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GOCE gravity satellite has plenty of fuel to keep flying

Posted: April 3, 2011

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Europe's GOCE satellite is collecting exquisite data on Earth's uneven gravity field and has enough xenon fuel to maintain its low-altitude orbit for continued operations through 2013, according to the mission's project manager.

GOCE data produced this geoid, or model of Earth's gravity field. Credit: ESA/HPF/DLR
The European Space Agency released last week a sneak peek of Earth's gravity field in a lumpy spherical representation called a geoid, a three-dimensional map of the planet's surface and oceans if tides and currents did not exist.

The Gravity Field and Steady-State Ocean Explorer, or GOCE, launched in 2009 aboard a converted Russian ballistic missile. GOCE flies about 160 miles above Earth, an unusually low orbital altitude necessary to measure gravity with the necessary precision.

Because the spacecraft orbits just above the atmosphere, it must maintain its orbit with a low-thrust, high-efficiency ion engine fueled by xenon propellant. Without the steady boost from the ion system, GOCE could drop from orbit within a handful of weeks and burn up in the atmosphere.

Rune Floberghagen, GOCE's mission manager, said low solar activity so far in GOCE's time in space has limited the ion engine's fuel consumption. Instead of running out of fuel in 2012, GOCE should be able to remain in space until at least 2013, or even longer.

The sun was in an extended lull of activity, illustrated by fewer sunspots adn solar flares, when GOCE launched in 2009. More solar activity tends to expand Earth's atmosphere, creating more drag for satellites in low Earth orbit.

"We are quite confident that late 2013 is a conservative estimate of the satellite's lifetime," Floberghagen told Spaceflight Now.

The GOCE satellite, built by Thales Alenia Space and Astrium, features an aerodynamic shape to help counteract drag. Small winglets and a tail fin will help stabilize the spacecraft as it circles Earth. Engineers want to limit use of chemical thrusters because they could disrupt GOCE's gravity studies.

GOCE stretches more than 17 feet long and 3 feet wide. The satellite has a low cross-section to diminish the force of drag.

ESA officials tout GOCE's early results as the best view yet of Earth's gravity field, eclipsing data from other European and U.S. satellite missions.

Artist's concept of the GOCE satellite in orbit. Credit: ESA/AOES Medialab
GOCE's primary instrument, called a gradiometer, is mounted near the satellite's center of gravity to take advantage of a pristine environment free of disturbances that could spoil the sensor's measurements.

The gradiometer includes three pairs of identical accelerometers that can detect minuscule gravity changes as small as one-ten-trillionth of the gravity experienced on Earth, according to ESA.

The accelerometers are mounted on three arms facing in the satellite's direction of travel, perpendicular to the flight path, and toward the center of the Earth.

"We see a continuous stream of excellent GOCE gradiometry data coming in," said Reiner Rummel, former head of the institute for astronomical and physical geodesy at the Technical University of Munich. "With each new two-month cycle, our GOCE gravity field model is getting better and better."

The next step is to analyze GOCE's raw data as a reference for ocean circulation and sea level changes related to climate change.

Scientists use the geoid's reference surface to weigh against measurements of ocean activity. The comparisons allow scientists to more accurately study ocean circulation and sea level changes.

"Now the time has come to use GOCE data for science and applications," Rummel said. "I am particularly excited about the first oceanographic results. They show that GOCE will give us dynamic topography and circulation patterns of the oceans with unprecedented quality and resolution. I am confident that these results will help improve our understanding of the dynamics of world oceans."

GOCE was out of commission for two months last year due to a telemetry glitch on-board the spacecraft, but Floberghagen said the satellite is now in good health.