By BBC News Online science editor Dr David Whitehouse

Just a few days into its mission, the Near Earth Asteroid Rendezvous (Near) mission is showing that Eros is no ordinary space rock.

Since Near went into orbit around the asteroid last week, scientists have pored over images and other early scientific returns.

It will take months to unravel the deeper mysteries of Eros, but the data from Near's final approach and first days of orbit offer tantalising glimpses of an ancient surface covered with craters, grooves, layers, house-sized boulders and other complex features.

"Work is just starting, but it's already clear that Eros is much more exciting and geologically diverse than we had expected," says Dr Andrew Cheng, of the Applied Physics Laboratory, Near's lead scientist.

Thanks to Near's orbital motion around the asteroid, scientists now know that Eros's mass is 2.4 grams per cubic centimetre - about the same density of Earth's crust and a near match of the estimates derived from Near's flyby of Eros in December 1998.

Not a 'rubble pile'

"With this new data, it now looks like we have a fairly solid object," says Near radio science team leader Dr Donald Yeomans. "There is no strong evidence that it's a rubble pile like Mathilde," the large asteroid Near passed and photographed in 1997.

Pictures taken with Near's Multispectral Imager offer several clues about Eros's origin, age and geography. The large number and concentration of craters point to an old asteroid. Uniform grooves across its craters and ridges hint at a global network of underground layers, which Cheng says could indicate Eros was once part of a larger body.

The digital camera has also captured brighter spots on the surface that Near scientists are keen to study.

"One patch is about 25% brighter than the rest of the asteroid, and that's a very large difference from the materials you expect to find on the surface," says Dr Mark Robinson, a Near imaging team member from Northwestern University. "That's a really neat feature to keep our eyes on."

For the next year, Near's instruments will continue to examine the potato-shaped asteroid's chemistry, geology, and evolutionary history.