Mathilde, a 52 kilometres asteroid, has been observed by the Near Earth Asteroid Rendezvous (NEAR) spacecraft. The spacecraft measured the mass of the asteroid when it passed within 1,200 km of Mathilde. Findings show that Mathilde is full of cavities. Researchers at the Jet Propulsion Lab, Pasadena, USA, say the mass of Mathilde is about a millionth of that of the Earth. The density of the asteroid is slightly more than that of water. Scientists are speculating that Mathilde is not solid but is a mixture of blocks and loosely condensed debris ( Nature , Vol 388, No 6639).