An innovative plasma rocket being built as a spare for one heading to the International Space Station may have a space mission of its own: visiting an asteroid.

Equipped with an electric propulsion system, the rocket, known as Variable Specific Impulse Magnetoplasma Rocket (VASIMR), is being developed to one day transport astronauts to Mars in 39 to 45 days — a fraction of the six to nine months the trip would take with conventional chemical rockets. Shorter travel time greatly reduces astronauts' exposure to potentially deadly cosmic and solar radiation, currently a show-stopper for human missions to Mars.

Setting sail for an asteroid would be a powerful demonstration of VASIMR technology, which uses radio waves to ionize propellant — such as argon, xenon or hydrogen — and heat the resulting plasma to temperatures 20 times hotter than the surface of the sun. In place of metal nozzles to control the direction of the exhaust, VASIMR uses magnetic fields.

"All of a sudden, the future is here," said VASIMR inventor and physicist Franklin Chang-Diaz, a seven-time shuttle flier who left NASA in 2005 to start a company and work full time developing the rocket.

Chang Diaz's Houston-based Ad Astra Rocket Co., which has raised millions of dollars from private investors, reached a significant milestone last year when it successfully operated a demonstrator VASIMR at full power in a vacuum chamber.

"The engine is actually firing right now," Chang-Diaz told Discovery News. "We have lots of hurdles and challenges; we have lots of work to do. But if you look at what has happened in the last five years since we left NASA, it's been amazing."

Ad Astra plans to launch its flight version VASIMR to the space station in 2014. As a backup, Chang-Diaz intends to manufacture two engines in case a launch accident or other major problem prevents the first from reaching the outpost.

Once the engine is safely installed outside the station, the spare could be tapped for a new mission — that did not require investment by NASA.

"I had this idea that maybe there's a way we can use this backup engine that he's already building," said Rob Kelso, a former shuttle flight director at NASA's Johnson Space Center in Houston now working to build innovative partnerships between NASA and commercial firms.

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