Planetary scientists weren't sure what sort of asteroid they were looking at when the European Space Agency's Rosetta spacecraft flew close by Lutetia on 10 July 2010. But online today in Science, they report that the 121-kilometer-long asteroid  is most likely the first object among the 10,000-plus known asteroids to be recognized as an intact planetesimal  of the sort that glommed together to form the planets 4.6 billion years ago. The tip-off was its high density, as gauged by the way Lutetia's gravity deflected the passing spacecraft. The rocky body—the largest yet to get a close flyby—seems to have been just large enough to avoid being reduced to a flying pile of rubble by eons of collisions with other asteroids. The quick look has left researchers still scratching their heads over the planetesimal's mineralogical makeup, but Rosetta has moved on to bigger things: a 2014 landing on comet Churyumov-Gerasimenko.
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