Word: dawns
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...duck of a spacecraft, scheduled to launch in September, is known simply as Dawn, and its destinations are the asteroid Vesta and the dwarf planet Ceres, mysterious bodies orbiting in the belt of rubble that circles the sun between Mars and Venus. NASA vehicles have been this way before, but they've usually been just passing through on their way to the planets in the outer solar system. This time the asteroid belt itself will be the destination, and the ship will get there courtesy of the young technology of ion propulsion...
Like any other spacecraft, Dawn will have to be muscled off its launchpad by a conventional rocket burning conventional propellant. Once it climbs to near Earth space, however, everything will change. Of all the things that add weight to a spacecraft, fuel presents the most problems. The farther you're going, the more propellant you need, but every pound of it you add means more mass the engine must propel, which requires more fuel still, and on and on. A spacecraft like Dawn, which is designed not just to fly by its two targets but also to settle into orbit...
...over time it adds up. "It's acceleration with patience," says Rayman. "In the four days it takes to increase speed by 60 m.p.h., we'll use only 2 lbs. of propellant. If we keep thrusting, however, we can achieve extremely high speed." Indeed they can. By the time Dawn completes its four-year journey to the neighborhood of Vesta, a trip made longer by the slow acceleration, it will have sped up by 24,500 m.p.h. (39,400 km/h) and will be tearing along as fast as any interplanetary ship has ever propelled itself...
There's good reason to spend so much time and money--Dawn carries a $446 million price tag--getting to Ceres and Vesta. The composition and reflectivity of the bodies suggest they were formed within the first 3 million years of the solar system's life, whereas Earth was something of a late arrival, coming along about 27 million years later. A close look at Ceres and Vesta, then, is a close look at a local cosmos that our planet wasn't even around to see. "These two objects are our best opportunity for going back into time," says Christopher...
...ensure that it can uncover the secrets of both worlds, Dawn will ease itself into a six-month orbit around Vesta, then climb gradually back out and fly on to Ceres, which it will orbit for about five months. This is the part that would have been simply too fuel intensive for an ordinary spacecraft. Dawn, by contrast, should have enough xenon left over after its Ceres stay that mission planners might even consider sending it on to a third destination...