21 Lutetia is a large main-belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter (120 km along its major axis). It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name of the city that stood where Paris was later built.
Lutetia has an irregular shape and is heavily cratered, with the largest impact crater reaching 45 km in diameter. The surface is geologically heterogeneous and is intersected by a system of grooves and scarps, which are thought to be fractures. It has a high average density, meaning that it is made of metal-rich rock.
The Rosetta probe passed within 3,162 km (1,965 mi) of Lutetia in July 2010. It was the largest asteroid visited by a spacecraft until the Dawn mission arrived at 4 Vesta in July 2011.
Read more about 21 Lutetia: Discovery and Exploration