GLAMOUROUS GOCE SATELLITE IS ALL SET TO FLY

The European Space Agency's (Esa) Goce satellite
touted as the 'The most beautiful satellite that has ever been built', is intended to map minute variations in the gravitational pull experienced across the planet.Scientists will use its data to improve their understanding of how the oceans move, and to frame a universal system to measure height anywhere on Earth. The super-sleek spacecraft will go into orbit on a modified intercontinental ballistic missile from the Plesetsk Cosmodrome in north-west Russia.

Lift-off for the Rockot vehicle is timed for 1421 GMT on Monday. Goce's striking good-looks are a requirement of the extremely testing environment in which it will have to operate.The arrow shape and fins are necessary to keep the spacecraft stable as it flies through the wisps of air still present at an altitude just under 270km.This orbit is much lower than for most Earth observation missions but will be essential if Goce is to sense the very subtle gravity anomalies that exist across the planet.

Gravity,the force which makes us stand on the ground is generally misconceivedto be the same everywhere on earth,but the fact is if we go to the North Pole we will weigh more than if we are at the equator."This extraordinary phenomenon is explained in part by the shape of the planet. It is not a perfect sphere - it is flatter at the poles, fatter at the equator. Its interior layers are also not composed of uniform shells of homogenous rock - some regions are thicker or denser.This leads to an irregular distribution of mass; and as everything that has mass is pulled by gravity, its tug becomes irregular, too.The variations, though, are miniscule - almost imperceptible

At the heart of the spacecraft is a device known as a gradiometer consisting of three pairs of "proof masses", or accelerometers. They are aligned at 90 degrees, across each axis. The entire set-up is mounted inside an ultra-stable casing.As Goce bumps through the Earth's gravity field, the accelerometers will sense the fantastically small disturbances. Goce employs an ion engine to maintain a steady path - a sort of cruise control. The engine is throttled up and down, producing exquisite levels of thrust by accelerating charged atoms of xenon through nozzles at the rear of the spacecraft."We are an enabling technology on this mission; it couldn't happen without us," said Neil Wallace from Qinetiq, the UK technology firm which supplied the engine. "But then this mission has many such technologies."

1. The 1,100kg Goce is built from rigid materials and carries fixed solar wings. The gravity data must be clear of spacecraft 'noise'

2. Solar cells produce 1,300W and cover the Sun-facing side of Goce; the near side (as shown) radiates heat to keep it cool

3. The 5m-by-1m frame incorporates fins to stabilise the spacecraft as it flies through the residual air in the thermosphere

4. Goce's accelerometers measure accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth

5. The UK-built engine ejects xenon ions at velocities exceeding 40,000m/s; Goce's mission will end when the 40kg fuel tank empties

6. S Band antenna: Data downloads to the Kiruna (Sweden) ground station. Processing, archiving is done at Esa's centre in Frascati, Italy

7. GPS antennas: Precise positioning of Goce is required, but GPS data in itself can also provide some gravity field information.
Goce's quest is to produce a snapshot of the Earth's gravity field at an unprecedented resolution. The data will inform a multitude of science disciplines:Understanding how the mass of ocean waters circulate, moving heat around the planet, will assist climate prediction and because gravity defines what is meant by "up", "down" and "level", the new data can underpin a truly universal system to compare heights the world .

OverSix missions have so far been approved; a seventh is in discussion. All will use cutting-edge space technology to acquire their data. Goce will be put into a sun-synchronous orbit, meaning the spacecraft will be kept in daylight for a sustained period of time. The Breeze-KM upper-stage booster will release Goce at an altitude of about 285km.The satellite will then gradually fall to its operational altitude of 263km, where its ion engine will maintain a steady orbit for the science campaign.Two major data-gathering periods are planned, each lasting about six months. The first should start in early September after all the in-orbit testing is complete.The mission will probably be extended if sufficient xenon is left, although some propellant will be needed to take the spacecraft safely out of the sky in a controlled burn-up over ocean waters.

1. Goce senses tiny variations in the pull of gravity over Earth

2. The data is used to construct an idealised surface, or geoid

3. It traces gravity of equal 'potential'; balls won't roll on its 'slopes'

4. It is the shape the oceans would take without winds and currents

5. So, comparing sea level and geoid data reveals ocean behaviour

6. Gravity changes can betray magma movements under volcanoes

7. A precise geoid underpins a universal height system for the world

8. Gravity data can also reveal how much mass is lost by ice sheets