21 Lutetia

21 Lutetia is a large asteroid in the main belt between Mars and Jupiter, and for a brief window it was the biggest asteroid any spacecraft had ever seen. When ESA’s Rosetta probe swept past on 10 July 2010 on its way to a comet, its OSIRIS camera caught the battered, irregular world this composite is built from.

Lutetia is an M-type asteroid, but it is not the simple lump of metal that classification once implied. Its surface is heavily cratered and blanketed by a regolith layer estimated to be kilometres thick, and its high bulk density points to metal-enriched silicate rock rather than pure iron. Many researchers read it as a primordial planetesimal — a leftover building block that never grew into a planet and survived largely intact for billions of years.

Measuring roughly 121 by 101 by 75 kilometres, Lutetia held the record as the largest asteroid visited by a spacecraft until Dawn reached Vesta in 2011. It remains one of the few asteroids we have resolved in high detail, a rare close look at the kind of ancient body the planets were assembled from.