Spaceprobe still active on asteroid

The probe is still working on the surface of Eros

By BBC News Online science editor Dr David Whitehouse

Astronomers are receiving data from the Near Earth Asteroid Rendezvous (Near) Shoemaker spacecraft that has landed on the surface of asteroid Eros.

One of its main instruments, the gamma-ray spectrometer, has been making measurements of the composition of the dusty surface. It is analysing a small patch of Eros' surface looking for the spectral signature of certain elements.

The last picture of Eros taken just before touchdown

Ground controllers are delighted at the contact, saying that the data sent back is of good quality and contains much information.

Additional time on Nasa's Deep Space Network of radio telescopes is being made available to communicate with the probe that is 315 million km (196 m miles) away.

Puzzling surface

From Near-Shoemaker's mission, the first to orbit an asteroid, astronomers have confirmed that Eros is made of material unchanged since the birth of the Solar System.

Its surface has been sculpted by innumerable impacts. But the size distribution of the craters on Eros is different from anything astronomers have seen before, with a remarkable deficit of craters with diameters below 100 m (328 ft) as well as a great many rocks and boulders strewn on its surface.

According to Jeff Bell of the University of Hawaii, the new data suggests that impacts and the behaviour of debris on asteroids is very different from that on other planetary objects.

The key observation, he says, is that an analysis of the size of craters on Eros' surface shows that small craters are about 200 times less abundant than expected.

He believes that the unusual distribution of crater sizes reflects the different population of objects that struck Eros in the past.

This population of rocky objects must be very different from the normal inner solar system projectile size distribution that produced the craters seen on the Moon, Mercury, and Mars.

It implies that the region of the Solar System where Eros was born, the main asteroid belt between the orbits of Mars and Jupiter, is depleted in objects smaller than a few metres in size.

The under abundance of small impactors may also provide an explanation for the puzzling number of boulders on Eros.

It is believed that they were created by ejection from larger impacts and gradually eroded into smaller boulders. It is an effect that can be seen on the Moon where fields of jagged boulders are seen around fresh craters.

Dr Bell proposes that the absence of smaller impactors allowed boulders to accumulate on Eros without being broken up or eroded.