GOCE's electric ion propulsion engine s...( GOCE's sophisticated electric ion pr...)

GOCE's sophisticated electric ion propulsion system has been switched on and confirmed to be operating normally, marking another crucial milestone in the satellite's post-launch commissioning phase.

The success of GOCE's ultra-sensitive gravity measurements depends on finely controlling the satellite's orbit and speed. The push from the thruster must be just enough to compensate for the tiny amount of drag generated by the few wisps of atmosphere at GOCE's orbital height. No normal jet engine could do this, but GOCE's sophisticated electric ion propulsion system can.

This cutting-edge system does not burn fuel like a regular rocket motor. Instead, it is supplied with xenon from a 40-kg tank, which is converted to fast-moving ions - naked xenon atoms that have had some of their electrons stripped away by an electric discharge generated from solar energy. The ions are ejected toward the rear, giving a very gentle, steady and smooth thrust.

The electric ion propulsion system comprises two redundant thruster units mounted externally on the last panel of the satellite. The thrusters can be throttled between 1 and 20 millinewtons (mN), which will be set automatically, depending on the actual real time drag that the satellite experiences once GOCE is in routine operations.

This is a fantastically small force - a thrust of a few mN is similar to the weight of a few drops of water on Earth. Yet, by thrusting continuously during GOCE's routine operations, it is sufficient to maintain a 'freefall' orbit.

"On Tuesday last week, we fired the B-unit of the electric ion propulsion, stepping it through 1, 3 and 8.3 mNs of thrust. On Thursday, we fired the A-unit. Both are performing nominally," said Spacecraft Operations Manager Juan Pieiro, speaking at, ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany.

"Next comes the 'wake up' for the instruments, which puts us in the position to start operating
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