By BBC News Online science editor Dr David Whitehouse

Scientists are bracing themselves for a historic rendezvous between a spacecraft and an asteroid. It should result in a probe going into orbit around a minor body of the Solar System for the very first time.

The Near Earth Asteroid Rendezvous (Near) spacecraft is currently closing on the asteroid Eros at a relative speed of just 35 kph (22 mph). The two objects are set for a Monday meeting some 260 million km (160 million miles) from Earth.

Already, Near's multi-spectral imager has been taking pictures to make sure that the spacecraft is on course as well as to check the rotation of the asteroid and to look for any tiny moons that might be orbiting it.

On 14 February, at 1533 GMT, when Near is 333 km (207 miles) from Eros, it will fire its hydrazine-fuelled engines to reduce its speed. This will allow the asteroid's feeble gravitational pull to capture Near.

Irregularly shaped Eros rotates once every 5.27 hours, that and its weak gravity makes it tricky for any spacecraft to go into orbit around it.

First time

Dr Robert Farquhar, Near mission director, said: "No one has ever orbited a small body in space. The orbital stability is rather tenuous, and as we travel around Eros, our navigation manoeuvres must be perfect to keep us from crashing into it."

When Near gets to Eros, a magnetometer will see if it has a magnetic field.

"This will give the scientific community the first definitive measurement of an asteroid's magnetism, which contains clues to its thermal and geologic history," said mission scientist Andrew Cheng.

"The results of these measurements and others that we will take over the next year will help us to determine the origin of the asteroid and give us an unprecedented understanding of asteroids in general," he added.

During its approach to Eros, Near is taking better images of it than have ever been obtained before. Several new features have been seen. One newly seen large crater has what has been described as a beautifully defined rim.

When it goes into orbit, Near will spend a year scrutinising the 33 by 13 by 13 km (21 by 8 by 8 miles) chunk of rock.

The day before orbital insertion, Near will fly directly between the Sun and the asteroid, enabling it make critical observations of the rock's northern hemisphere under near-perfect lighting conditions. This data will allow it to characterise the asteroid's mineral composition.

Busy year

For the first two months, Near will slowly descend to within 50 km (31 miles) from the asteroid's surface. During this low-orbit phase, Near's X-ray/gamma-ray spectrometer will measure the composition of the surface.

In late August, the spacecraft will begin to climb from 50 to 500 km (31 to 311 miles) above Eros. During this ascent, the multispectral imager will continue to take images of Eros that will be compiled into a complete map of the asteroid.

In December, the spacecraft will descend, possibly to within a couple of kilometres from the surface of the asteroid. This will enable it to collect extremely high-resolution data of the surface.

It is possible that Near will be commanded to crash onto Eros when its mission is ended.

Near's encounter with Eros is its second chance to go into orbit around the rock. It was to have orbited Eros in January 1999, but a computer glitch ordered Near's rocket to misfire.

Scientists managed to salvage the mission by placing the spacecraft into a trajectory that let it fly by the asteroid and have another chance the following year.

"The fact that the mission is still on track is a tribute to the robustness of the spacecraft and a mission design that included planning for adversity," says Dr Robert Farquhar.