Asteroid 21 Lutetia is a large, heavily cratered main-belt asteroid (located between the orbits of Mars and Jupiter), made of metal rich rock. It has an irregular shape with estimated dimensions of 132 × 101 × 76 kilometers, and a mass of approximately 1.700 × 1018 kilograms.
The surface of Lutetia is covered by numerous impact craters, ranging from 600 m to 55 km, and intersected by fractures, scarps and grooves thought to be surface manifestations of internal fractures. The most heavily cratered surfaces have a crater retention age of about 3.6 billion years.
It has one of the highest densities seen before in asteroids and exceeds that of a typical stony meteorite, meaning that it is made of metal rich rock. There is evidence of hydrated materials on its surface, abundant silicates, and a thicker regolith than most asteroids, made of loosely aggregated dust particles. It is estimated to be 3 km thick and may be responsible for the softened outlines of many of the larger craters.
Lutetia orbits the Sun at the distance of approximately 2.4 AU. Its orbit is moderately eccentric and the orbit period of Lutetia is 3.8 years.
The Rosetta probe passed within 3,162 km (1,965 mi) of Lutetia in July 2010 and its cameras took about 400 pictures of the asteroid during the flyby. It was the largest asteroid to be visited by a spacecraft until the Dawn mission arrived at 4 Vesta in July 2011.