NASA's Dawn spacecraft aims for the curious Vesta and Ceres
A half-dozen spacecraft launched by the United States and Europe have flown past or landed on asteroids. A Japanese mission that attempted to collect samples of an asteroid is due back on Earth in three years.
But NASA's Dawn spacecraft, scheduled for launch near dawn Thursday, is designed to bring a new day to asteroid science.
Dawn will aim for the solar system's vast asteroid belt, a collection of rocky materials left over from the formation of the planets, for closeup studies of Vesta and Ceres, two of the largest celestial bodies in the belt region.
Scientists expect Dawn's eight-year, $449 million mission to provide new insight into the early phase of the planet-building process that began more than 4.5 billion years ago.
Dawn was designed to steer into orbit around Vesta to map the terrain and study the mineral makeup, then depart for Ceres to conduct a second orbital reconnaissance.
Just hints so far
The spacecraft is scheduled for launch from Cape Canaveral Air Force Station, Fla., aboard a Delta II rocket.
If successful, Dawn will become the first spacecraft to orbit a solar system body and then travel through space to circle another.
"To go to one body, leave and go to another is sort of what science fiction has always been about," said Christopher Russell, the University of California, Los Angeles space physicist who serves as the Dawn mission's chief scientist.
The Hubble Space Telescope and other powerful observatories offer only scant clues of what Vesta and Ceres must be like, revealing hints of past volcano activity on the first and water on the second.
Most of the asteroid belt orbits the sun between Mars and Jupiter.
"So, we are going out to two bodies, which are quite different as far as we can tell. One seems dry like our moon. The other seems to have a lot of water in it," Russell said. "On our planet, water is very important. We will try to understand why some bodies are very wet out there and some very dry."
Plans to launch the mission earlier this year encountered a series of problems with the assembly of Dawn's rocket launcher, bad weather and difficulties establishing a ground tracking network.
The first stop on Dawn's journey is Vesta, an asteroid the size of Arizona that offers a glimpse at the processes that produced the solar system's rocky inner planets - Mercury, Venus, Earth and Mars.
Ceres, the final stop, is about the size of Texas and was recently reclassified from asteroid to dwarf planet by astronomers. Ceres, which may have a thin atmosphere, could offer clues about the processes that folded water-born minerals into the final assembly of the icy moons of the outer planets - Jupiter, Saturn, Neptune and Uranus.
Far-off sling shot
Scientists believe the thousands of rocky objects in the asteroid belt were once destined to collide and clump together until they became a full-fledged planet. However, the assembly process was interrupted by the strong gravitational forces of Jupiter, the solar system's biggest planet.
Dawn's liftoff will initiate a 3.2 billion mile journey that will swing the spacecraft into orbit around Vesta in August 2011.
After circling Vesta for seven months, Dawn will depart for Ceres. The final leg of Dawn's long journey will take nearly three years. Reaching Ceres in February 2015, the probe will orbit for at least six months of observations.
In April 2009, Dawn will speed close enough to Mars for the gravitational field of the Red Planet to sling the spacecraft outward with additional velocity.
Ceres was discovered in 1801 by the Italian astronomer Giuseppe Piazzi. Largest of the planetary fragments in the asteroid belt, Ceres is curiously planetlike. Ceres is spherical and possesses enough of a gravity field to pull the heaviest of its minerals to the core. It may have a weak atmosphere, a thick layer of water frozen below a dusty surface and perhaps frost-covered polar caps.
Last year, those qualities convinced the International Astronomical Union to upgrade Ceres' status from asteroid to dwarf planet - the same reclassification that resulted in the more controversial demotion of Pluto from planet to dwarf planet.
Vesta was discovered a half-dozen years after Ceres by the German astronomer Heinrich Wilhelm Olbers. Distant observations reveal a somewhat spherical shape with a surface of frozen lava that oozed from a hot interior shortly after Vesta formed.
At Vesta's south pole is a large crater that was gouged out by a collision with another asteroid. Some of the material blasted away from Vesta by the powerful impact may have reached the Earth as meteorites.