"I want to go to there." (Henry Guttmann/Getty Images)
At last count, exoplanet hunters have dead-eyed 495 distant worlds. Granted, none of them are "Earth-like," but what will we do if such a world finally pops up? Will we be able to travel there in person or send a probe? Or are our interplanetary relationships doomed to remain long distance?
The Tau Zero Foundation takes this question very seriously. The nonprofit group encompasses scientists, engineers, entrepreneurs and writers who all share a common goal: to conduct research into interstellar flight technologies.
According to Centauri Dreams writer Paul Gilster, one of the founders of Tau Zero, getting to another star isn't beyond reach. Getting there within a human lifetime? Now that's a problem.
"Technically, if we could point one of our Voyagers at Alpha Centauri, we could get it there," Gilster says. "But it would take over 70,000 years. The problem comes with how we can shrink those travel times."
Yet the prospect of traveling to another star within a human lifetime does not violate the laws of physics. Several interstellar travel methods are mere centuries away from becoming a reality, provided it remains something that human societies want to achieve, according to Gilster.
One way to travel to another star is to catch a ride on waves from our own sun. As laid out by solar sail pioneer Gregory Matloff, photons from the sun could push against lightweight, reflective sails to propel a spacecraft. Pick up an initial gravitational assist from the sun, and you'd arrive at Alpha Centauri in a mere 1,000 years.
In order to speed the whole process up, physicist Robert Forward proposed pushing on the sail with a laser beam.
"Some of Forward's designs got to speeds up to 10 percent of light speed," Gilster says. "And if you are talking about 4.3 light-years away from Earth, which is where the Centauri primary stars are, that gets you there in about 43 years."
Other theoretical models of sail-based interstellar propulsion systems call for a magnetic bubble instead of a physical sail. By tethering the bubble to a spacecraft and lining it up with a particle beam, scientists could propel a probe up to considerable speeds.
Interstellar propulsion inevitably comes down to energy, and few future energy sources are as promising as fusion power, the joining of atomic nuclei to produce a single nuclei and a release of energy.
"Fusion is another possibility, particularly deuterium/helium-3 fusion," Gilster says. "We haven't yet figured out how to this reaction on Earth, but it's possible that in the next 50 or 100 years we'll learn how to tap this kind of fusion for propulsion."
As such, several different interstellar propulsion models depend on fusion. Physicist Robert Bussard suggested an "interstellar ramjet," by which a speeding spacecraft would scoop up widely distributed interstellar hydrogen to serve as fusion fuel.
Another model, called the fusion runway, expanded on this principle by calling for a string of fusion fuel pellets to trail off from a spacecraft's point of departure. Each time the ship hit a pellet, it would instigate a fusion reaction that would propel it even faster toward the next pellet. Eventually, the craft would reach the cruising speed it needs to reach its destination - perhaps reaching Alpha Centauri in as few as 40 years, according to Gilster.
Is fusion propulsion a reasonable means of reaching other stars? That's what the British Interplanetary Society Project's Daedalus project set out to answer in the 1970s.
Daedalus researchers outlined plans for fusion pulse propulsion system, which would expend pellets composed of a mix of Deuterium Helium-3 at a rate of 250 per second. The resulting explosion would propel the unmanned craft to an estimated 12-percent light speed, or 22,354 miles per second (35,975 kilometers per second).
The Daedalus project's target destination was Barnard's star, located about 6 light years from Earth. The journey was expected to take an estimated 50 years.
Today, the Icarus Project picks up where Daedalus left off and, according to project leader Richard Obousy, the time is right to re-evaluate fusion pulse propulsion.
"Let's look at Daedalus but with the hindsight of 35 years of new technology," Obousy says, "and couple that to the excitement of extrasolar planets and the rapidly improvements in telescopic technology, meaning that there is a good chance we'll actually be able to image an Earth-like planet (if they're discovered) within the next two decades."
Should that happens, Obousy believes the mere act of seeing another blue green world will fuel the desire to send a space craft to it. Until that happens, however, the Icarus project will continue to safeguard Daedalus's findings and refine them for the future.
"What we really want to do is keep that dream and that knowledge of interstellar propulsion alive," Obousy says. "So worst-case scenario, we'll know the Daedalus project very, very well and pass that knowledge onto the next generation. Best-case scenario, we'll completely redesign it and come up with a more efficient and optimized fusion-based propulsion spacecraft. The physics is there. The technology is being proven."
Travel to another star is within the grasp of human technology. The question is whether we'll remain committed to the dream in the years to come.