IF ALL goes to plan, Moon power will this week help bring a wayward satellite
into a useful orbit. Similar manoeuvres could in future see the Moon become a
staging post for a series of missions to Mars at a tenth the price of last
year鈥檚 Pathfinder mission.
A handful of spacecraft have already used the Moon鈥檚 gravity to help them on
their way into space. But the latest spacecraft to try the trick is a lost
satellite trying to get into a geostationary orbit. 鈥淣o one has ever tried it to
bring a communications satellite back into Earth orbit,鈥� says Ronald Swanson,
president of Hughes Global Services, a division of Hughes Electronics in Los
Angeles.
Asiasat-3, a communications satellite made by Hughes Electronics, was
launched on Christmas Day 1997, on a Russian Proton launcher. The rocket鈥檚
fourth stage failed, so instead of ending up 36 000 kilometres above Singapore
in a circular geostationary orbit, Asiasat-3 was left in an elliptical transfer
orbit that took it up to the required 36 000 kilometres and then back down to
within 350 kilometres of Earth. This orbit was also inclined at 51掳 to the
desired orbit in the plane of the equator.
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Asiasat-3 was built for a long life, so it carried plenty of fuel, but not
quite enough to push it into the desired orbit. Instead the satellite was
written off, the insurers paid up and that appeared to be that鈥攗ntil Rex
Ridenoure, a specialist in space mission design at a company called Microcosm in
Los Angeles, contacted Hughes in January.
Ridenoure suggested that Hughes look at some out-of-the-ordinary orbits that
might achieve the desired result while burning less fuel. Hughes鈥檚 orbital
dynamics specialists didn鈥檛 follow the orbits suggested by Ridenoure, but
spurred on by his ideas they hit upon a rescue plan involving the Moon.
Since 10 April, Hughes engineers have fired the satellite鈥檚 engines several
times to make the high end of its orbit higher still. On 7 May, they gave it a
final kick to send it to the Moon. Lunar gravity should fling the satellite back
to Earth and change the inclination of its orbit, so that it will return on a
trajectory from which, with careful use of its burners, it should be possible to
enter a geostationary orbit.
With little fuel left over, Asiasat-3 will not be up to the job it was
designed for, but officials at Hughes still hope to sell the satellite鈥檚
services and make a bit of money鈥攕ome of which will go to the insurers.
The satellite has been renamed HGS-1.
Similar manoeuvres are being studied to slash the cost of missions to Mars.
Besides its main payload, the European Space Agency鈥檚 new Ariane 5 launcher can
carry eight 100-kilogram 鈥減iggyback鈥� science payloads, which will be left in
transfer orbits much like that in which Asiasat-3 found itself. ESA will charge
just $1 million a time for these launch slots.
Getting from there to other planets is a tall order for a tiny spacecraft,
but the Moon should make it possible. Paul Penzo and his colleagues at NASA鈥檚
Jet Propulsion Laboratory in Pasadena have already come up with a 鈥渢hree burn鈥�
strategy for getting to Mars. First, you lengthen the transfer orbit until it
extends beyond the Moon. Then you change the orbit so that the spacecraft flies
back past the Moon, gaining momentum from its gravity in the process. Finally,
as the satellite skirts in a near parabolic path around the Earth, you fire the
rockets one last time to send it on its way to Mars. This should allow a mission
to reach the Red Planet for just $25 million.
