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A diamond in the sky

Rosetta captures asteroid Steins.
Provided by ESA, Noordwijk, Netherlands
Steins
This anaglyph image of Steins was taken around the time of Rosetta's closest approach to Steins on September 5.
ESA/MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
September 8, 2008
The first images from Rosetta's OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) and VIRTIS (Visual and Infrared Thermal Imaging Spectrometer) were derived from raw data this morning and have delivered spectacular results. The images featured asteroid Steins, which is located in the asteroid belt between the orbits of Mars and Jupiter.

"Steins looks like a diamond in the sky," says Uwe Keller, principal investigator for OSIRIS from the Max Planck Institut.

Visible in the image are several small craters on the asteroid, and two huge craters, one of which is 2 kilometers in diameter, indicating that the asteroid must be very old.

The images are 50 to 60 pixels in diameter, enough to characterize the shape and other characteristics of the body of the asteroid.

"In the images is a chain of impact craters, which must 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," says Rita Schulz, Rosetta project scientist.
steins
This image shows asteroid Steins seen from a distance of 800 kilometers, taken by the OSIRIS imaging system from two different perspectives. The effective diameter of the asteroid is 5 km, approximately as predicted. At the top of the asteroid (as shown in this image), a large crater, approximately 1.5-km in size, can be seen.
ESA/MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
The chain is composed of about 7 craters. To determine the age of the asteroid, a count of the craters on the asteroid's surface has been started (the more the number of craters, the older the asteroid). So far, 23 craters have been spotted.

From the images, scientists will try and understand why the asteroid is unusually bright, and how fine the grains in the surface layer are. This will tell them more about 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."

The OSIRIS imaging system's Wide Angle Camera (WAC) worked perfectly through the fly-by.

The OSIRIS team expects that the images that they will retrieve from the Narrow Angle Camera (NAC) will be of comparable resolution. This will add to the detailed color information and hence to greater knowledge of the surface composition.

Science team members noted that the NAC appears to have switched to safe mode a few minutes before closest approach, but switched back on after a few hours. The software is programmed to switch to safe mode when certain parameter thresholds are crossed to protect the camera. The team will concentrate on investigating the reasons for this anomaly once the science data has been analyzed.

After analysis of the Rosetta data, Steins will be one of the best-characterized asteroids so far.
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