NASA's Dawn satellite has reached the asteroid belt. First stop: Vesta.
Vesta and Ceres, the two largest objects in the asteroid belt, have mystified scientists for centuries. With NASA satellite Dawn on final approach to Vesta, the wait is almost over.
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The impact is thought to have spawned a family of asteroids known as V-type asteroids, as well as untold numbers of meteoroids, many of which have landed on Earth.Skip to next paragraph
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Indeed, one in every 20 meteorites that scientists find come from Vesta, says Christopher Russell, a planetary scientist with the University of California at Los Angeles and the leader of the international team of researchers behind the mission.
Analyses of these meteorites have led researchers to conclude that Vesta formed about the same time Jupiter did. Jupiter is widely thought to be the first planet that grew from the disk of dust and gas that surrounded the early sun.
The composition of those Vesta meteorites also suggests that at the time its development was arrested, Vesta already had started to experience enough heating from its interior – and from impacts by other objects – to begin the process of separating dense materials from lighter materials. Like the iron in Earth's core, the heavier, denser materials gravitated toward Vesta's center, while lighter ones rose toward the surface.
That geophysical separation of materials within Vesta has prompted many scientists to promote it from asteroid to protoplanet.
What paused Vesta's development before it could mature from protoplanet to planet? One leading suspect is Jupiter, whose relatively rapid growth and gravitational influence on the neighborhood triggered a bumper-car-like free-for-all in the asteroid belt.
"It would have grown into a planet had it been allowed to continue," Dr. Russell says.
These are among the processes scientists hope to confirm once Dawn arrives next month.
Dawn's next destination: Ceres, queen of the asteroid belt
Ceres, by contrast, is now considered a dwarf planet, a step up in rank from a protoplanet.
Ceres is some 600 miles across and accounts for roughly a third of the mass of all the objects in the main asteroid belt. And unlike Vesta, Ceres is spherical.
Ceres, too, appears to have undergone partitioning of its interior by the density of the materials it contains. Like most of the moons orbiting the gas giants, Ceres is thought to have a rocky core covered by an icy mantle – and, like Saturn's moon Enceladus, may host a mini sea under the ice.
With Vesta and Ceres nothing more than fuzzy blobs from our best telescopes, it's little wonder planetary scientists are eager to visit these objects via an orbiting spacecraft. What they think they know now about these two objects is built on models and out-of-focus images.
Dawn is giving scientists "an unprecedented opportunity to spend a year at a body we really know nothing about," Dr. Raymond says.