Hubble Space Telescope image of Ceres. Credit: NASA/ESA/J. Parker (Southwest Research Institute)/ P. Thomas (Cornell University)/ L. McFadden (University of Maryland, College Park)/ & M. Mutchler & Z. Levay (STScI)

Jan 6, 2015 Surveying Ceres

Sen—A gentle puff of glowing green xenon gas is all that's propelled the Dawn spacecraft on a grand tour of the inner Solar System, spiraling outward past Mars, then Vesta, and finally Ceres.

Ceres, the largest member of the asteroid belt, will be Dawn's final destination. In two months, on March 6, Dawn will cross ahead of Ceres in its orbit, then rotate to push itself toward Ceres' path.

For sixteen months, Dawn's engines have worked to match its trajectory to that of Ceres'; only a tiny final effort will allow Ceres' gravity to reach out and grab Dawn into its first, elliptical orbit. Dawn will spend the rest of 2015 orbiting Ceres on a series of ever-closer circular paths.

Beginning with a high orbit and dropping to successively lower orbits, Dawn can survey Ceres from afar, establishing the world's global geography and allowing scientists to create base maps for later, more detailed work.

Dawn will begin with three photographic studies where it will take advantage of Ceres' rotation to bring every bit of its surface to Dawn's view over one Ceres day, about 9 hours. The first two of these "Rotation Characterizations" happen while Dawn is still on approach to Ceres on January 13 and 26, and will already produce images that are better than any Hubble has ever taken (or ever will). But they'll still be too distant to reveal more than fuzzy details.

Ceres will finally come into focus with Dawn's third Rotation Characterization phase, beginning April 23, with Dawn in a polar orbit that takes it once around the asteroid in 15 days. At an altitude of 13,500 kilometers, Dawn will still be far enough away that all of Ceres will fit comfortably within its camera frame, but the photos will be 20 times sharper than Hubble's, revealing details as small as a few kilometers across. A week after thoroughly photographing the sun-facing hemisphere of Ceres, Dawn's orbit will carry it to the world's night side, where the asteroid will form a dramatic crescent against the blackness of space beyond.

What will Dawn find at Ceres? We know already that it will look fundamentally different from Vesta. Dawn's first asteroid was noticeably lumpy, its once-round shape now scarred by two enormous overlapping impact basins that obliterated its southern hemisphere. Ceres is round, and its surface is made of different materials with a much more water-rich chemistry. What we do know about Ceres makes it much more like the mid-sized moons of the outer planets than it is like the potato-shaped rocky asteroids we've visited before. But it's a completely different world, whose history and tectonics have not been affected by a nearby giant planet. The fact is, we have no idea what it's going to look like. We'll all be discovering this strange new world together, come April.

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This Cassini photo of Saturn's moon Tethys (which is 1,060 kilometers in diameter, or about 12% larger than Ceres) demonstrates the kinds of detail Dawn will be able to see at Ceres during the Rotation Characterization 3 phase of the mission, beginning April 23. Image credit: NASA / JPL / SSI

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