SMART-1, the European
Space Agency craft currently in orbit around the moon, makes use of a technology
that was pure science fiction until the 1960s - the ion drive. An ion drive is a
method of propulsion that uses electricity to create charged ions and then
accelerate them with a magnetic field, pushing them out the rear of a
spacecraft.
The ion drive was first described in 1947 by Jack
Williamson in a short story published in Astounding Science Fiction
called "The Equalizer." In the story, the spacecraft achieves a significant
fraction of the speed of light, returning decades later due to time dilation.
George Lucas makes use of ion drives in Star
Wars: the old Empire makes use of T.I.E. Fighters as small,
manuverable spacecraft. T.I.E. is an acronym for Twin Ion Engines:
The hexagonal solar panels supply power to
a unique propulsion system. Microparticle accelerators propel Ionized gasses
at a substantial fraction of lightspeed...
(Read more about T.I.E. Fighters)
US astronaut Edward Gibson published Reach in
1989; this novel features a spacecraft with an ion drive that accelerates atoms
of mercury to one percent of the speed of light before sending them out the rear
of the craft.
SMART-1 has a stationary plasma thruster using xenon
gas with 1190 watts of power available, giving a nominal thrust of 68 mN. The
spacecraft contains 48 liters of xenon gas at 150 bar. The lifetime of the
thruster is 7,000 hours at maximum power. The thrust is equivalent to two
pennies resting in the palm of your hand.
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SMART-1's Ion
Drive
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If this seems like a propulsion system that is a bit
short of "warp speed," you're right. However, in the frictionless environment of
space, even a gentle acceleration like this can produce real velocity if applied
constantly over a long enough period of time. Take a look at the path of SMART-1
as it gradually increased its speed, spiralling further and further from Earth
before breaking free of Earth's gravity and heading toward the moon. Since a
solar panel-equipped ion drive craft only carries the weight of the propellant,
and can eject the propellant at significantly greater velocity, such a craft
will outperform a spacecraft that uses conventional chemical propulsion over the
long run. (Chemical propulsion depends on carrying the weight of the reactant
chemical; the exhaust velocity is also significantly less.)
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SMART-1's leisurely path to
the moon.
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The US probe Deep Space 1 also used an ion drive,
succeeding in its dual missions of flybys of asteroid Braille and Comet Borrelly
in 1999. Ion drives have been used in satellites as a way of gently nudging them
into higer orbits; the Irridium satellite network made use of ion drive
propulsion for positioning and to prevent orbit decay.
SMART 1 has traveled a total of 80 million kilometers
in 13 months to reach the moon (due to its spiraling path); Apollo 11 traveled
400,000 kilometers in four days during the first Moon landing mission. On the
other hand, the washing machine-sized SMART 1 cost just $85 million.
The basic principles of the ion drive were tested as
early as 1959 at NASA's Glen Research Center; the first working drive was
created in 1970. Deep Space 1, launched in 1998, was the first ion drive craft sent into space (read
the final technical evaluation of its propulsion system).
The European Space Agency has a very nice website
about SMART-1; along with mission objectives and updates, it includes a set of
articles about electric spacecraft
propulsion. You'll also find some information
about ion drives and other science-fictional technologies in the short book
Innovative Technologies from Science
Fiction. Thanks to David Raitt for pointing
these out.
(This Science Fiction in the News story used
with permission from Technovelgy.com - where science meets fiction.)
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