NASA’s Dawn spaceprobe is approaching the asteroid Vesta. At a distance of 483 000 km, the spacecraft has made images rivalling the best Hubble Space Telescope views of this fascinating little world.
Between Mars and Jupiter lies the Asteroid Belt. Relics of the Solar Nebula which birthed the planets, these asteroids are the leftovers of the Solar System. Churned by giant Jupiter’s gravity, they are stuff that never had the chance to become a planet. There are about a million asteroids with a diameter of 1 km or more.
Only 200 or so of the asteroids in the Belt are more than 100 km across. The biggest object in the asteroid belt member is Ceres, so large it was up-graded from asteroid to Dwarf Planet in 2006. The three largest asteroids (Vesta, Pallas and Hygiea) plus Ceres together make almost half the total mass within the main belt. Ceres alone accounts for one-third of that total mass. Note that if Ceres and all the asteroids in the Belt combined into one lump the resulting body would be less than a twentieth the size of our Moon. The idea that the Asteroid Belt was once a planet destroyed in some cataclysm belongs to vintage science fiction works.
As they are ‘living fossils’ from the earliest days of the Solar System, asteroids are fascinating to astronomers. The largest bodies out there have, entirely through chance, survived billions of years of possible collisions and have remained intact ever since they formed. Asteroid 4 Vesta (the “4” indicates it was the fourth asteroid to be discovered) which orbits on average 350 million km (2.4 AU) from the Sun is a unique survior the Solar System’s turbulent past. Observations of Vesta suggest it has a complex geology suggesting an intriguing history. Its surface features include basaltic lava flows – implying that it once had a molten core – and a deep impact crater near its southern pole. Vesta is not a simple lump, it is a structured, differentiated body believed to have a core of nickel-iron, an olivine mantle, and a basaltic crust. Vesta seems to be bone-dry. In contrast, smaller asteroids have a simple primordial surfaces coated with carbonaceous materials with evidence of water ice.
Vesta is potato-shaped. At widest it is about 580 km across (only slightly more than the distance from Belfast to London) but about 460 km from pole to pole. Vesta has probably enjoyed an eventful life. In fact Vesta has had huge fragments splintered off it. How do we know this? Astronomers can use a technique called reflection spectroscopy to identify the presence of chemical elements on an astronomical body by the ‘fingerprints’ they leave in the sunlight reflecting off the surface. Spectroscopic analysis indicated that several small asteroids, the Vestoids, have surfaces which are highly similar or identical to Vesta’s. The best explanation is that they are indeed fragments blasted off Vesta by an ancient collision, possibly the very one which left the great crater near the asteroid’s south pole. But that is not all, the visible and near-infrared reflectance spectra of Vesta and the basaltic achondrites, a class of meteorite which fall on Earth, are so similar they must be connected. Presumably these are fragments from the massive impact that have found their way to our world. We almost certainly already have samples of Vesta here on Earth!
The Dawn space probe is expected to go into orbit around Vesta on 16 July, from there it will return some detailed images and other data, staying at the asteroid for about a year before setting off to visit Ceres.