Science: Big days on little planets: visitors from Earth call in

IMAGINE you are exploring the surface of a near-Earth asteroid. Your world might be tiny, but you are its ruler, free to bound across its surface. Take care though not to use too much effort - for it would be all too easy to jump off this tiny world for ever. The gravity is one 10,000th less than Earth: you can launch yourself off this place for ever. It's a warped and mountainous world, where every step kicks up dust that lingers for minutes.

"Asteroids are the fossils of the solar system," says Akira Fujiwara of Japan's Institute of Space and Astronautical Science, who wants to collect a sample a sample from one. The spacecraft to do so, scheduled for launch in January 2002, will be a technological tour-de-force. It will give scientists their first hands-on contact with ancient rock from a known source, rock that has remained relatively unchanged since the earliest days of the solar system.

But we will not have to wait that long to visit one of these strange little worlds. The spacecraft that will do it first is already cruising towards its target. We are a year away from placing an artificial moon around one of these strange objects. The Near Earth Asteroid Rendezvous (Near) spacecraft, built by Nasa, will manoeuvre in early 1999 into the first orbit about a low-gravity planet, the Earth-approaching asteroid 433 Eros. It is roughly nine miles by 25 miles - though that gives its dimensions a regularity that doesn't exist.

Near will circle 19 - 60 miles above the surface for a year or more, making a range of measurements including returning unprecedented colour images which will transform little-known Eros into one of the most exhaustively explored members of our solar system, and the first body in that size range to be understood to any significant extent.

Near will circle an asteroid but what about landing on one? The Japanese Muses-C spacecraft will, after a journey of 20 months, reach Nereus, a near-Earth asteroid about a mile in diameter. Muses-C will drift alongside it for two months, periodically firing rocket thrusters to draw it closer to the asteroid. As the rocky surface approaches the spacecraft will use radar to make a soft landing. Of the worlds on which spacecraft have landed, none have been anything like Nereus. Too small to have been modified by large-scale geological forces, its landscape hides the secrets of the larger body from which it fragmented when the solar system was young.

Japan has many dreams for its space programme: a mission to the Moon is already being planned. But none of its ambitions will be as difficult to achieve or require as much advanced technology as the trip to the minor planet Nereus. It will be the start of a trend for the next century.

Dr David Whitehouse is the BBC's Science Correspondent.