Dawn En Route to Vesta and Ceres
Saturday, September 29th, 2007The Dawn spacecraft successfully launched yesterday en route to two of the largest asteroids in the solar system. Ceres, the largest asteroid and now termed a “dwarf planet”, like Pluto, under the new IAU nomenclature, has one-quarter the mass of the entire asteroid belt. Dawn’s other target, Vesta, is a bit smaller It will first visit Vesta, arriving there in September 2011 and orbiting along with it for about 6 months before using its low-thrust ion propulsion system to put it on a rendezvous course for Ceres with a February 2015 arrival.
Asteroids are interesting objects because they are leftover building blocks from the era of planet formation. Those objects, called planetesimals, ended up becoming part of a planet or an asteroid if they had rocky composition or a comet if they were primarily icy. That is, comets and asteroids are both leftover planetesimals that didn’t make it into a planet due to the whims of dynamics over the past 4.5 billion years or so. Of course, every now and then, an asteroid or comet takes the plunge into a planet, such as the Shoemaker-Levy 9 comet impact into Jupiter in 1994 or the infamous asteroid that struck the Yucatan peninsula 65 million years ago, beginning a really bad year for a lot of species. Dawn will give us a look at two of the largest of these leftover planetesimals. Both Vesta and Ceres are large enough to be differentiated, meaning that the dense metals have sunk to the center (like on the Earth and other planets). Interestingly, we actually have samples of Vesta here on the Earth. Vesta’s basaltic (lava-like) crust has a recognizable and distinct spectrum that matches certain classes of meteorites. Just as some meteorites are fragments of the planet Mars, identified by the relative abundances of isotopes of gases characteristic of Mars’ atmosphere, certain meteorites can be traced to Vesta by their spectra. Dawn is basically a mission of exploration to learn as much as possible about these two protoplanets, hopefully providing clues to the conditions at the dawn of the solar system in the process.



