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.
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.
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.
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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