NASA's Dawn Probe moved into orbit around the asteroid Vesta on 16 July this year (2011) and has been beaming information back to Earth ever since. A picture has emerged of an oblate spheroid (squashed ball-shaped) world measuring 578 × 458 km, with a surface dominated by a large crater 460 km in diameter (Rheasilvia) at the southern pole; a series of grooves around the centre of the asteroid appear to be stress structures caused by the Rheasilvia impact.
A false-colour image of the surface of Vesta, centered on the Rheasilvia Crater.
This month NASA scientists attending the Fall Meeting of the American Geophysical Union report the discovery of of a metallic core and magnetic field in Vesta. This allows some interesting inferences about the history of Vesta to be made. In order to have assumed a spherical shape with a differentiated metal core Vesta must have taken a considerable time to cool, far longer than can be explained by its current size. The most likely explanation for this is that Vesta lost an appreciable amount of it's mass as a result of the Rheasilvia impact.
Vesta is currently designated as an Asteroid, rather than a Dwarf Planet, a body large enough to assume a spherical structure due to its own gravity, as it was not thought to be massive enough to achieve this, but Vesta is clearly roughly spherical, so either our theories on how massive a body must be to assume a spherical shape are wrong, or Vesta has lost some mass since it formed, something which the Rheasilvia impact can explain. It is likely that Vesta will be redesignated as a Dwarf Planet in the near future.
A map of the magnetic field of Vesta; like the surface this clearly shows the effects of the Rheasilvia impact.
The Dawn Probe is due to leave Vesta in July 2012 and move on to Ceres, which it will reach in February 2015. Ceres was formerly considered to be the largest asteroid in the Solar System; though since the introduction of the Dwarf Planet classification it has been considered the smallest Dwarf Planet, with Vesta, formerly the second largest asteroid promoted to largest asteroid.
See also The surface of Vesta, Visiting Vesta, Is the Jebel Hadid structure an impact crater? and Dwarf Planets on Sciency Thoughts YouTube.