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. It is 96,000 miles away from the protoplanet 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.
Both Vesta and Ceres are thought to be the remnants of a planet whose formation was disrupted by the gravitational effects of nearby Jupiter.
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.