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Dawn departs one asteroid bound for another
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: September 6, 2012


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Gently driven by ionized xenon gas from its electric propulsion system, NASA's Dawn spacecraft departed the giant asteroid Vesta on Wednesday after a year of science observations which unmasked the world's tortured, cratered surface.


Artist's concept of the Dawn spacecraft. Credit: NASA/JPL-Caltech
 
Dawn is now en route to Ceres, an icy, unexplored dwarf planet lying between the orbits of Mars and Jupiter. Powered by highly-efficient ion thrusters, the probe will reach Ceres in early 2015 to begin a mapping and observing campaign.

"As we respectfully say goodbye to Vesta and reflect on the amazing discoveries over the past year, we eagerly look forward to the next phase of our adventure at Ceres, where even more exciting discoveries await," said Robert Mase, Dawn's project manager at NASA's Jet Propulsion Laboratory.

The probe left the gravitational influence of Vesta at 2:26 a.m. EDT (0626 GMT) Wednesday, according to data transmitted from Dawn and received by ground stations on Earth.

Dawn finished its science observations at Vesta on July 25, but its departure from the asteroid was delayed by an anomaly with one of the craft's reaction wheels, which are used to control the vehicle's orientation in space.

It was the second time Dawn had trouble with a reaction wheel. In both instances, on-board software shut down a wheel after it developed excessive friction. The wheels spin rapidly to precisely point the spacecraft in the right direction.

Engineers are evaluating the reaction wheel problem and could institute a fix while Dawn cruises toward Ceres. In the meantime, the craft will use attitude control thrusters to manage its orientation.

Dawn is NASA's first mission to orbit two large bodies in the solar system.


One of the final images of Vesta captured by Dawn's framing camera shows the asteroid's sunlit north pole on Aug. 26. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
 
The spacecraft extensively studied Vesta after it arrived in orbit in July 2011, mapping the surface and charting the history of the giant asteroid, which measures about 359 miles by 285 miles.

"We went to Vesta to fill in the blanks of our knowledge about the early history of our solar system," said Christopher Russell, Dawn's principal investigator at UCLA. "Dawn has filled in those pages, and more, revealing to us how special Vesta is as a survivor from the earliest days of the solar system. We can now say with certainty that Vesta resembles a small planet more closely than a typical asteroid."

Scientists say Vesta and Ceres are relics left over from the birth of the solar system. Formed in the hot soup of rock and gas which coalesced into the planets and moons, the asteroids are cousins of the primordial building blocks which formed Earth, Mars, and other terrestrial planets.

First identified in 1807, Vesta was the fourth asteroid ever discovered.

Before the Dawn probe arrived at Vesta, scientists only knew the asteroid as a fuzzy dot viewed through the apertures of telescopes. They had no idea about Vesta's landslides or lumpy gravity field. Scientists did not even know what Vesta might look like once Dawn arrived in its vicinity.

Dawn approached within 130 miles of Vesta, revealing scarring from mammoth cosmic collisions in the last two billion years. The impacts left ripple-like marks across Vesta's southern hemisphere and ejected material which later fell to Earth as meteorites.

Reconnaissance of Vesta and Ceres could give researchers clues about how Earth and other planets formed.

Ceres - almost twice the size of Vesta - is the largest object in the asteroid belt. Crude imagery and telescopic observations of Ceres indicate water ice may be buried beneath its crust, which may contain water-bearing minerals.

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