European Space Agency’s (ESA) GOCE Satellite Mission to Map Earth’s Gravity Draws to a Close

After mapping variations in Earth’s gravity with unprecedented detail for four years, the GOCE satellite has run out of fuel and the end of mission has been declared.

Since March 2009, the Gravity field and steady-state Ocean Circulation Explorer – GOCE – has been orbiting Earth at the lowest altitude of any research satellite. Its ‘gradiometer’ – the sensitive instrument measuring gravity in 3D – was the first in space and has mapped variations in Earth’s gravity with unrivalled precision. The result is a unique model of the ‘geoid’, which is essentially the shape of an ideal global ocean at rest and therefore critical for accurate measurements of ocean circulation and sea-level change.

The initial map, which is based on two months of data, illustrates the varying strength of gravity. Blue indicates weak areas, green is average and orange is strong. The deviations occur because the Earth isn’t entirely spherical, which means that the gravity isn’t equal across the planet.

GOCE has provided dynamic topography and circulation patterns of the oceans with unprecedented quality and resolution, improving our understanding of the dynamics of world oceans. Scientists further exploited GOCE’s data to create the first global high-resolution map of the boundary between Earth’s crust and mantle – called the Moho.

The satellite also became the first seismometer in orbit when it detected sound waves from the massive earthquake that hit Japan on 11 March 2011.

Although the planned mission was completed in April 2011, the fuel consumption was much lower than anticipated because of the low solar activity, enabling ESA to extend GOCE’s life. Last week, on 21 October, the mission came to a natural end when it ran out of fuel. The satellite is expected to re-enter Earth’s atmosphere in about two weeks.

Facts and Figures – GOCE

 

 

Full name: Gravity field and steady-state Ocean Circulation Explorer

Launched: 17 March 2009

Launcher: Rockot (with Breeze-KM upper stage) by Eurockot Launch Services GmbH

Launch site: Plesetsk Cosmodrome, Russia

Mission control: European Space Operations Centre (ESOC), Darmstadt, Germany

Data download: to Kiruna (Sweden) and Svalbard (Norway) ground stations

Processing & archiving: ESA’s Earth observation centre in Italy (ESRIN) and the distributed High-level Processing Facilities

Number of instruments: 3

Nominal life: 20 months

Mission cost: €350 million (including launcher and operations)

Orbit: about 260 km altitude, polar, Sun-synchronous

Mass: 1100 kg

Size: 5.3 m long, about 1 m body diameter

Propulsion tank: 40 kg of xenon

Geoid accuracy: 1 – 2 cm vertically with 100 km spatial resolution

Gravitational acceleration at Earth’s surface is about 9.8 m/s², varying from a minimum of 9.788 m/s² at the equator to a maximum of 9.838 m/s² at the poles. The six accelerometers (three pairs in three orthogonal directions) are some 100 times more sensitive than any previously flown in space. 

GOCE Data

GOCE will gather data to map the Earth’s gravity field for around 20 months. In order to turn the raw data into products that are suitable for users worldwide, there are a number of steps that have to be taken. This part of the mission involves the so-called ground segment, which is essentially made up of three main components – mission control, data processing centres & long-term data storage and services to provide data to users.

GOCE is controlled and operated by the Flight Operations Segment at ESA’s European Space Operations Centre (ESOC), in Darmstadt, Germany. From here, the health and status of GOCE is monitored and commands are generated and sent to the satellite.