Gravity-measuring satellite at mission end | Science| In-depth reporting on science and technology | DW | 30.10.2013
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Gravity-measuring satellite at mission end

The mission of the Gravity field and steady-state Ocean Circulation Explorer satellite, or GOCE, has come to an end. Data from it will be analyzed for years to come as other satellites continue examining Earth.

GOCE satellite

The "Ferrari of space"

The European Space Agency (ESA) announced an official end to the GOCE mission after the satellite ran out of fuel on October 21. Designed to measure variations in the Earth's gravitational field, the satellite is expected to reenter the Earth's atmosphere in the first week of November.

Launched in March 2009, the satellite was originally designed to serve for 20 months - but low solar activity allowed it to continue operating for far longer.

"We have squeezed about four-and-a-half years out of it," said Rune Floberghagen, the mission manager for GOCE. "The mission has delivered more than it promised - we have a lot of excellent applications of data and scientific results," Floberghagen told DW.

Our irregular Earth

GOCE's single purpose was to measure variations in the planet's gravitational field, and it did so in unprecedented resolution.

Since the Earth is not a perfect sphere, the Earth's gravitational field is not constant, instead rather mirroring the distribution of mass across the surface. The Indian Ocean, for example, is a kind of pit, due to mass displaced from building up the Himalayas.

Using data from GOCE, exactly these variations have been mapped out in one of the mission's crowning achievement to date, the geoid - or the shape the globe would be if all oceans were completely at rest.

Geoid (Credit: European Space Agency (ESA)/DLR)

Geoid: blue areas indicate lower gravitation, red higher - so you'd weigh more on the Indian Ocean than in South Africa

The geoid is useful for measuring ocean circulation and sea-level change, as well as in planning across nations for large-scale infrastructure projects like pipelines. The data can even be analyzed to find out more about the Earth's interior, including seismic activity: it was able to detect the earthquake that hit Japan in March 2011, for example.

GOCE is one of seven satellites in the ESA's "Earth Explorers" mission, which includes the CryoSat and SMOS satellites that are currently observing the Earth's icy regions, and soil moisture and salt content of its oceans, respectively. The Swarm mission will soon examine the Earth's magnetic field.

'Ferrari of space'

The GOCE satellite was initially set in orbit at 255 kilometers (158 miles) above the Earth - the lowest orbit of any research satellite to date. In August of 2012, the orbit was further lowered, to 224 kilometers.

The lower orbit made it possible to achieve even greater resolution in measurements of the Earth's gravity, Floberghagen explained. And that itself was made possible by a "drag-free altitude control system" unique to GOCE.

The agency had dubbed it the "Ferrari of space" due to its sleek and aerodynamic design. "Like any racecar, it's designed to cut through the air without any resistance," Floberghagen said.

Reentry being monitored

The ESA estimates that the satellite will reenter the Earth's atmosphere at some point during the first week of November. "My personal guess is around Friday (08.11.2013)," Floberghagen said.

Metal object discovered on a farm in Cheepie, southwestern Queensland in November 2007. The ball of twisted metal is believed to be space junk, possibly from a rocket used to launch satellites into space. (Photo: EPA/James Striton)

This metal mass found on a farm in Australia was likely once part of a rocket

The craft, though out of fuel, still has an intact power supply and electrical system, and is still able to communicate with mission control. It's not yet known where the craft might reenter.

An inter-governmental body, the Inter-Agency Space Debris Coordination Committee, is jointly monitoring the satellite, and the ESA space debris office will issue an alert when reentry is imminent.

"The Americans are looking at it, the Russians are doing the same," Floberghagen said, adding that the Fraunhofer Institute in Germany is also monitoring the craft's location.

Most of the satellite will burn up upon reentry. About 100 tons of space debris return to Earth every year, but the risk this presents to humans is minimal: In more than 50 years of space flight, there hasn't been a single report of such injury. "Your chances of getting hit by a meteorite are higher," Floberghagen said.

Data collected by GOCE presents a valuable legacy for scientists, Floberghagen said, adding that there are many plans for it, and that the best is yet to come.

"We still have a few aces up our sleeve," Floberghagen concluded.

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