There's a growing chorus of calls to send astronauts to Mars rather than the moon, but critics point out that such trips would be long and gruelling, taking about six months to reach the Red Planet. But now, researchers are testing a powerful new ion engine that could one day shorten the journey to just 39 days.
Traditional rockets burn chemical fuel to produce thrust. Most of that fuel is used up in the initial push off the Earth's surface, so the rockets tend to coast most of the time they're in space.
Ion engines, on the other hand, accelerate electrically charged atoms, or ions, through an electric field, thereby pushing the spacecraft in the opposite direction. They provide much less thrust at a given moment than do chemical rockets, which means they can't break free of the Earth's gravity on their own.
But once in space, they can give a continuous push for years, like a steady breeze at the back of a sailboat, accelerating gradually until they're moving faster than chemical rockets.
Several space missions have already used ion engines, including NASA's Dawn spacecraft, which is en route to the asteroids Vesta and CeresMovie Camera, and Japan's spacecraft Hayabusa, which rendezvoused with the asteroid Itokawa in 2005.
But a new engine, called VASIMR (Variable Specific Impulse Magnetoplasma Rocket), will have much more "oomph" than previous ones. That's because it uses a radio frequency generator, similar to transmitters used to broadcast radio shows, to heat the charged particles, or plasma.
The engine is being developed by the Ad Astra Rocket Company, which was founded in 2005 by plasma physicist and former space shuttle astronaut Franklin Chang-Diaz.
As hot as the sun
VASIMR works something like a steam engine, with the first stage performing a duty analogous to boiling water to create steam. The radio frequency generator heats a gas of argon atoms until electrons "boil" off, creating plasma. This stage was tested for the first time on 2 July at Ad Astra's headquarters in Webster, Texas.
The plasma could produce thrust on its own if it were shot out of the rocket, but not very efficiently. To optimise efficiency, the rocket's second stage then heats the ions to about a million degrees, a temperature comparable to that at the centre of the sun.
It does this by taking advantage of the fact that in a strong magnetic field – like those produced by superconducting magnets in the engine, ions spin at a fixed frequency. The radio frequency generator is then tuned to that same frequency, injecting extra energy into the ions.
High power
Strong magnetic fields then channel the plasma out the back of the engine, propelling the rocket in the opposite direction.
Thanks to the radio frequency generator, VASIMR can reach power levels a hundred times as high as other engines, which simply accelerate their plasma by sending it through a series of metal grids with different voltages. In that setup, ions colliding with the grid tend to erode it, limiting the power and lifetime of the rocket. VASIMR's radio frequency generator gets around that problem by never coming into contact with the ions.
"It's the most powerful superconducting plasma source ever, as far as we know," says Jared Squire, director of research at Ad Astra.
Scientists at Ad Astra began tests of the engine's second stage – which heats the plasma – last week. So far, team members have run the two-stage engine at a power of 50 kilowatts. But they hope to ramp up to 200 kW of power in ongoing tests, enough to provide about a pound of thrust. That may not sound like much, but in space it can propel up to two tonnes of cargo, reaching Jupiter in about 19 months from a starting position relatively close to the sun, says Squire.
Orbital boosts
Ad Astra and NASA have agreed to test fire the rocket in space, attached to the International Space Station in 2012 or 2013. Potentially, VASIMR could provide the periodic boosts needed to keep the ISS in its orbit.
At its current power level, VASIMR could be run entirely on solar energy. Squire says it would make a good Earth-orbit tugboat, pulling satellites to different orbits. It could also shuttle cargo to a lunar base, and because it could travel relatively quickly, it could be deployed to dangerous asteroids to gravitationally nudge them off course years before they would reach Earth.
To travel to Mars in 39 days, however, the engine would need 1000 times more power than solar energy could provide. For that, VASIMR would need an onboard nuclear reactor. Early versions of the reactor technology were used from the 1960s to the 1980s by the Soviet Union, but have not been used in space since and would take time to develop. "That would be quite a ways down the line," Squire says.
'Game changer'
But the possibility of such a short trip to Mars was recently lauded by Charles Bolden, NASA's new chief. He said NASA had provided a small stipend towards VASIMR's development, and said the collaboration was a good example of a partnership with private industry that could help the agency meet its goals after the space shuttles are retired in 2010.
John Muratore of the University of Tennessee Space Institute and a former lead engineer for NASA's space shuttle programme, says engines like VASIMR could enable the first human trips to Mars.
"The bottom line is with the current propulsion technology, Mars missions are undoable for humans," he says, explaining that such long trips outside of the Earth's protective magnetic field would expose astronauts to greater amounts of dangerous space radiation.
If engines, such as VASIMR, could be developed to take people to the Red Planet in 40 days, "that puts it inside the range of what we feel comfortable of doing with humans," he told New Scientist. "Something like VASIMR – that's a game changer."
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