The NASA Dawn spacecraft is now in its low altitude mapping orbit (LAMO) 210 km above the surface of the asteroid 4 Vesta (image: NASA).

The NASA Dawn mission continues to send back breathtaking images of the asteroid 4 Vesta, which it has been studying in detail since July 2011. In mid-December the spacecraft successfully manoeuvred to its lowest orbit around the giant asteroid and now has an average altitude above the surface of 210 kilometres. During this phase of the mission, known as LAMO: low altitude mapping orbit, Dawn will be collecting detailed information about Vesta’s surface and interior features. In particular, it will collect data using its gamma ray and neutron detector (GRaND) and gravity sensors.

Initial results from Dawn were presented in early December at the American Geophysical Union (AGU) Fall meeting in San Francisco. The new data suggests that Vesta is more like a small planet than a large asteroid. It appears to have a layered and differentiated internal structure with density increasing inwards and shows clear evidence for the existence of a dense metallic core. These are all features typical of a large differentiated body, such as the Earth, the Moon or Mars. However, one major unresolved problem is the lack of clear volcanic features on the surface. A differentiated planet might be expected to show features indicative of volcanic activity, such as cones or lava flows. So far, such features have not been observed on Vesta.

But perhaps this is less of a surprise than it might at first seem.

The heavily cratered surface of Vesta as imaged by the NASA Dawn spacecraft (image NASA).

Because the most striking feature of the new Dawn data is the extent to which Vesta was pounded by impacts. The southern basin on Vesta has turned out to be the result of at least two major impacts. The surface of Vesta is highly cratered and there are distinct equatorial ridges that appear to have formed during the same impact event that produced the giant southern crater. In fact the mystery is why Vesta is there at all? The evidence from Dawn suggests that it must have got close to the point of complete catastrophic breakup.

Vesta is located in the asteroid belt which lies between the orbits of Mars and Jupiter. Modelling suggests that in the earliest stages of solar system history the asteroid belt contained many more worlds like Vesta, which were later destroyed by impacts. In fact the asteroid belt once had a mass many thousands of times greater than present. This original mass may have exceeded that of the Earth.

So why is Vesta still there?

One suggestion is that Vesta did not originally form in the asteroid belt at all, but migrated there later on. It is called a main belt interloper. Vesta may have formed in the inner solar system and then migrated outwards to its present position.

It is clear from the presentations at AGU that there is much we still have to learn about the origin and early evolution of Vesta. With Dawn now gathering more detailed data expect a lot more surprises before it completes its work later this summer and heads off for its rendezvous with Ceres in 2015.