Launched on 17 March 2009, ESA's Gravity field and steady-state Ocean Circulation Explorer (GOCE) is bringing about a whole new level of understanding of one of Earth's most fundamental forces of nature – the gravity field.

Dubbed the 'Formula 1' of satellites, this sleek high-tech gravity satellite embodies many firsts in its design and use of new technology in space to map Earth's gravity field in unprecedented detail.

As the most advanced gravity space mission to date, GOCE is realising a broad range of fascinating new possibilities for oceanography, solid Earth physics, geodesy and sea-level research, and significantly contribute to furthering our understanding of climate change.

Although invisible, gravity is a complex force of nature that has an immeasurable impact on our everyday lives. It is often assumed that the force of gravity on the surface of Earth has a constant value, but in fact the value of 'g' varies subtly from place to place. These variations are due to a number of factors such as the rotation of Earth, the position of mountains and ocean trenches and variations in density of Earth's interior.

GOCE is mapping these variations in the gravity field with extreme detail and accuracy. This will result in a unique model of the 'geoid', which is the surface of equal gravitational potential defined by the gravity field – crucial for deriving accurate measurements of ocean circulation and sea-level change, both of which are affected by climate change.

GOCE-derived data are also much needed to understand more about processes occurring inside Earth and for use in practical applications such as surveying and levelling.

Since the gravitational signal is stronger closer to Earth, the 'arrow-like', five metre-long GOCE satellite was designed to cut through what remains of Earth's atmosphere at just 250 km above the surface.

To help avoid drag and ensure that the gravity measurements are of true gravity, the satellite has to be kept stable in ‘free fall’. Any buffeting from residual air at this low altitude could potentially drown out the gravity data. To help achieve this, the satellite carries an electric ion thruster system that continuously generates tiny forces to compensate for any drag the satellite experiences at this low altitude.

This low-orbiting satellite is the first mission to employ the concept of 'gradiometry' – the measurement of acceleration differences over short distances between an ensemble of proof masses inside the satellite.

GOCE is equipped with three pairs of ultra-sensitive accelerometers arranged in three dimensions that respond to tiny variations in the 'gravitational tug' of Earth as it travels along its orbital path. Because of their different positions in the gravitational field they all experience gravitational acceleration slightly differently. The three axes of the gradiometer allow the simultaneous measurement of six independent but complementary components of the gravity field.

In order to measure gravity, there can be no interference from moving parts so the entire satellite is actually one extremely sensitive measuring device.

Mission objectives

• to determine gravity-field anomalies with an accuracy of
  1 mGal (where 1 mGal = 10^–5 ms^–2).
• to determine the geoid with an accuracy of 1-2 cm.
• to achieve the above at a spatial resolution better than
  100 km.