The European Space Agency's Rosetta flew past Lutetia on 10 July 2010 at a speed of 54 000 km/hr and at a close distance of 3170 km. At the time, the 130 km-long asteroid was the largest encountered by a spacecraft. Since then, scientists have been analysing the data taken during the brief encounter.
All previous "flybys" went past objects, which were fragments of once-larger bodies. However, during the encounter, scientists thought that Lutetia might be an older, more primitive 'mini-world'.
Now they are much more certain. Images from the OSIRIS camera reveal that parts of Lutetia's surface are around 3.6 billion years old. Other parts are young by astronomical standards, at around 50-80 million years old.
Astronomers estimate the age of airless planets, moons, and asteroids by counting craters. Each bowl-shaped depression on the surface is made by an impact. The older the surface, the more impacts it will have accumulated. Some parts of Lutetia are heavily cratered, implying that it is very old. Some impacts must have been so large that they broke off whole chunks of Lutetia, gradually shaping it into the battered wreck we see today.
Bibliography:
www.sciencedaily.com
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