LOS ANGELES — NASA's pioneering Dawn spacecraft, a year late in being launched and 20 percent over budget, is slowly creeping up on the protoplanet Vesta and is expected to enter orbit around it about July 16, the first stop on a remarkable journey that will later take the craft to the larger dwarf planet Ceres.
The craft, the largest probe ever launched by NASA, is about halfway through its three-month approach phase to Vesta, 96,000 miles away and closing in at the sedate speed of about 260 mph.
The whole procedure is happening so slowly, in terms of normal asteroid flybys and planetary encounters, that scientists at the Jet Propulsion Laboratory in La Canada Flintridge will not be able to calculate precisely when the craft entered orbit until after the fact.
The craft's visit to Vesta will be the first prolonged encounter with an object in the main asteroid belt that lies between Mars and Jupiter and the first trip to a protoplanet, a large body that almost became a planet.
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Both Vesta and Ceres are thought to have been formed when the solar system itself was being assembled from the dust that condensed into both sun and planets. Researchers once thought that both were remnants of a planet that formed and then was destroyed by a massive collision with a moon or some other object.
They now believe, however, that the asteroid belt contains the remnants of a planet whose formation was disrupted by the gravitational effects of nearby Jupiter. As such, its composition should mirror that of the earliest objects orbiting the sun.
"This tiny world... is a window into the early origin of the solar system and the terrestrial planets," said Carol Raymond of JPL, deputy principal investigator for Dawn. Learning about it "will give us better tools to understand the thousands of fragments that are out there in the asteroid belt."
Dawn was originally scheduled to launch in the summer of 2006, but testing problems with the engines delayed the actual launch until September 2007. Those problems also raised the cost of the mission from a planned $373 million to $446 million at launch.
Dawn is a unique craft and mission on a variety of accounts. For one, it is not powered by a conventional rocket engine, but by three ion engines. Electrical fields produced by two 27-foot-long solar panels accelerate xenon ions to high speed, expelling them out the three engines and providing a thrust about the same as "a single piece of paper pressing down on your hand," according to JPL's Robert Mase, the Dawn project manager.
Dawn will go into orbit around Vesta and stay there for a year before climbing out of orbit and proceeding on to Ceres, which it is expected to reach in 2015. No spacecraft has accomplished such a feat before.