Rosetta finds a rocky jewel

Spacecraft images asteroid during close encounter

On September 5, the European Space Agency’s Rosetta mission became the first spacecraft to take a close-up portrait of a rare type of asteroid that lies in the main belt, between the orbits of Mars and Jupiter. The craft captured images of a 5-kilometer-wide asteroid, called 2867 Steins, while flying within 800 kilometers of the rocky body’s surface.

Rosetta found that the grayish asteroid is shaped like a diamond, measuring about 5.9 kilometers by 4 kilometers. “We observed a new jewel in the solar system,” said Rosetta scientist Uwe Keller during a Sept. 6 press briefing at ESA headquarters in Darmstadt, Germany.

A camera on Rosetta identified 23 craters bigger than 200 meters across, with the largest crater, on the north side of Steins, measuring 2 kilometers in diameter. The camera also spotted a chain of small craters near the rim of the largest crater, said Keller, of the Max Planck Institute for Solar System Research in Katlenburg-Lindau. Crater chains have also been seen on Earth’s moon and Jupiter’s large moons and may be produced by debris excavated when impactors slammed into the bodies and formed the large craters. The asteroid reflects about 35 percent of the sunlight striking its cratered surface, slightly less than scientists calculated but still brighter than most asteroids.

DIAMOND IN THE SKY The first close-up portraits of asteroid 2867 Steins were taken by the Rosetta spacecraft during a flyby on September 5. The large crater seen at the top of Steins in this series of images is 1.5 kilometers wide. ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Additional analysis of images and spectra recorded during the flyby is expected over the next days to weeks.

Asteroids are rocky leftovers from the planet-building process. These chunks of material never got incorporated into planets when they formed some 4.5 billion years ago and therefore offer clues about the birth of the solar system.

“It’s a little like a piece of DNA from the solar system,” says Rosetta project scientist Rita Schulz. Steins belongs to a rare class of main-belt asteroids known as E-type, which are distinguished by their high reflectivity and a silicate-rich composition that includes the mineral enstatite. E-type asteroids are thought to be fragments chipped off the outer edge of larger chunks of rock during an impact.

Spacecraft have obtained detailed images of only eight other asteroids among the hundreds of thousands of rocks in the main asteroid belt.

The flyby and a scheduled visit to another asteroid, 21 Lutetia, in 2010, are mere stopovers en route to the craft’s main mission. In 2014, Rosetta will rendezvous with the comet 67P/Churyumov-Gerasimenko, releasing a small lander onto the comet’s icy surface when it is far from the sun and inactive. Then, for the next two years, Rosetta will chase the comet and lander as they head toward the inner solar system at speeds of over 100,000 kilometers per hour. Rosetta will observe the changes in the comet as it nears the sun’s warming rays. The sun’s heat causes ice on the comet to suddenly and violently evaporate. Jets of gas blasted from the comet’s surface drag out dust grains along with it, forming the coma, or tail.

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