Rheasilvia
Rheasilvia is an enormous impact basin, about 500 kilometres across, near Vesta's south pole and the likely source of many HED meteorites.
Rheasilvia is an impact basin on the asteroid Vesta, roughly 500 kilometres in diameter, almost as wide as Vesta itself, which is about 525 kilometres across. Its central peak rises roughly 20 kilometres above the basin floor, making it one of the tallest mountains in the Solar System.
The basin was first seen as a giant hole in Hubble Space Telescope images in 1997 and mapped in detail by NASA's Dawn spacecraft in 2011 and 2012. Dawn showed that it partly overlies an older basin, Veneneia. Crater counts suggest Rheasilvia formed about a billion years ago, though estimates vary.
The impact that formed Rheasilvia blasted vast amounts of material into space. Some formed the Vesta family of small asteroids, the vestoids, which share Vesta's spectrum. Fragments of these that reach Earth are thought to be the source of many HED meteorites, which is why you can hold a piece of Vesta in your hand.
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