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Vesta's Visitor

Sunday - 7/17/2011, 10:22am  ET

This Week's Astrophoto: NASA's Dawn spacecraft obtained this image with its framing camera on July 9, 2011. It was taken from a distance of about 26,000 miles (41,000 kilometers) away from the protoplanet Vesta. (NASA)

Greg Redfern,

On Saturday, July 16, at approximately 1 a.m. EDT, a visitor from Earth entered orbit around Vesta, the second largest object in the asteroid belt. Confirmation that NASA's Dawn spacecraft had achieved this space exploration milestone was not received until approximately 24 hours later, during a planned communications pass.

It took Dawn 1.7 billion miles in 4 years to reach the gravitational pull of Vesta to enter orbit. Because we do not precisely know the mass of Vesta we cannot specify the time of orbit entry or the orbital parameters of the spacecraft - yet.

Dawn will orbit Vesta for the next year, starting at about 9,900 miles above the surface and work its way down to only 120 miles above the lava covered surface. Cameras and scientific instruments will give us our best ever determination of the composition and structure of a protoplanet - the building block necessary to create planets.

Vesta is 326 miles across and has undergone geological differentiation, a process involving a molten core and creation of geological boundaries - layers - within its spherical boundary which mimic those of our own planet Earth. We actually have pieces of Vesta that reached Earth as meteorites which have confirmed that Vesta has undergone this process.

During the next year we will learn a lot about Vesta, protoplanets and planetary evolution. But this is only half of the story as Dawn will then depart Vesta in July 2012 to travel to Ceres, the largest protoplanet in the asteroid belt at about 600 miles across. From February to July 2015, Dawn will study this dwarf planet which is covered with water bearing minerals and may have a 60-mile layer of water or water ice.

Dawn will provide scientists with data from two distinctly different worlds located at different locations in the asteroid belt. Studying these two worlds will help determine the role of size and water in planetary evolution. Dawn will also add needed knowledge to help prepare for President Obama's call for a manned mission to an asteroid in the coming decades.

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