A satellite that can measure tiny variations in the Earth's gravity field will be one of Europe's most challenging space missions to date. Goce, due for launch next year, looks like a spyplane from a movie.

Its arrow shape, fins, and electric engine help keep the satellite stable as it flies through the wisps of air still present at an altitude of 260km. Goce data will have many uses, probing hazardous volcanic regions and bringing new insight into ocean behavior. The latter, in particular, is a major driver for the mission. 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. ""If we want to improve our climate models then we need to improve our knowledge of how the oceans move, and Goce will help us do that,"" mission scientist Dr Mark Drinkwater, from the European Space Agency (Esa), told BBC News. Most people are taught at school that the acceleration due to gravity at the 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 underfoot. We're smearing out the interpretation of the currents 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. The Gravity Field and Steady-State Ocean Circulation Explorer (Goce) will map these differences. This information will then be used to fashion what is, in essence, an idealized globe. The 'standard' acceleration due to gravity at the Earth's surface is 9.8m per second squared In reality the figure varies from 9.78 (minimum) at the equator to 9.83 (maximum) at the poles 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. If local gravity differences are not creating these features, then other factors such as currents, winds and tides must be responsible. With the help of the Goce geoid, scientists will be able to tease out these details with a precision and at a resolution not obtainable with current satellite technology. ""At the moment we can see structures down to the size of 150-200km,"" explained Dr Jakob Flury, formerly of the Technical University Munich, Germany, and now with the University of Texas at Austin, U.S. ""That's nice but the oceanographers want more detail. Goce will have spatial resolution which is finer, down to 80-100km. There are 'fronts' and currents in the oceans that are at width scales of 100km."" This will help scientists to characterize boundary currents, such as the Gulf Stream, which flow along the edges of deeply sloping continental shelves. This is cruise control for a spacecraft, but at an unbelievable level of precision. (European Space Agency