Asteroid Vesta is 'last of a kind' rock

A computer generated perspective of Rhea Silvia basin, which was
dug out by an impact a billion years ago 

Vesta is the only remaining example of the original objects that came together to form the rocky planets, like Earth and Mars, some 4.6 billion years ago.

This assessment is based on data from the Dawn probe which has been orbiting the second largest body in the asteroid belt for the past 10 months.

The findings from the Nasa mission are reported in Science magazine.

They confirm that Vesta has a layered interior with a metal-rich core, just as Earth, Mars, Venus and Mercury do.

Using information about the shape of the asteroid and its gravity field, scientists can even say something about the likely size of this core.

The Dawn team calculates it to be about 220km (135 miles) across, representing about 40% of the radius of Vesta, or roughly 18% of its total mass.  (BBC.co.uk)