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"Dawn will be history's first mission to go out into the solar system, orbit and explore a distant body, and then go on to a totally different celestial body and explore that one," said Dawn project manager Keyur Patel of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "To do all that you need a spacecraft with a lot under the hood."
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"In the end it is about the science," added Patel. "What we find when Dawn gets to Vesta and Ceres will re-write the history books on the beginning of our solar system. But how we get there is almost as remarkable, 1.8 billion miles to Vesta, months flying around it performing science adjusting our orbits as we go. Then we travel another billion miles to Ceres where we do it all over again. That is a lot to ask of a beam of blue light."
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"In the end it is about the science," added Patel. "What we find when Dawn gets to Vesta and Ceres will re-write the history books on the beginning of our solar system. But how we get there is almost as remarkable, 1.8 billion miles to Vesta, months flying around it performing science adjusting our orbits as we go. Then we travel another billion miles to Ceres where we do it all over again. That is a lot to ask of a beam of blue light."
Their thrust is low, but over the course of the mission the total change in velocity from the ION drives will be similar to that delivered by all the chemical rockets used for the launch; 9 SRB's + the Delta II's first, second and third stages.
For all of this each ION drive is about the size of a basketball and weighs about 9 kg (20 lbs).
[NOTE: VASIMR, a variable power plasma electric rocket ultimately capable of thousands to millions of times the thrust of any ION drive while maintaining ION's efficiency, is due to fly its first checkout mission by 2011 if not sooner. Ad Astra just built one of the largest vacuum chambers in the world for testing the prototype for that flight, but smaller ones have already been fired for several hours continuously.]
The 8 year mission's goal is to investigate in detail two of the largest protoplanets remaining intact; Ceres and Vesta. Dawn will use its ION drives to speed its trip and to maneuver into orbit around each object for extended examinations.
Ceres is a minor planet with a mass about 4% that of the Moon but containing ~200 million cu/km of water, more than the amount of fresh water on Earth, frozen over its rocky core. This could make Ceres an important way-station for extended manned missions around the solar system as it could provide both water and rocket fuel (water split into H2 and O2).
Vesta is the 2nd largest object in the asteroid belt after Ceres and thought to be a metallic iron-nickel core, an olivine mantle with a surface crust of basalts and other types. A few meteorites are thought to be fragments of Vesta blown off in collisions, and they raised many interesting questions.
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