On July 15th, NASA’s Dawn spacecraft will be entering orbit around the asteroid Vesta – and I’m really looking forward to it! This is a new frontier – while we’ve seen a few small asteroids (e.g. Ida, Gaspra), we’ve never seen a big asteroid up close, and Vesta is the third largest asteroid in the asteroid belt between Mars and Jupiter (it’s also the second most-massive after Ceres, which will be visited later by Dawn). We’ve also found several meteorites on Earth that we think were blasted off from Vesta’s surface by impacts on the asteroid – these indicate that Vesta should have differentiated into a rocky mantle and metallic core, and I’m curious to see if there is evidence of volcanic activity on its surface (my Ph.D. supervisor published some papers about Vesta’s early history, and some of his interest and enthusiasm about it has rubbed off on me!).
Asteroids are interesting beasts – they’re mostly found in the space between Mars and Jupiter, and are remnants from the formation of the solar system. The rocky planets in the solar system formed by accretion in the solar nebula about 4.6 billion years ago – put very simply, dust grains clumped together due to gravity to form planetesimals, which themselves clumped together to form protoplanets, which then clumped together to form larger planets. However, Jupiter’s gravity affected (and still affects) the orbits of nearby objects, which prevented the asteroids from accreting into planets. As a result of this, we now have a band of asteroids between Mars and Jupiter – it’s essentially debris that was never allowed to form into a planet. What’s more, if you could combine all the asteroids currently in the asteroid belt, the resulting object would be smaller than our own moon (part of this is due to the fact that many asteroids have been ejected from the belt by gravitational interactions or destroyed by collisions)! Since we’ve not seen a large one up close before, a lot of planetary scientists will be interested to see what Dawn reveals at Vesta (and later when it moves on to orbit Ceres, which is the largest asteroid and also a dwarf planet).
As Dawn has been approaching Vesta over the past few weeks, features have been slowly coming into view that we couldn’t see from Earth. The latest picture (shown above) hints at interesting features – for one thing, there don’t appear to be an awful lot of large craters visible, which would imply that its surface may be young (possibly evidence for volcanic resurfacing?). Some curvilinear features are also visible in the southern hemisphere – maybe they’re tectonic in origin, or perhaps they’re something else (crater-chains? flow fronts?). Right now it’s all very tantalising, but we’ll find out more when Dawn goes into orbit in a few days!