Astronomers have discovered an Earth-sized exoplanet orbiting a star in Alpha Centauri, just 4.3 light-years from Earth. With surface temperatures in excess of 2,000 degrees fahrenheit, this particular planet would be inhospitable, but the fact that it exists at all raises hopes of finding another, more human-friendly planet within the same star system.
Just for fun, let's say we found that planet tomorrow. How would we get there?
Top image, an artist's rendering of the exoplanet in Alpha Centauri, via L. Calcada/European Southern Observatory
Some of the most ambitious spacefaring scenarios out there put a cap of 1/10th the speed of light on our maximum travel velocity. We're obviously nowhere near achieving such celerity, but even if we could, the trip to Alpha Centauri would still take upwards of 40 years. By definition, that means we're going to need a generation ship.
Think of a generation ship like an interstellar ark. It's slow-moving (in the sense that it would likely travel much more slowly than the speed of light) but colossal and built to last. Granted, it's built to last because it has to last. The word "generation" in "Generation Ship" is a nod to the unfortunate reality that such a vessel would take quite some time to reach its destination — as in "many generations will live and die on this ship en route to Alpha Centauri."
If we built a generation ship that could travel as quickly as the fastest thing we've put in space, we'd still be looking at tens of thousands of years between wheels up and wheels down. Excluding orbiting vessels like NASA's Space Shuttles (which could circle Earth at up upwards of 17,500 mph/28,000 kph) and the Helios 2 spacecraft (which zips around the Sun at around 150,000 mph / 240,000 kph), the record-holder for fastest solar escape velocity belongs to the Voyager 1 probe, which for the last several decades has been coursing through space at speeds upwards of 38,000 mph / 62,000 kph. That's over 17 kilometers per second — but ven at those speeds, Voyager 1 is only just now poised to exit our solar system; were it heading in the right direction, it would still take over 70,000 years to reach Alpha Centauri.
Given that a generation ship would most likely need to be self sustaining — generating its own food, water and supplies — a quicker flight time would obviously be preferable.
Revolt Against the Tyranny of the Rocket Equation
Quicker flight times demand faster travel, and faster travel implies novel forms of more powerful propulsion technology.
It's important to point out here that novel technology is essential. Even if we could produce more powerful versions of the rockets that we use today, we'd still need to power them with fuel. But fuel takes up space. A lot of it. The famous line from NASA is that your typical soda can is 94% soda and 6% can, by mass; and that the external tank for the Space Shuttle is 96% propellant and 4% structure — and that's just to escape into low-Earth orbit.
NASA astronauts and scientists often refer to the limitations of our current propulsion technologies as "The Tyranny of the Rocket Equation." Using a simple example, Astronaut Don Pettit encapsulates the extent of our suboptimal relationship with rocket technology in an excerpt from a recent essay:
If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.
If we are to revolt against the tyranny of the rocket equation, Pettit says, new technology will be needed. "The discovery of some new physical principle," he continues, "could break the tyranny and allow Earth escape outside the governance of the rocket paradigm."
"A Spaceship in Twilight rev 5," by eRe4s3r, via Deviantart
Novel Propulsion Methods
So what "paradigm-shifting" technologies are on the table?
- Antimatter engines — which would use the meeting of matter and antimatter to generate massive amounts of energy — are one option, though we are missing a key ingredient: antimatter. At present, we just can't produce enough of it.
- Ramscoop drives are another commonly cited option. Proposed by the late physicist Robert Bussard back in the 60s, the ramscoop would use electromagnetic fields to gather and compress hydrogen from the interstellar medium. Bussard theorized that this process would beget thermonuclear fusion, the energy of which could be used in the form of propulsion. Unfortunately, Bussard's ramscoop can be torpedoed by a number of limiting factors, not the least of which being that interstellar hydrogen is actually relatively scarce.
- A warp drive, à la Star Trek? Turns out that "may not be as unrealistic as once thought." Experiments are still very much preliminary, and the concept is certainly "outside the box," to say the least — but then again, "If we're ever going to become a true spacefaring civilization," says Richard Obousy, president of Icarus Interstellar, a non-profit group of scientists and engineers devoted to pursuing interstellar spaceflight, "we're going to have to think outside the box a little bit, we're going to have to be a little bit audacious."
Forget Propulsion, Bring on the Lasers
Then again, maybe spacecraft-mounted propulsion isn't the way to go about solving this problem at all. That's where light sails (aka solar sails) come in. This technology uses incredibly large, incredibly lightweight reflective sails to exploit the matter-moving properties of light. That light could be generated by the Sun, other stars, or even lasers beamed from Earth. The point is your energy source is coming from outside the spacecraft itself. And because you're in space, any momentum you pick up is preserved — you could, in theory, continue gaining speed almost indefinitely, riding on "gust" after "gust," produced by lasers, or microwave transmitters.
Artist's conception of the Japanese IKAROS spaceprobe in flight, via Wikimedia Commons
Of course solar sails also have their limitations. A spacecraft outfitted with solar sails would require a lot of energy and guidance from Earth. According to NASA:
A precursor space mission, carrying a 1 kg (2.2 lb) payload on a 10x10-meter sail would take 20 hours to accelerate. In three weeks, it would pass the orbit of Pluto and continue outward to the Oort cloud of comets surrounding the solar system. Reaching a star would take 400 years.
If humans wanted to reach Alpha Centauri any faster, we'd need to continue accelerating the craft from Earth for significantly longer than three weeks, which would likely require lasers with higher precision than is currently possible (laser beams tend to disperse over distance). But the promising thing about solar sails is how achievable it feels.
"It's still science fiction," says Geoffrey Landis, a NASA physicist and scifi author, "but it's near-term science fiction."