NASA’s Dawn spacecraft was launched September 27, 2007, on a unique mission: to explore the two most massive objects in the asteroid belt, Vesta and Ceres. Propelled by an ion engine, it will, if successful, break new ground in spaceflight by becoming the first probe to orbit two separate extraterrestrial bodies.
Dawn’s journey to Vesta took almost four years and involved a flyby of Mars, to get a gravity assist, and several extended periods of ion engine thrusting. On July 16, 2011, it entered orbit around the potato-shaped, 525-kilometer-wide Vesta for a detailed study of the asteroid lasting more than a year. On September 5, 2012, it fired its thrusters again to break free of Vesta’s gravity and began the long haul to its next target—the dwarf planet Ceres.
Ceres is by far the largest body in the asteroid belt and the first to be discovered, by Guiseppe Piazzi in 1801. It measures 950 km (590 miles) across and, as distinct from almost every other asteroid, is round like a planet, its self-gravity having been strong enough when it was still molten to pull it into a spherical shape. In 2006, it was given dwarf planet status by the International Astronomical Union, putting it in the same category as Pluto.
The Hubble Space Telescope has captured images of Ceres revealing a dozen or so large, fuzzy features, some of which are craters. But Dawn will provide us with the first clear pictures of a dwarf planet—five months before New Horizons sweeps past Pluto, opening our eyes to that small world as well.
Dawn is scheduled to go into orbit around Ceres in February 2015, starting out at an initial altitude of 5,900 km. After five months at that height, the spacecraft will spiral down to a lower orbit 1,300 km high, and then, after another five months, down to a minimum orbit taking it to within just 700 km of the surface.
Dawn’s main goal, as at Vesta, will be to shed light on the early history of the solar system and the process by which planets are formed. These most massive of asteroids are perfect for this objective because they’re the largest surviving protoplanets that we know about—and quite different in character. Whereas Vesta is mainly rocky and highly evolved in a geological sense, Ceres is icy and more primitive, a result of them having formed at different distances from the Sun.
Dawn’s instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These will be trained on the dwarf planet to determine its exact shape, the nature of its topographical features—down to a resolution of about 60 meters per pixel (a vast improvement on the 18 km per pixel available with Hubble)—and the composition of its surface.
The end of Dawn’s primary mission is scheduled for about July 2015, and after that an extended mission is possible which could involve further study of Ceres or moving on to a third object in the asteroid belt. Originally, Pallas, another giant asteroid, was considered as an extended target, but delays in launching Dawn have probably put it out of range. A limited fuel supply for its ion thrusters may make it impractical to break orbit from Ceres, and the spacecraft may end its days continuing to send back information from a few hundred kilometers above the dwarf planet’s surface.
Want to keep up-to-date with all things space? Be sure to “Like” AmericaSpace on Facebook and follow us on Twitter: @AmericaSpace