News & events
Culham’s role in global gravity mapping | 07/07/2010
Currently orbiting the earth at such a low altitude that it's riding along the fringes of the atmosphere is the Gravity Field and Steady-State Ocean Circulation Explorer – the GOCE satellite. Its aim is to produce precise maps of the Earth's gravitational fluctuations. Back in the 1970s, Culham had a hand in the early development of the technology being used to power the GOCE spacecraft.
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Because gravity is stronger closer to Earth, GOCE needs to be as close to the surface as possible. The instrumentation aboard GOCE needs to be as stable as possible to measure accurately and the GOCE team has created a sleek, aerodynamic satellite boasting a unique propulsion system that orbits a mere 158 miles up, extremely low for an earth observation satellite but high enough to eliminate most drag.
The gravity measurements require the craft remain stable in free fall; any interference from air at this altitude could skew the gravity data and jeopardize the mission's quality. So GOCE was fitted with an electric ion thruster that continuously offers tiny bursts of thrust to compensate for any drag the satellite encounters.
Until the development of ion propulsion, Earth satellites and deep space probes could be manoeuvred only by thruster rockets, using large amounts of fuel, but with ion thrusters the exhaust velocity, typically 1 km per second, is much higher so the mass of fuel is much reduced; the inert and naturally occurring gas xenon is injected into the thruster, where electrons are removed to form electrically charged atoms known as ions. These are ejected in a narrow beam producing a force in the opposite direction, with the number and speed of ions adjusted to assure pinpoint accuracy.
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Ion propulsion in spacecraft has been developed over the years all around the world – including seminal work at Culham. Peter Harbour, who worked on the Culham ion thruster projects in the 1970s, is delighted to see their successors being used in this ground-breaking experiment:
'The results from GOCE will revolutionise understanding of oceanographic currents and temperatures, important in understanding climate change; the results will also improve understanding of plate tectonics.
'When funding for Culham's electric propulsion work stopped in 1976, we were world leaders, with the best diagnostics, the best scientific understanding, and Culham's Special Techniques Laboratory pioneered the development of the critical components – accelerating grids, insulators and cathodes. Our contract research involved interacting with five space technology companies. Work resumed at Culham for a period in the late 1980s but funding again ceased a few years later, which was disappointing. But the great and lasting satisfaction comes from seeing our work actually put to good use.'
Images courtesy of ESA. For more information on GOCE, see:
http://www.esa.int/SPECIALS/GOCE/SEMY0FOZVAG_0.html
