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ESA proposes ion drive Sun-dodge Mars commsat ploy

Electric rockets to 'hover' above, below ecliptic

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Experts in interplanetary navigation have hit upon a novel scheme for maintaining communications with Mars, should a need to do so eventuate - as in the case of a manned mission to the red planet, for instance.

Diagram of the proposed Mars commsat orbits, looking in the plane of the ecliptic. Credit: Universities of Strathclyde and Glasgow

Forget geosynchronous, powered B-orbits are where it's at for commsats.

The problems in communicating with Mars come when the Sun lies between it and Earth, which happens at intervals of a bit more than two years. Direct radio communications are cut off for weeks.

A natural solution would be to use a communications relay satellite in a suitable orbit, which could carry signals from planet to planet around the sun much as Earth-orbiting satellites carry them around the world.

According to François Bosquillon de Frescheville of the European Space Operations Centre, Darmstadt, it's not as simple as that.

"It has been known for some time that, due to the natural orbital motions of the Sun, Earth and Mars, any communication relay satellite that orbits Mars in a traditional, unpowered Keplerian orbit will, at some point, be blocked by the Sun. That's not good for any astronauts on Mars," he says.

It might be possible to use a relay spacecraft in orbit round the Sun, but this would probably involve serious increases in the time taken for messages to travel back and forth - already some hours when Earth and Mars are far apart.

Thus de Frescheville consulted with a number of top space-navigation engineers at the Universities of Strathclyde and Glasgow. The idea was to see if it might be possible to make use of ion drives, electrically powered rockets which produce very low amounts of thrust (about the same as you feel when you blow on your hand, apparently) but which can keep on doing so for a long time using very small amounts of fuel.

Probe missions using such drives have already been sent out into the solar system, and there is a low-orbiting Earth survey satellite which uses ion propulsion to counteract the minuscule - but nonetheless significant - amounts of atmospheric drag it suffers due to being in the extreme upper reaches of the atmosphere.

"What we have shown is that if you can provide continuous thrust, a pair of spacecraft could 'hover', respectively, over a point leading, and under a point trailing, the Mars orbit, and provide continuous radio communications between Earth and Mars. You would need two relay spacecraft to cover both halves of Mars," says de Frescheville.

"You would get, in effect, full-time communications to almost anywhere on the Red Planet's surface. When the Earth–Mars conjunction season is over, the spacecraft could stop thrusting, save fuel and take up regular, unpowered or near-Keplerian orbits until the following conjunction approaches, and then take up their relay positions again for the next conjunction."

According to de Frescheville and his Scottish colleagues, the relay satellites would only have to switch on their thrusters for 90 days in each 2+ year conjunction cycle, and they would only increase the one-way signal travel time by one minute.

The plan could be very useful in the event of a manned mission to Mars, which was until recently an official goal for NASA - perhaps as soon as 2030. However, at the moment the US space agency is struggling even to assemble a coherent plan for its future manned space efforts, and nobody else has anything more concrete.

However, the relay sats could also help with unmanned missions. According to de Frescheville, similar cunning low-thrust "B-orbit" tricks could be employed in other contexts.

The ion-drive relay sat study was funded by the European Space Agency, and the results were presented this week at the International Astronautical Congress held in Daejeon, South Korea. You can read it here (pdf). ®

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