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:


For more information about the Dawn mission, visit:



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