It will also be used to test ways of landing rockets on asteroids that are in danger of colliding with Earth, to shift them into safer orbits.
Scientists from the American National Aeronautics and Space Administration (Nasa) yesterday fired the rocket engines of a space probe flying beyond Mars to accelerate it towards its final target: Eros, an asteroid the size of London.
By the end of next month the Near spacecraft will have come alongside its quarry and begun the complex series of manoeuvres designed to bring it within a few miles of the surface. The Near probe, which cost pounds 138m, will for the next 12 months photograph, measure, monitor and map Eros from orbit to make it the most studied asteroid to date.
At the end of the year-long mission, in the first weeks of the new millennium, Nasa scientists hope to bring the probe to within a few feet of Eros, and even test land it on the asteroid's rock-hard surface.
The most difficult part of the mission began yesterday with a main engine burn to accelerate the spacecraft towards a rendezvous with the faster- moving asteroid.
Carl Pilcher, a Nasa scientist, said that knowing more about asteroids and how to approach them may one day be useful if the Earth is ever threatened by a collision. "It is prudent to learn the properties of these objects, if one day we find one with our name on it and we have to do something about it," he said.
Scientists estimate there are about 1,500 asteroids that pass close enough to Earth and are big enough to pose a threat if they ever hit the planet. A small asteroid just 33ft wide exploded over the Tunguska region of Siberia in 1908, destroying over half a million acres of forest. In 1989, an asteroid 0.25 miles wide and weighing 50 million tons came within 400,000 miles of Earth, passing the same point in space just six hours apart.
Eros, a potato-shaped body 25 miles long and 9 miles wide, is 240 million miles away and poses no threat to Earth. But it offers scientists the best opportunity to find out more about the mysterious lumps of rock left over from the formation of the planets.
Eros is an S-type asteroid composed of silicates enriched with metallic iron, The analysis of its composition will help to determine the part it played in the birth and evolution of the solar system, said Professor William Boynton, a Near scientist at Arizona University.
"This is the first time ever a spacecraft will orbit an asteroid. There have been fly-bys and snapshots, but not much in the way of quantitative scientific data," he said.
Robert Farquhar, of the Johns Hopkins University in Baltimore, where some of the Near instruments were built, said: "What we know of asteroids is very limited. But now we're going to go into orbit around an asteroid and study it intensely for a year. We expect to get astounding information."
Although Eros is 240 million miles from Earth, the Near probe has travelled more than 1.5 billion miles since its launch in February 1996. It has flown an indirect route, which included a return trip to Earth to use the planet's gravitational pull as a "slingshot" to throw the probe back into space.