Mission Extension Likely for ESA’s Gravity-measuring GOCE Satellite

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MUNICH — European Earth observation managers said the GOCE satellite measuring the Earth’s gravity field has returned such exceptional data in the two years since its launch that its mission will almost certainly be extended through 2014, if not longer.

GOCE, or the Gravity Field and steady-state Ocean Circulation Explorer, is the winged, arrowhead-shaped satellite launched in March 2009 packed with key never-flown systems, and operating in an orbit — about 255 kilometers — that no science satellite had ever used before.

Key to its success, among other novel elements, was a suite of six accelerometers developed by the French aerospace research institute, Onera, and a drag-free control system designed using ion-electric propulsion to compensate for the drag produced by the air molecules present at such a low operating altitude.

After a tense period when GOCE’s mission seemed compromised by a computer glitch, the satellite has been operating as designed and, by chance, has taken advantage of an exceptionally long period of low solar activity.

During high-activity periods of the sun’s usual 11-year cycle, the Earth’s atmosphere expands. That would have thickened the atmosphere in GOCE’s ideal orbit, making it difficult to fly drag-free.

Addressing a GOCE results conference here March 31, Reiner Rummel of the Technical University of Munich, who is considered a prime force behind GOCE’s development, said the satellite’s full mission will be completed in 2012, even if solar activity picks up substantially, as it is expected to do.

Rummel and other GOCE managers presented fresh images from GOCE of the Earth’s geoid — a theoretical shape the Earth would assume if it were covered by an ocean at rest, meaning no currents or tidal activity. The result, as European Space Agency (ESA) Earth Observation Director Volker Liebig put it, resembles a misshapen potato.

Officials said they hope GOCE’s data ultimately will become a gravity and sea-height reference worldwide. They noted that there are dozens of altitude measurements used today, a fact that caused some embarrassment when French and British engineers, each digging toward each other to create the Channel Tunnel, discovered that their points of reference resulted in a difference of 50 centimeters in height.

Rune Floberghagen, GOCE’s mission manager at ESA, said the low solar activity has permitted the satellite to economize its original 41 kilograms of xenon fuel. After two years in orbit, 28.4 kilograms of fuel remain, assuring GOCE operations for at least two years, and probably more than three years even if increased solar activity forces it into an orbit 10 kilometers higher than where it is now.

Floberghagen said GOCE’s expected data volume, even if the satellite were to be retired at the end of 2012, will be three times the expected amount. He said operating GOCE beyond 2012 would cost about 7.5 million euros ($10.5 million) per year, including data handling.