'Formula One' of spacecraft ready for launch
14 March 2009
Dubbed the 'Formula One' of spacecraft, a new satellite to map variations in the Earth's gravity field with unprecedented accuracy is due to launch at 14:21 GMT, Monday 16 March 2009.
The satellite's sleek design will help it skim the top of the atmosphere.
The sleek-looking satellite will give UK scientists vital information about ocean circulation and sea level change needed to improve climate forecast models.
The Gravity field and steady-state Ocean Circulation Explorer (GOCE) will be the first of European Space Agency's (ESA) Earth Explorer missions to be launched. Data from GOCE will allow scientists to create a detailed model of the Earth's gravity field and provide a universal way of determining height.
'GOCE will yield details of the Earth's gravity field to an accuracy and resolution that is simply unobtainable by existing terrestrial and space techniques,' says Professor Philip Moore of Newcastle University, who specialises in gravity research.
The new satellite will map the Earth's geoid (pictured). This is the shape of the Earth if sea level was distributed more evenly over the continents and allowed to rest undisturbed. A ball placed on any point on this hypothetical surface will not roll, even on what look like slopes.
An accurate model of the Earth's gravity field is crucial for defining exactly what height above sea-level actually means. With GOCE, scientists will be able to say if two points are at the same height, however far apart they are.
At school, most of us learn that acceleration due to gravity is 9.8 metres per second squared wherever you are on the planet. It turns out it's not quite as simple as this. The shape of the Earth, mountains, trenches deep beneath the ocean and the ground beneath our feet all affect the gravity field, meaning it's not identical everywhere.
The aerodynamic satellite, designed to skip over the top of the atmosphere in a very low Earth orbit, will also map ocean currents. By comparing a model of the Earth's gravity field with ocean surface height from other satellite data, oceanographers can track the speed and direction of currents around the globe.
'Ocean circulation is important in climate forecast models,' says ESA's GOCE mission scientist, Mark Drinkwater. 'Currents carry large quantities of heat from the equator to the poles.'
Combined with more than 15 years of existing data on sea-surface height, the data that GOCE delivers will help scientists more accurately measure the role of ocean currents in transporting heat and water around the globe.
Professor Keith Haines, an expert in ocean circulation at the National Centre for Earth Observation (NCEO) says, 'Researchers at NCEO are keen to start using the data that GOCE delivers. We've been working with ESA to make sure we can use the data in the UK's world-leading ocean and climate prediction models.'
'The Met Office and the European Centre for Medium-Range Weather Forecasts have been working with us on this as well. One of the things I'm excited about is finding out about ocean currents in remote regions such as the southern hemisphere for the first time,' added Haines.
Universal height measurements
GOCE has other uses. An accurate model of the Earth's gravity field is crucial for defining exactly what height above sea-level actually means. Different countries have their own definitions of sea level, meaning that height is defined differently country to country. And of course, sea level is rising.
With GOCE, scientists will be able to say if two points are at the same height, however far apart they are. This will be important for large-scale surveying and engineering projects such as bridge, tunnel or pipeline building between islands or across seas.
The instrument that GOCE will use to measure gravity is called a gradiometer. It is made up of three pairs of accelerometers that measure tiny differences in gravity at many points as GOCE orbits the Earth. Because the strength of gravity decreases with altitude, the satellite will be in a much lower orbit than other orbiting spacecraft. This makes GOCE one of ESA's most challenging missions to date.
The torpedo-shaped satellite is designed to cut through the edge of the Earth's atmosphere at just 150 miles above the surface of the planet.
GOCE will be launched from the Plesetsk Cosmodrome in Russia into a polar sun-synchronous orbit - its solar panels will always face the sun. It will use the sun's energy to charge and accelerate 40g of xenon particles, which the satellite's ion thrusters will use to combat the effects of air drag. The mission will last around 20 months.
The GOCE mission - costing €350m - involves a large collaboration of European organisations, including UK scientists and engineers working for QinetiQ, Logica and SciSys. The UK's Natural Environment Research Council invests around £45m in ESA annually. Most of this is used to support Earth Explorer missions under ESA's Earth Observation Envelope Programme.
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