On February 17 at about 7:28 P.M. Eastern Standard Time, the spacecraft Dawn received a speed boost and a change of direction in the form ofa gravity-assist from Mars. The craft slingshot past the planet, using it (rather than its own precious fuel) to adjust its trajectory, and will continue to the first stop in its journey, the asteroid Vesta. The Dawn mission has gone so smoothly that after almost a year and a half of travel, it required no course correction as it neared the Red Planet. Now that it has passed Mars, it will coast for about four months before powering up its ion drive engine and traveling another 26 months to Vesta.
Mission controllers have compared the Dawn mission with that of the fictional space voyages depicted on Star Trek. Dawn project system engineer Marc Rayman says, "Dawn is like the first real interplanetary spaceship! Many spacecraft have gathered data at multiple bodies, but Dawn will be the first to go somewhere, go into orbit, and be able to linger there, and then travel to another body and do the same thing." To do this, Dawn is powered by an ion plasma propulsion engine. Unlike on the starship Enterprise, however, where venting plasma was sometimes a bad sign, Dawn will rely on the controlled venting of its plasma thrust to continuously accelerate toward Vesta. On its way, the spacecraft's ion engine will speed it up to about 24,600 miles per hour (11 kilometers per second), far more than any spacecraft has ever achieved.
Dawn will place itself in orbit around Vesta in August 2011 and will spend some nine months collecting data that will help us to better understand the origin of our solar system. It will then leave orbit and begin the second, and longest, leg of its trip: a two-year, nine-month journey to the dwarf planet Ceres, the largest body in the asteroid belt. After several more months of gathering data, the Dawn mission will come to an end.
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And what will happen to Dawn? According to Rayman, "because of the possibility of there being a substantial inventory of water at that mysterious body, we have 'planetary protection' requirements." (Did someone say "Prime Directive"?)
Essentially this means that Dawn will not be sent hurtling into Ceres. Rayman continues: "Now there is no reason whatsoever to believe there is life on Ceres, but there may be chemistry occurring there that is related to the chemistry that preceded the development of life on Earth—and perhaps elsewhere! Therefore, to protect that fascinating environment, we are currently required to leave the spacecraft in an orbit in which it will not crash into Ceres for at least 20 years."
Will Dawn, then, go out with a whimper? Maybe not. If more funding became available, and there is enough leftover xenon fuel in the ion engine's tank, new objectives for Dawn might be considered—perhaps to again boldly go on where no spacecraft has gone before.