| By Jonathan Amos
Science reporter, BBC News |   Goce has fins to keep it stable as it flies through whisps of air | Europe's gravity mission has been bumped to next year because of ongoing technical problems with its launcher. The arrow-shaped Goce satellite will map tiny variations in the pull of gravity experienced across the world. The information will give scientists a clearer insight into how the oceans move, and provide a universal reference to measure height anywhere on Earth. But concerns about the reliability of its Russian rocket mean a lift-off is now unlikely before February. It is a frustrating delay for the Gravity Field and Steady-State Ocean Circulation Explorer (Goce). The satellite was already two years behind schedule when it was sent to the launch pad because engineers had to work through immense technical difficulties in building it. The super-sleek spacecraft was due to go into orbit on a modified intercontinental ballistic missile, known at the Rockot, from the Plesetsk Cosmodrome in north-west Russia in the spring. It was held on the ground while an investigation was undertaken into the performance of a different, failed rocket system that shared key components. Once that was cleared, Goce's own launcher then became the focus of separate investigations that have now resulted in a further, extended delay. The problem is centred on the guidance and navigation units of the Breeze upper stage that has been added to the ICBM to inject satellite payloads into their correct orbits. Goce is a remarkable mission by any standards. It will sense the very subtle gravity anomalies that exist across the planet. There is a misconception that Earth's pull is the same everywhere. In reality, it differs ever so slightly from place to place. One of the reasons is that our planet 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, that tug becomes irregular, too. The variations, though, are tiny - almost imperceptible. Goce will carry a special device known as a gradiometer to measure these anomalies. The gradiometer incorporates precision-built accelerometers that sense accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth. But for optimal performance, Goce needs to fly low - below 300km. This is difficult because the residual atmosphere at that altitude can jostle the spacecraft and introduce "noise" into the data. Just how much air is present at the flight altitude is linked indirectly to activity on the Sun, which heats the upper reaches of the atmosphere, altering its density on an 11-year-cycle. If, as expected, the Sun gets more active in the coming months and years, Goce may have to fly higher than was anticipated, limiting the resolution of its data. | 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. This 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 |
| 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 | | 
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