Sleek Probe To Map Earth’s Gravity

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the first of three Earth Explorer core missions in ESA’s Earth Observation Envelope Programme. Data from GOCE will allow scientists to create a detailed model of the Earth's gravity field, or geoid.

“GOCE will yield details of the Earth's gravity field to an accuracy and resolution that is simply unobtainable by existing terrestrial and space techniques,” says Professor Philip Moore from Newcastle University, who specialises in gravity research.

By comparing a model of the geoid with ocean surface height from other satellite data, oceanographers can track the speed and direction of ocean currents around the globe.

At school, most of us learn that acceleration due to gravity is 9.8 metres per second squared wherever you are on the planet. It turns out it’s not quite as simple as this. The shape of the Earth, mountains, trenches deep beneath the ocean and the ground beneath our feet all affect the gravity field, meaning it’s not identical everywhere.

“Ocean circulation is important in climate forecast models. Currents carry large quantities of heat from the equator to the poles, such as the system in the north Atlantic, which helps to keep Europe's climate relatively mild. Combined with more than 15 years of existing data on sea-surface height, the new GOCE geoid will help us more accurately measure the role of ocean currents in transporting heat and water around the globe,” says GOCE mission scientist, Mark Drinkwater, from ESA.

“Data from GOCE will provide information on ocean currents in remote regions such as the southern hemisphere for the first time,” says Keith Haines from the University of Reading, an expert in ocean circulation.

GOCE has other uses. An accurate geoid is crucial for defining exactly what height above sea-level actually means. Different countries have their own definitions of sea level, meaning that height is defined differently country to country. With GOCE, scientists will be able to say if two points are at the same height, however far apart they are. This will be important for large-scale surveying and engineering projects such as bridge, tunnel or pipeline building between islands or across seas.

The instrument that GOCE will use to measure gravity is called a gradiometer. It is made up of three pairs of accelerometers that measure tiny differences in gravity at many points as GOCE orbits the Earth. Because the strength of gravity decreases with altitude, the satellite will be in a much lower orbit than other orbiting spacecraft, cutting through the edge of the Earth’s atmosphere just 150 miles above the surface of the planet. This makes GOCE one of ESA’s most challenging missions to date.

The torpedo-shaped satellite will launch from the Plesetsk Cosmodrome in Russia into a polar sun-synchronous orbit – its solar panels will always face the sun. Unlike other spacecraft, the panels are in a fixed position on GOCE. This means that the whole satellite will have to turn to face the sun. To do this, GOCE has ion thrusters which act like a cruise control, making minute adjustments to its position to combat the effects of air drag at the edge of space. The sun’s energy will charge and accelerate xenon atoms into ions to power the thrusters.

Earth observation is crucial for monitoring changes in the environment. The Natural Environment Research Council makes a large investment in scientific Earth observation programmes, investing around £45m in ESA annually. Most of this is used to support Earth Explorer missions under ESA’s Earth Observation Envelope Programme. The Earth Explorer missions are designed to answer important scientific questions as well as demonstrate breakthrough technology in Earth observation.

The GOCE mission - costing €340m - involved a large collaboration of European organisations, including UK scientists and engineers working for QinetiQ, Logica and SciSys.

For more information, visit: http://www.esa.int/goce