Science reporter, BBC News
Neil Wallace reveals the inner workings of Goce's ion engine
Europe is set to launch one of its most challenging space missions to date.
The Goce satellite will map minute variations in the pull of gravity 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.
 |   This is the most beautiful satellite that has ever been built  Reiner Rummel, Technical University of Munich |
Most satellites launched into space are ugly boxes. The European Space Agency's (Esa) Goce satellite is very different.
"This is the most beautiful satellite that has ever been built - and for good reason," enthused one of the scientists who conceived the mission, Reiner Rummel, from the Technical University of Munich, Germany.
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.
"Our current knowledge of the Earth's gravity is incomplete," explained Danilo Muzi, Esa's Goce programme manager.
"Gravity is the force we experience daily; it keeps our feet on the ground. But there is this general misconception that it is constant everywhere on the globe, which is not true. If we go to the North Pole we will weigh more than if we are at the equator."
|  Goce data will be used to construct an idealised surface called a geoid  |
More detailsThis 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 minuscule - almost imperceptible.
Meeting the measurement challenge in itself resulted in two years' delay for the Gravity Field and Steady-State Ocean Circulation Explorer (Goce). Engineers have had to work through immense technical difficulties.
At the heart of the spacecraft is a device known as a gradiometer.
"This is a very complex instrument," said Andrea Allasio, who led the production of the satellite at Thales Alenia Space in Italy. "It is, for sure, the most sophisticated gradiometer which has ever been prepared for a satellite."
It consists 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.
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As Goce bumps through the Earth's gravity field, the accelerometers will sense the fantastically small disturbances.
"We have one comparison that we often make," explained Rune Floberghagen, Esa's Goce mission manager.
"Imagine a snowflake, which has a fraction of a gram, slowly falling down on to the deck of a supertanker. The acceleration that the supertanker experiences from that snowflake is comparable to the sensitivity of our instrument," he told BBC News.
There is however a potential showstopper: the low altitude Goce must fly to get the detail it seeks in the gravity signal. The constant buffeting the satellite receives from the residual air still present in the thermosphere would ordinarily drown out the data.
So 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."
| GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER  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
a better knowledge of the way mass is distributed inside the Earth will be useful to those who study geo-hazards such as volcanoes and earthquakes
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 over
Goce is the first of Esa's Earth Explorers, a series of spacecraft that will provide quick answers to key environmental questions.
Six missions have so far been approved; a seventh is in discussion. All will use cutting-edge space technology to acquire their data.
|  Cryosat has been re-built and will launch later this year  |
The Goce mission has experienced a series of frustrating delays. It was sent to Plesetsk in August last year and should have orbited in September, but the satellite was then held on the ground because of niggling concerns about the readiness of its launcher system.
The ghost that haunts this mission is the Cryosat satellite. The Esa spacecraft built to map the world's ice fields was supposed to be first Earth Explorer but it was destroyed on launch in 2005 when its Rockot failed and ditched in the Arctic Ocean.
"From the information we have seen from Eurockot (operator) and Khrunichev (manufacturer), we have seen they have done extensive testing," said Danilo Muzi.
"On the basis of all the testing that has been done, and the fact that these tests were successful, then the confidence in the good status of the launcher has been restored," he told BBC News.
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.
| GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER  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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