How to get to Alpha Centauri

By Charles Q. Choi

updated 1:30 p.m. ET March 12, 2008

If the nearest star system, Alpha Centauri, does harbor rocky planets similar to Earth as new findings suggest, there exist a host of ways to get us there, in theory.

Sending a person to Alpha Centauri within a human lifetime wouldn't be easy. Alpha Centauri is 4.37 light-years away — more than 25.6 trillion miles, or more than 276,000 times the distance from the Earth to the sun.

"Interstellar travel is extremely hard," said science fiction author and NASA physicist Geoffrey Landis.

But the lure has never been stronger. Scientists last week said the Alpha Centauri system has the ingredients for an Earth-like planet, and they think they can spot it.

Conventional rockets are nowhere near efficient enough. At a maximum speed of about 17,600 mph, it would take the space shuttle, for example, about 165,000 years to reach Alpha Centauri. In any case, "the problem with conventional rockets is that if you're carrying fuel, you need fuel just to carry all the fuel you bring with you, and it just gets exponentially worse," Landis said.

But antimatter engines might work. These drives rely on the extraordinary amount of energy released when antimatter and matter annihilate each other. The problem, however, is creating enough and storing any antimatter for the trip.

"All of the current methods of manufacturing antimatter require enormous particle accelerators and produce antimatter in very small quantities," Landis said. "And to store antimatter, if you need a ton of magnets for one gram of antimatter, the entire idea of a lightweight way to store immense amounts of energy is no longer lightweight."

MSX/IPAC/NASA New findings suggest Alpha Centauri might harbor rocky planets similar to Earth.

Although one could in principle freeze anti-hydrogen and thus bypass the need for magnets, "if even the tiniest amount ever leaked out and touched the walls surrounding it, you'd produce a lot of heat, which in turn would heat up the frozen anti-hydrogen, and the whole thing catastrophically goes away," Landis said.

Antimatter could nevertheless perhaps find use in interstellar spaceships as a way to help trigger nuclear reactions. "That's something that hasn't been ruled out yet, and a little antimatter could help go a long way," Landis said.

Instead of rockets that carry all their fuel with them, spaceships might scoop it up along the way. One design proposed by physicist Robert Bussard (who died last year) would employ giant electromagnetic fields to suck in hydrogen to fuel a nuclear rocket.

Unfortunately, this "ramscoop" or Bussard ramjet, probably could not work. "The interstellar medium is not as dense as Bob Bussard thought it would be," Landis said. "And so far all attempts to design some kind of scoop had the unfortunate effect of producing more drag than you get back thrust, working kind of like parachutes."

Moreover, "we don't really have any notion of how to use the pure hydrogen we find in interstellar space as fusion fuel," Landis added. All of the proposals for fusion in the lab use deuterium-tritium (two isotopes of hydrogen) or deuterium with helium-3 (an isotope of helium) — "we don't have any suggestions for pure hydrogen in a fusion reaction," he said. "It was a clever idea, but the devil's in the details."

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