astroengine writes: "Asteroids visited by spacecraft have all turned out to be piles of rubble or chunks broken off of larger bodies, but that's not the case with 21 Lutetia, a 75-mile long, 47-mile wide body circling in the main belt of asteroids between Mars and Jupiter. Europe's comet-bound Rosetta probe flew by Lutetia last year and gave scientists a big surprise. With its dense body and an interior that seems to have survived intact, the large asteroid appears more like a protoplanet — a leftover building block from the formation of the solar system."
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