The Dawn spacecraft had a difficult birth: it was threatened by cost overruns and technical concerns, cancelled, reinstated and scaled down. Now, after a four-year journey spiralling out from Earth's orbit, the probe is set to explore the beginnings of the Solar System. On 16 July, Dawn will enter orbit around Vesta (see 'Dawn patrol'), an asteroid that, at 500 kilometres wide, is the second largest in the Solar System. It will spend a year there before flying on to Ceres, the Solar System's largest asteroid at nearly 1,000 kilometres wide. There are hundreds of thousands of bodies in the main asteroid belt, which sprawls between the orbits of Mars and Jupiter and is a storehouse of material that formed early in the Solar System's history. But because Vesta and Ceres have apparently survived in one piece since then, "they are like time capsules telling us about the earliest stages of planet formation", says Carol Raymond, deputy principal investigator of the mission and a planetary scientist at NASAs Jet Propulsion Laboratory in Pasadena, California. Dawn's comparative study of the two bodies should also help to show how similarly sized objects can evolve very differently. Glimpses of Vesta suggest that its structure is like that of a miniature Earth, with a metallic core and a rocky mantle and crust, but that its growth was halted when Jupiter's far-reaching gravi-tional influence prevented asteroids in the belt from coalescing any further. Vestas composition, deduced from afar through its spectral properties, suggests that after its formation, the asteroid was initially hot enough for lava to ooze out onto its surface. By contrast, Ceres contains many water-bearing minerals, and with an average density lower than that of Earth's rocky crust, it may be one-quarter ice beneath its dust-coated surface. The asteroid could even hold a subsurface ocean, long frozen or perhaps still liquid.
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