Asteroid 4, diameter 576 km, discovered by H. W. M. Olbers in 1802. It is the third-largest asteroid known and the brightest of all, sometimes reaching a visual magnitude of 6, when it is just detectable to the unaided eye under optimum observing conditions. Vesta’s brightness is due to its high albedo of 25 per cent. As it rotates every 5.43 hours, regular changes in the colour and spectrum are observed, reflecting the fact that the surface is not uniform. Vesta appears to be a true mini-planet which has survived largely intact since the solar system formed, rather than being a fragment from a larger body. Hubble Space Telescope images reveal details down to 80 km 50 miles across, including impact craters. One large crater seems to have torn away part of the crust completely, exposing the mantle below. There is evidence for ancient lava flows dating from 4 billion years ago when the interior was hot and molten. It is thought that Vesta may be the parent body of the eucrite type of meteorites.