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One of the problems with any plan to deflect or destroy an asteroid on a collision course with Earth is that astronomers do not know whether asteroids are rubble or monolithic. Several space missions are now trying to learn more about what asteroids are made from.

Last month, a spacecraft called Deep Space 1 took photos of the near-Earth asteroid 1992 KD from six miles away. During the fly-by, it took pictures and measured the asteroid's composition, size, shape, surface features and brightness.

The first spacecraft to study an asteroid close up was an American mission called Galileo. In 1991, on its way to Jupiter, it passed the asteroid 951 Gaspra at a distance of 1,000 miles. Gaspra is believed to be composed of a mixture of rocky and metallic minerals. Two years later, Galileo crossed paths with asteroid 243 Ida at a distance of 1,500 miles. Ida, an irregular, cratered object, has a tiny moon, about 1.5 kilometres across.

The American space agency Nasa's NEAR mission will be the first spacecraft to orbit an asteroid when it arrives at 433 Eros in February 2000. It will orbit Eros for at least a year, determining its mass, structure, geology, composition, gravity and magnetic field. Eros is the largest of the near-Earth asteroids.

Meanwhile, Japan is planning to take samples from an asteroid and return them to Earth, on a mission in collaboration with Nasa. Launching in 2002, MUSES-CN will arrive at asteroid 4660 Nereus in April 2003. The spacecraft will also drop a rover onto the surface of the asteroid, where it will take high-resolution close-up images of the surface.