than Daedalus.)

Like Daedalus, Valkyrie would begin its outward trip with fusion power, quickly accelerating up to 20% of the speed of light. As it accelerates, however, it would gradually transition from nuclear fusion to antimatter — kind of like a hybrid car, except with fantasy fuels. The antimatter would be used to accelerate the ship to a stunning 92% of the speed of light. As it approached Centauri, Valkyrie would decelerate using an identical antimatter engine mounted on the opposite end of the spacecraft.

Of course, antimatter is harder to come by than helium-3. According to the math at Wikipedia, we could fulfill the entire energy needs of the human race for 80 years with the amount of power it would take to manufacture the fifty tons of antimatter needed by Valkyrie’s engines.

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