Space

The comet-chasing Rosetta spacecraft has taken the first close-up snapshots of the asteroid Steins, revealing it is an ancient, cratered rock shaped like a diamond.

Launched by the European Space Agency four years ago, Rosetta came within 800 kilometres of the tiny space rock on Friday. Steins, a rare type of asteroid located in the main asteroid belt between Mars and Jupiter, is Rosetta's first scientific target on its way to rendezvous with Comet 67P/Churyumov-Gerasimenko in 2014.

As Rosetta started its approach at 1858 GMT, its wide-angle OSIRIS camera started snapping a series of images of the 6-kilometre-wide asteroid.

"Steins looks like a diamond in the sky," says Uwe Keller of the Max Plank Institute for Solar System Research in Lindau, Germany. Previous observations with the Spitzer space telescope and ground-based telescopes had hinted at such a shape and size.

A large crater 2 kilometres across covers the northern half of Steins.

A chain of seven other craters surrounds that crater and probably formed in a single event. "The impact may have been caused by a meteoroid stream, or fragments from a shattered small body," says Rita Schulz of ESA.

Unknown glitch

At least 15 smaller pockmarks are scattered across the surface. Impacts with other space rocks are rare on an object this small, so the observations suggest that Steins is very old - it has survived long enough to accumulate the craters.

Information about the composition of the space rock, collected by the spacecraft's VIRTIS spectrometer, will continue to be analysed over the next few weeks.

Preliminary results show the asteroid to be brighter than most, reflecting 35% of the sunlight that falls on it. Further analysis may provide more clues about how the asteroid formed.

A glitch prevented the spacecraft from seeing the asteroid in even greater detail. OSIRIS's narrow-angle camera, which has a higher resolution, stopped collecting data minutes before the closest approach.

ESA scientists are still investigating why the instrument switched to this 'safe' mode, designed to protect the camera from injury.

Rosetta's team is already looking forward to the spacecraft's next encounter with the larger asteroid 21 Lutetia, scheduled for 2010.

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