ESA gravity mission slips

to 2009
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 24, 2008

The launch of Europe’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) has slipped to February 2009 due to ongoing technical faults with its launcher.

GOCE has been developed to help understand one of the most fundamental forces in the Universe: gravity. The most advanced gravity space mission to date, GOCE will open new possibilities in a range of fields from oceanography to solid Earth physics and climate change.

GOCE will orbit the Earth at an altitude of just 250km to map the gravity field of our home planet in greater detail than ever before. Image: ESA - AOES Medialab.

Often assumed to be constant across the surface of the Earth at 9.81 metres per second squared, the value of gravity in fact varies subtly from place to place according to the location of mountains or deep sea trenches, the density of the Earth’s interior, and the Earth’s rotation. GOCE will map these variations throughout its 20 month mission in order to determine the geoid – the surface of equal gravitational potential defined by the gravity field – and crucial for mapping ocean circulation and sea level change, to an accuracy of just 1-2 centimetres. As well as scientific benefits, the mission will also have practical applications for mapping and surveying, providing a universal reference to measure height anywhere on the Earth.

The five metre long satellite will orbit Earth at an altitude of 250 kilometres and 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 the Earth as it travels along its orbital path. Because of their different position in the gravitational field they all experience the gravitational acceleration of the Earth slightly differently. The three axes of the gradiometer allow the simultaneous measurement of six independent but complementary components of the gravity field.

GOCE's instruments will allow six different but complementary components of the gravity field to be measured as the satellite moves over the Earth's surface. Image: ESA-AOES Medialab.

Since it is vital to ensure that the measurements taken are of true gravity and not influenced by any movement of the satellite, the entire spacecraft is actually one extremely sensitive measuring device, and has none of the moving parts often seen in other spacecraft. Indeed, the gradiometer will be sensitive to accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth.

A technical hitch was uncovered during launch preparations in September by the Russian authorities responsible for the launch, forcing a delay in the mission from the proposed launch date of 27 October to February 2009 at the earliest.

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