080908 Steins “a diamond in the sky”

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Steins “a diamond in the sky”
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 08, 2008

Four years into its ten year journey to comet 67/P Churyumov-Gerasimenko, Rosetta completed a successful fly-by of asteroid (2867) Steins at 20:58 CEST on 5 September, only the ninth asteroid to be studied up close by a passing spacecraft.

Click for animation as Rosetta approaches Steins. The sequence was captured with the OSIRIS imaging system's wide angle camera and begins about three minutes before closest approach and ends four minutes after closest approach. Image: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA.

"Steins looks like a diamond in the sky," says Uwe Keller, Principal Investigator for Rosetta’s OSIRIS imaging system. OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) comprises a wide-angled camera and a narrow-angle camera to obtain high resolution images.

Steins is a small asteroid of irregular shape with a diameter of just 4.6 kilometres. It belongs to the rare class of E-type asteroids, which had not been directly observed by an interplanetary spacecraft before. Although such asteroids are generally found in the inner part of the main Asteroid Belt, scientists speculate that they are derived from the mantle of larger asteroids destroyed in the early history of the Solar System. Analysis of the data collected by Rosetta this weekend as it swooped past the asteroid’s surface at a distance of 800 kilometres will finally unveil the true nature of Steins.

Asteroid Steins, as seen from a distance of 800 kilometres. A large crater at least 1.5 kilometres wide can be seen at the 'top' of the five kilometre wide asteroid. Image: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/ID.

Images were streamed back to Earth into the morning of Saturday 6 September, and revealed Steins as a heavily pockmarked asteroid, covered with impact craters that imply an old age, since the more cratered a body is, the older it is. So far, 23 craters have been identified, and a particularly large crater, around two kilometres in diameter can be seen at the ‘top’ of the asteroid in the images shown here. A chain of at least seven impact craters can also be discerned which, according to Rosetta project scientist Rita Schulz, may have formed from recurring impact as the asteroid rotated. “The impact may have been caused by a meteoroid stream, or fragments from a shattered small body," she says.

The project team also found that Steins was unusually bright and hope to find out why as they plough through the data. They also want to know how fine grains of the surface regolith are, to shed light on how the asteroid formed.

"It looks like a typical asteroid, but it is really fascinating how much we can learn from just the images,” says Gerhard Schwehm, Mission Manager for Rosetta. “This is our first science highlight; we certainly have a lot of promising science ahead of us. I’m already looking forward to encountering our next diamond in the sky, the much bigger Lutetia."

Science observations of Steins will continue for the next two days before Rosetta’s attentions turn to the next rendezvous: asteroid (21) Lutetia, scheduled for 10 July 2010, prior to the final destination of comet 67/P Churyumov-Gerasimenko, where a lander will be deployed to the comet’s surface while the orbiter will map the comet’s surface and study changes in activity.