Close encounter with asteroid

The Near-Shoemaker spacecraft has spent eight months in orbit around Eros

By BBC News Online science editor Dr David Whitehouse

A spacecraft is on course for the most daring part of its mission to explore a giant asteroid.

The Near (Near Earth Asteroid Rendezvous) - Shoemaker spacecraft will fly to within 5 km (3 miles) of the rocky surface of the asteroid Eros on Wednesday and Thursday.

This is the closest any spacecraft has ever come to an asteroid.

"We expect to get clear images of boulders as small as two feet across and see ridges and craters in exquisite detail," said Near project scientist Dr Andrew Cheng.

The closest images yet. But the new ones will be six times better

Over the past few weeks, Near-Shoemaker has performed a series of thruster firings in preparation for the fly-by. The final burn will be made just a few hours before the attempt, allowing the spacecraft to pass closer to Eros than commercial airliners cruise over land.

The mission is designed to answer fundamental questions about the many asteroids and comets which come close to Earth's orbit. It may contain clues about how the Earth and other planets were formed.

Almost a landing

On Wednesday, the Near-Shoemaker spacecraft will begin a descent from its current 50 km (31 mile) orbit above Eros to perform a low altitude swoop on the following day.

Previously scientists have seen many boulders on the surface of Eros and they want to see how small they are.

Astronomers expect the spacecraft to take more than 200 pictures. Exactly what it will capture will depend on Near-Shoemaker's trajectory and the final altitude of its camera.

Near-Shoemaker has orbited at various altitudes since it started investigating the giant asteroid at the start of the year.

It went to within 35 km (21 miles) of the asteroid for two weeks in July. For the past five weeks it has been mapping Eros at an altitude of 100 km (62 miles).

Near-Shoemaker's descent has already increased scientists' knowledge of the surface features of Eros. The close fly-by, however, will reveal a new level of detail in the shapes of rocks and patterns of brightness variation that will be visible.

Immediately after the swoop, Near-Shoemaker will conduct another manoeuvre to lift it to a safer 200 km (124 mile) orbit.