NASA spacecraft set to probe two planet wannabes

The first clues about what these objects are like came from studies using ground-based telescopes. Estimates of the densities of the two objects suggested that Ceres is about 25 percent water, while Vesta is probably as dry as a bone. Spectroscopic studies indicate that Vesta's surface is covered in volcanic basalts. Indeed, studies show a strong likeness between Vesta's makeup and that of a class of meteorites scientists have picked up in Antarctica and vast deserts. Vesta has an enormous ding at its south pole – a crater some 285 miles across and 8 miles deep. At its center sits a peak that rises 11 miles from the crater floor. The lost chunk could be the source for the meteorites found on Earth.

If Dawn confirms the link, "that would be wonderful," says Thomas Prettyman, a planetary scientist at the Los Alamos National Laboratory in New Mexico and a member of the Dawn team. It would mean that scientists could conduct more-detailed studies of Vesta's evolution as a protoplanet without leaving Earth.

Hints of water on Ceres

Perhaps more intriguing is Dawn's second target, Ceres. It, too, gives hints of undergoing internal changes that sent heavy material to its center and lighter material to its surface. But its nearly spherical shape also hints at a layer of liquid water beneath the crust.

Recent spectroscopic studies found that Ceres's surface is covered with minerals – carbonates and iron-rich clays – that require water to form. Ceres may have an iron core, overlaid with minerals called silicates, then liquid water, then ice, and finally, the crust, a modeling study McCord published two years ago suggests. Or, the core may be made entirely of silicates. Either way, materials are likely to be present – heat, water, organic compounds, and salts – that could yield an environment hospitable for simple forms of life to develop.

"It's fair to speculate that if you have liquid water over a warm core, you might have hydrothermal vents," he says.

As for Ceres's and Vesta's different compositions, that could be a function of their distances from the sun. Ceres is located just across what astronomers dub "the snow line," where sunlight is weak enough for ices and other readily evaporated materials to remain stable. Or Vesta could merely be older, giving its internal heat engine more time to dry things out.

A groundbreaking mission

Whatever the explanation, Dawn is a potentially groundbreaking mission, researchers say. As a technical feat, it's the first craft to orbit two planetary objects back to back. It's also the first formal US science mission to use ion-drive, a kind of electric motor NASA tested during its Deep Space 1 mission, launched in 1998. Dawn will spend 10 months at Vesta, beginning in August 2011, then travel to Ceres for a six-month visit.

"I really think this is going to give us some exciting science," Dr. Prettyman says.

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