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Gravity Satellite Blasts Off on Climate Mission

The spacecraft will measure the earth’s gravitational field with new accuracy.

Today the European Space Agency (ESA) launched one of the most advanced Earth-observing satellites ever built. The GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite blasted off from the Plesetsk Cosmodrome in Northern Russia aboard a modified intercontinental ballistic missile.

Liftoff: ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) will measure the earth’s gravity field. The satellite underwent final checks in a clean room (top) before being mounted to a rocket. It was launched today from the Plesetsk Cosmodrome in Northern Russia. An artist’s illustration (bottom) shows the satellite in orbit. It’s five meters in length and one meter wide, with four body-mounted and two wing-mounted solar panels.

GOCE will orbit the earth at an altitude of around 260 kilometers for 24 months, circling from pole to pole as the planet turns beneath it. From August, the satellite will measure the earth’s gravitational field with better accuracy and higher resolution than any previous mission.

Measuring the earth’s gravitational field will provide scientists with new insights into the composition of the planet, the movements of its oceans, and the thickness and movement of polar ice sheets. Crucially, it will also help scientists build computer models to predict the impact of climate change more accurately.

Monitoring ice coverage, sea-level changes, and ocean circulation will be a particularly important part of the mission. “One of the major impacts of climate change is loss of ice mass and increase in sea-level rise,” says Prasad Gogineni, the director of the Center for Remote Sensing of Ice Sheets (CReSIS) at the University of Kansas. Data from the new satellite will be coupled with sea-level models to better predict regional and global sea-level rises, adds Claude Laird, a research associate at CReSIS.

Danilo Muzi, project manager of GOCE, says that the spacecraft’s sensors are 100 times more sensitive than anything flown up until now. The spacecraft will measure the earth’s gravitational field using a gradiometer. This type of instrument is already used for ground-based measurements and on airplanes, but it has never before been placed on a satellite. The gradiometer consists of six ultrasensitive accelerometers–sensors that measure acceleration and were specifically built for the mission. These accelerometers are arranged in three pairs and are aligned along three different axes of the gradiometer. By measuring subtle differences in the gravitation pull felt by each pair of accelerometers, the satellite will produce a better map of gravity measurements for the entire globe.

GOCE’s instruments will also let scientists measure the earth’s gravity field down to very small spatial scales. “The smaller detail you have, the more you can learn about the earth underneath the surface,” says John Wahr, a professor of physics at the University of Colorado. The satellite will provide a resolution better than 100 kilometers. “To get the whole earth–including the oceans–at the small scales and the amazing resolution that GOCE is going to provide is just remarkable and extremely useful,” Wahr says.

NASA currently has a pair of satellites that measure the earth’s gravity field in orbit. These twin spacecraft, known as GRACE, were launched in 2002 and orbit 500 kilometers above the earth’s surface. They have already provided valuable information about how Antarctic and Greenland ice-sheet mass is changing, but their spatial resolution is not as good as GOCE, says Gogineni. “The [new] satellite will provide an improved data set that will allow us to do a much better job of measuring the ice-sheet mass, particularly in Greenland,” he says.

Smooth sailing: GOCE will use a gradiometer featuring sensitive accelerometers to measure the earth’s gravity gradient. It will also use an ion engine on its tail end to compensate for the deceleration caused by the atmosphere in low-earth orbit.

In order to make the measurement as accurate as possible, the researchers had to compensate for the air drag created by the atmosphere at low-earth orbit. This drag creates a tiny deceleration of the satellite, which would be sensed by the accelerometers as acceleration. Therefore, the researchers added an ion engine to the tail of the satellite that will emit ions at a rate that perfectly matches this deceleration.

GOCE is part of a larger ESA mission called the Living Planet Program, which will involve launching seven more satellites over the next two years, each designed to measure a different feature of the planet. For example, this summer, ESA plans to launch a satellite called SMOS to measure the earth’s moisture and ocean salinity. Another satellite, called CrySat-2, will blast off at the end of the year to map ice coverage. In the past, both ESA and NASA have focused on launching larger satellites carrying many instruments. In 2002, ESA launched Envisat, a 10-instrument satellite, and NASA has an ongoing program called Landsat, which started in 1972 and is considered the gold standard for earth-science missions.

Wahr, who worked on the GRACE mission, says that the new mission is very exciting. “For those of us in the business, it is going to be wonderful,” he says.

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