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21 Lutetia

Lutetia was the Roman name for the city that would become known as Paris, from which this asteroid was discovered in 1852 by the astronomer and artist Hermann Goldschmidt, the first of fourteen asteroids that Goldschmidt would identify over a period of eleven years. Following Goldschmidt's choice of a Roman name, regions and craters on Lutetia have also been given the names of Roman provinces and cities, including one crater named Roma for Rome itself.

Lutetia's orbit carries it through the inner parts of the Solar System's Asteroid Belt, and at its perihelion the asteroid comes close to the Belt's inner edge, some two Astronomical Units from the Sun. It is notable as being one of the few asteroids to have been closely approached by a spacecraft, in this case the Rosetta probe in 2010. We thus have clear and detailed images of much of the Lutetia, showing a rough and irregular body averaging about a hundred kilometres in diameter, pocked with craters and fracture lines.

The pattern of Lutetia's fractures implies that a huge crater exists on the surface, though this was not imaged directly by the Rosetta probe. This theoretical crater would have a diameter approaching half that of the entire asteroid, though as its existence is at present only suspected, it has been given the Latin name Suspicio.

Despite Rosetta's observiations, the physical structure of Lutetia is not well understood, though its surface is evidently covered by a thick layer of dust that is probably several kilometres thick. Beneath that dusty regolith, the asteroid is unusually dense, a fact usually implies the presence of metals in a body's make-up. In Lutetia's case, however, such metals have not been directly detected, and the asteroid may instead have an unusually dense stony core.