GOCE- Another Gravity Probe

In my earlier post we talked about NASA's Gravity Probe B, not to be left behind European Space Agency (ESA) is getting ready to launch their gravity probe-- The Gravity field and steady-state Ocean Circulation Explorer (GOCE) scheduled for launch in spring 2008.GOCE is dedicated to measuring the Earth’s gravity field and modelling the geoid with extremely high accuracy and spatial resolution.GOCE data will also be used for probing hazardous volcanic regions and bringing new insight into ocean behaviour. By combining the gravity data with information about sea-surface height gathered by other spacecraft, scientists will be able to track the direction and speed of ocean currents.

What will it do?
Most of us know that the acceleration due to gravity on Earth's surface is 9.8m per second squared - but, in truth, this figure varies around the planet depending on the nature of the material underneath the earth's surface.The figure varies from 9.78 (minimum) at the equator to 9.83 (maximum) at the poles.The planet is far from a smooth sphere; the radius of the globe at the equator is about 20km longer than at the poles. This ellipsoid is then marked by tall mountain ranges and cut by deep ocean trenches.
The Earth's interior layers are also not composed of perfect shells of homogenous rock - some regions are thicker or denser.
Such factors will cause the gravitational force at the surface to deviate from place to place by very small but significant amounts. GOCE will map these differences. This information will then be used to fashion what is, in essence, an idealised globe. Scientists call it the geoid.

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 describes where gravity is equal; 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

It is a critical reference; it defines the horizontal, tracing a surface on which the pull of gravity is everywhere equal. Put a ball on this hypothetical surface and it will not roll.
The geoid is of paramount interest to oceanographers who study the causes of the "hills" and "valleys" on the sea surface.

To make its gravity map, GOCE will use a gradiometer. This unique instrument consists of three pairs of accelerometers that will sense the tiny variations in the tug of gravity over different parts of the Earth.
The instrument's performance is phenomenal: it will register accelerations that are less than one millionth of a millionth of the g-force we experience when standing on the Earth.
But to make the most of this sensitivity, GOCE has to fly so low it will flirt with the top of the atmosphere; and that has proved to be a headache for the engineers because any buffeting on the spacecraft from air molecules will introduce noise into the data.

The Aircraft:

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

Acknowledgements: Jonathan Amos, Science reporter, BBC News