By DAVID PERLMAN
San Francisco Chronicle
July 09, 2007
Dawn had been scheduled to blast off from Cape Canaveral on Monday. But late last week, NASA announced the spacecraft would take off in September on its long voyage to a deeply cratered space rock called Vesta and then for an intriguingly round dwarf planet named Ceres -- largest of all the hundreds of thousands of objects in the asteroid belt.
Ceres and Vesta are two among the rocky swarms of long ago that might ultimately have become planets, but their development stopped early in solar-system evolution and they never made it, scientists believe.
"It's sort of like an archaeological expedition," said Christopher Russell, a space physicist at UCLA and the principal investigator for the Dawn mission. "We're going back in time some 4.6 billion years so we can try to understand how these early bodies evolved and why they formed as they did when the solar system was very young.
"We want to see what our ancestors were like," he said, "because these are objects that never made it to become true planets, but they're basically the building blocks of the same material that became our Earth."
Vesta, an asteroid shaped a little like a blocky potato, is only 300 miles wide and bone-dry with an iron core, but its southern side holds the remains of a huge crater -- evidence of a violent collision long ago between two large young rocks that might have stuck together to form a planet on their own -- but for some reason never did.
Ceres is round and smooth, and a little more than 600 miles in diameter. Fuzzy images from the Hubble Space Telescope show that it appears to have a thick surface coating of clay with a layer of solid ice beneath -- and from evidence of its mass and density, astronomers deduce that it holds a deep ocean beneath the ice.
When Ceres was discovered in 1701, astronomers decided it was a true planet, but when they began to find more and more chunks of rock -- some large and some small -- in the same neighborhood, they decided to call them all asteroids instead. Ceres was just the biggest asteroid of them all.
Then, a few years ago, came the big astronomical flap about the status of Pluto, long considered the ninth planet of the solar system. When the International Astronomical Union -- arbiter of all such things -- demoted Pluto to the status of a mere "dwarf planet" in August, it also ruled that Ceres was no longer just another asteroid, but promoted it as a "dwarf planet," too.
But Ceres the dwarf has something intriguing: water. Whenever a heavenly body shows any sign of water, scientists instantly think of the possibility of life.
Europa, the icy moon of giant Jupiter, is almost sure to contain a deep ocean beneath its fractured icy crust, and there are even brownish signs along the cracks in Europa's surface ice indicating that organic compounds may lie there. Astronomers are already designing future missions to probe Europa's crust, to reach the ocean below and even to seek for signs of life there.
After a 2 billion-mile flight from Earth, the Dawn spacecraft would arrive at Vesta in 2011. After orbiting that target three times for seven months, plans call for it then to fly on to Ceres -- about a billion miles away -- and arrive there in 2015 for a five-month mission, during which it would orbit the dwarf planet three times, too.
This will mark the first space mission ever to reach orbits in the asteroid belt, and the first to orbit two bodies one after the other after leaving Earth, mission engineers say.
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