NASA’s Dawn spacecraft eyes mysterious dwarf

It’s the home stretch for NASA’s Dawn spacecraft, which after a 3-billion-mile journey has finally got the dwarf planet Ceres in its sights. Now, Dawn’s newest images reveal fascinating features on Ceres’ surface that will only grow clearer in the run-up to the spacecraft’s arrival March 6.

Dawn’s newly released images of Ceres are 27 pixels across; that may not sound like much, but it’s about three times better than the images it took in December. Those were being used for calibration; these, which cover more than half the planetoid’s surface, will be used for navigation as Dawn closes in on its target.

At 590 miles across, Ceres is the largest asteroid in the belt of rocky debris between the orbits of Mars and Jupiter, and one of five dwarf planets (a list that includes Pluto). Its nature has long remained a mystery. The best images of Ceres were taken by the Hubble Space Telescope more than a decade ago, and those are still quite blurry.

But the Dawn spacecraft, set to enter orbit March 6, is soon to change that. The just-released navigation images are about 80 percent of the resolution of the Hubble portraits. Taken when the spacecraft was about 238,000 miles from the surface (close to the average Earth-moon distance), Dawn’s fuzzy images reveal surface structures that could be craters.

A few of the dwarf planet’s features — a bright spot in the northern hemisphere, and two larger dark spots in the southern hemisphere — have been identified by Hubble before. But the images also feature extensions near the dark spots’ upper edges that hadn’t been previously seen.

When Dawn takes its next set of images in late this month, the quality should surpass that of Hubble’s images, according to officials at NASA’s Jet Propulsion Laboratory.

Ceres is not Dawn’s first target. That honor goes to Vesta, another asteroid in the main belt, which the spacecraft circled from July 2011 to September 2012. Vesta is the second most massive asteroid after Ceres, but the two heavyweights are very different in character: Vesta is dry and elongated in shape, while Ceres is round and thought to be very wet and icy.