NASA Dawn Spacecraft Reveals Secrets of Large Asteroid

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May 10, 2012

Dwayne Brown 
Headquarters, Washington 
202-358-1726 
dwayne.c.brown@xxxxxxxx 

Jia-Rui C. Cook 
Jet Propulsion Laboratory, Pasadena, Calif. 
818-354-0850 
jccook@xxxxxxxxxxxx 

RELEASE: 12-153

NASA DAWN SPACECRAFT REVEALS SECRETS OF LARGE ASTEROID

WASHINGTON -- NASA's Dawn spacecraft has provided researchers with the 
first orbital analysis of the giant asteroid Vesta, yielding new 
insights into its creation and relation to the terrestrial planets 
and Earth's moon. 

Vesta now has been revealed as a special fossil of the early solar 
system with a more varied, diverse surface than originally thought. 
Scientists have confirmed a variety of ways Vesta more closely 
resembles a small planet or Earth's moon than another asteroid. 
Results appear in today's edition of the journal Science. 

"Dawn's visit to Vesta has confirmed our broad theories of this giant 
asteroid's history, while helping to fill in details it would have 
been impossible to know from afar," said Carol Raymond, deputy 
principal investigator at NASA's Jet Propulsion Laboratory (JPL) in 
Pasadena, Calif. "Dawn's residence at Vesta of nearly a year has made 
the asteroid's planet-like qualities obvious and shown us our 
connection to that bright orb in our night sky." 

Scientists now see Vesta as a layered, planetary building block with 
an iron core - the only one known to survive the earliest days of the 
solar system. The asteroid's geologic complexity can be attributed to 
a process that separated the asteroid into a crust, mantle and iron 
core with a radius of approximately 68 miles (110 kilometers) about 
4.56 billion years ago. The terrestrial planets and Earth's moon 
formed in a similar way. 

Dawn observed a pattern of minerals exposed by deep gashes created by 
space rock impacts, which may support the idea the asteroid once had 
a subsurface magma ocean. A magma ocean occurs when a body undergoes 
almost complete melting, leading to layered building blocks that can 
form planets. Other bodies with magma oceans ended up becoming parts 
of Earth and other planets. 

Data also confirm a distinct group of meteorites found on Earth did, 
as theorized, originate from Vesta. The signatures of pyroxene, an 
iron- and magnesium-rich mineral, in those meteorites match those of 
rocks on Vesta's surface. These objects account for about 6 percent 
of all meteorites seen falling on Earth. 

This makes the asteroid one of the largest single sources for Earth's 
meteorites. The finding also marks the first time a spacecraft has 
been able to visit the source of samples after they were identified 
on Earth. 

Scientists now know Vesta's topography is quite steep and varied. Some 
craters on Vesta formed on very steep slopes and have nearly vertical 
sides, with landslides occurring more frequently than expected. 

Another unexpected finding was that the asteroid's central peak in the 
Rheasilvia basin in the southern hemisphere is much higher and wider, 
relative to its crater size, than the central peaks of craters on 
bodies like our moon. Vesta also bears similarities to other 
low-gravity worlds like Saturn's small icy moons, and its surface has 
light and dark markings that don't match the predictable patterns on 
Earth's moon. 

"We know a lot about the moon and we're only coming up to speed now on 
Vesta," said Vishnu Reddy, a framing camera team member at the Max 
Planck Institute for Solar System Research in Germany and the 
University of North Dakota in Grand Forks. "Comparing the two gives 
us two storylines for how these fraternal twins evolved in the early 
solar system." 

Dawn has revealed details of ongoing collisions that battered Vesta 
throughout its history. Dawn scientists now can date the two giant 
impacts that pounded Vesta's southern hemisphere and created the 
basin Veneneia approximately 2 billion years ago and the Rheasilvia 
basin about 1 billion years ago. Rheasilvia is the largest impact 
basin on Vesta. 

"The large impact basins on the moon are all quite old," said David 
O'Brien, a Dawn participating scientist from the Planetary Science 
Institute in Tucson, Ariz. "The fact that the largest impact on Vesta 
is so young was surprising." 

Launched in 2007, Dawn began exploring Vesta in mid-2011. The 
spacecraft will depart Vesta on August 26 for its next study target, 
the dwarf planet Ceres, in 2015. 

Dawn's mission to Vesta and Ceres is managed by JPL for NASA's Science 
Mission Directorate in Washington. Dawn is a project of the 
directorate's Discovery Program managed by NASA's Marshall Space 
Flight Center in Huntsville, Ala. UCLA is responsible for overall 
Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed 
and built the spacecraft. The German Aerospace Center, the Max Planck 
Institute for Solar System Research, the Italian Space Agency and the 
Italian National Astrophysical Institute are international partners 
on the mission team. 

For images and videos related to the findings, visit: 

http://www.nasa.gov/mission_pages/dawn/news/dawn20120510.html 

For more information about the Dawn mission, visit: 

http://www.nasa.gov/dawn 

	
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