Page last updated at 23:07 GMT, Monday, 6 April 2009 00:07 UK

Gravity satellite feels the force

By Jonathan Amos
Science reporter, BBC News

Artist's impression of Goce in orbit (Esa)

Europe's innovative Goce satellite has switched on the super-sensitive instrument that will make ultra-fine measurements of Earth's gravity.

The sophisticated gradiometer will feel the subtle variations in Earth's tug as it sweeps around the globe.

The spacecraft has also fired up the British-built engine that will help maintain its orbit.

Goce needs tiny but continuous levels of thrust to keep it stable and prevent it from falling out of the sky.

European Space Agency (Esa) mission manager Rune Floberghagen said all systems on the spacecraft had now been activated following the launch from Russia last month.

Esoc control room (Esa)
Goce is being overseen from Esa's "mission control" in Germany

"The big news today is that the gradiometer is fully working; all its accelerometers have survived the launch and they are producing meaningful data," he told BBC News.

"Now we must learn to drive our super-satellite."

To acquire its data, Goce carries a set of six state-of-the-art high-sensitivity accelerometers. These have been arranged in pairs and sit across the three axes of the spacecraft.

As Goce "bumps" through Earth's gravity field, the accelerometers will sense fantastically small disturbances - as small as one part in 10,000,000,000,000 of the gravity experienced at the Earth's surface.

This exquisite measurement capability meant some very fragile mechanisms had to be built into the gradiometer, and developing these delicate technologies so they could also survive the intense shaking experienced at launch proved to be one of the major design challenges of the mission.

Monday's switch-on will be seen as vindication of the extraordinary engineering work on the gradiometer and its accelerometers, led by the Thales Alenia Space and Onera companies (France).

Volcano (Esa)
Gravity data can tell scientists about the nature of the Earth's interior

"This was a pivotal moment in the mission, for sure," said Dr Floberghagen.

"What's very important in this first phase is that we see some consistency between the measurements from the six sensors onboard; and we do see that, which is all very exciting. But still, we need to characterise each one of these very precise sensors, and that process is not over."

The other major milestone in the commissioning of Goce has been the successful start-up of its electric propulsion system.

Built by UK technology firm Qinetiq, the T5 ion engine is a critical part of the mission. The satellite flies so low in order to get a good gravity signal that it actually brushes through the top of the atmosphere.

Without the constant force applied from the T5 unit, the drag on Goce would rapidly pull it out of orbit. But the engine's presence is also integral to the acquisition of the gravity data itself.

The buffeting from air molecules would ordinarily upset Goce's gradiometer instrument, so the British engine is designed to throttle up and down to counteract this disturbance and leave a clean signal.

ELECTRIC PROPULSION ON GOCE
Artist's impression of Goce in orbit (Esa)
Satellite carries two engines; one is back-up in case of failure
T5 unit draws power from solar panels on side of spacecraft
Electrons are stripped off xenon atoms to give them charge
An electric field then hurls the xenon ions through rear nozzle
Xenon exits at speeds in excess of 40,000m/s to provide thrust
Amount of thrust is moderated by gradiometer information

The levels and range of thrust needed, however, are tiny - a continuously variable force of anywhere between one and 20 millinewtons during the science phase of the mission.

This is similar to the force a postcard will exert when laid down on a surface.

Put another way, you would need to strap together 650 million Goce spacecraft to achieve the same amount of thrust as Europe's mighty Ariane rocket at launch.

Commissioning last week saw both T5 "chains" (there are two engines; one is a back-up) perform precisely to specification.

The levels of thrust delivered were shown to be within 10 micronewtons of what was being demanded at any one time. The drive is also very straight, with the spacecraft deviating offline by only 0.6 of a degree.

"You work on these things for so many years that you should be cold and confident that it will all work, but there's always a risk that it won't," Neil Wallace, who leads the Qinetiq electric propulsion team, told BBC News.

"One of the tests we did was to demand a thrust ramp, from one to 20 millinewtons, as quickly as possible. This is one of the most critical requirements and the most difficult to achieve, and both chains did it perfectly."

The T5 was then switched off to allow controllers to concentrate on the gradiometer's behaviour.

"The other reason was to let the spacecraft's orbit decay. All the time we are thrusting, Goce is going up. We did one orbit at 8.3mN and we went up by 150m."

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Neil Wallace reveals the inner workings of Goce's ion engine

Goce was placed initially by its Rockot launcher in an orbit some 283km above the Earth. Spacecraft operators are allowing it to fall by between 150m to 200m a day.

It is now just above 275km and will continue to drop to its target science altitude of 263km.

By then, the satellite will have been put in a "closed loop" mode whereby the gradiometer and the engine will be working in tandem to fly a stable path and gather the gravity data.

Scientists will use Goce to help them construct high-resolution maps of the geoid, which, simply put, is a specialised map that traces "the level" on Earth.

Geoid information has many applications but perhaps the biggest knowledge gains will come in the study of ocean behaviour.

Understanding better how gravity pulls water - and therefore heat - around the globe will improve computer models that try to forecast climate change.

GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER
GOCE (BBC)
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

Jonathan.Amos-INTERNET@bbc.co.uk



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