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Asteroids and Comets
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The strong gravitational attraction of Jupiter prevented the formation of planets in the region that is currently occupied by the main asteroid belt. But the drama of the arrested planetary development turns out to have fortunate consequences: The asteroids in the main belt preserve the original state of the young solar system.
The NASA Dawn Mission aims to explore the two large asteroids Vesta and Ceres. With help of Dawn, we hope to learn more about the early formation of the solar system. There are three instruments onboard the spacecraft: a spectrometer (VIR), a gamma ray and neutron detector (GRaND), and a camera (FC) built with the participation of DLR Berlin. Dawn was launched in September 2007 and visited Vesta from August 2011 to August 2012. It will arrive at Ceres in April 2015.
Vesta is the third largest, and second-most massive asteroid in the main belt, having a mean diameter of about 525 km. Vesta orbits the Sun at a distance of 350 million kilometers and rotates once around its axis in 5 hours and 20 minutes. As Vesta is relatively close to the Sun, liquids and ices that were once exposed on the surface evaporated, sublimated, and escaped into space. Thus, Vesta is currently “dry”.
It takes Ceres 4.6 Earth-years to orbit the Sun, and 9 hours and 7 minutes to rotate around its axis. It is about 450 million km away from the Sun and formed in a cooler region of the early solar system than Vesta. For this reason, Ceres has a higher abundance of light elements and thereby a lower density. Due to the presence of water ice, possibly even liquid water, below the surface, Ceres shares some characteristics with comets. The water abundance is estimated to be between 17% and 27% by mass, and thus Ceres is considered “wet”. The surface has not yet been seen up-close, but images of the Hubble space telescope show variations in brightness that hint at morphological or chemical differences..
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