[ExI] Hubble Images of Asteroids Help Astronomers Prepare for Spacecraft Visit

[Amara Graps]

http://www.nasa.gov/mission_pages/hubble/news/vesta.html

Hubble Images of Asteroids Help Astronomers Prepare for Spacecraft Visit
06.20.07

Images of the asteroids Ceres and Vesta

+ Click for an MPEG of Vesta's rotation

These Hubble Space Telescope images of Vesta and Ceres show two of the
most massive asteroids in the asteroid belt, a region between Mars and
Jupiter. The images are helping astronomers plan for the Dawn
spacecraft's tour of these hefty asteroids.

On July 7, NASA is scheduled to launch the spacecraft on a four-year
journey to the asteroid belt. Once there, Dawn will do some
asteroid-hopping, going into orbit around Vesta in 2011 and Ceres in
2015. Dawn will be the first spacecraft to orbit two targets. At least
100,000 asteroids inhabit the asteroid belt, a reservoir of leftover
material from the formation of our solar-system planets 4.6 billion
years ago.

Dawn also will be the first satellite to tour a dwarf planet. The
International Astronomical Union named Ceres one of three dwarf planets
in 2006. Ceres is round like planets in our solar system, but it does
not clear debris out of its orbit as our planets do.

To prepare for the Dawn spacecraft's visit to Vesta, astronomers used
Hubble's Wide Field Planetary Camera 2 to snap new images of the
asteroid. The image at right was taken on May 14 and 16, 2007. Using
Hubble, astronomers mapped Vesta's southern hemisphere, a region
dominated by a giant impact crater formed by a collision billions of
years ago. The crater is 285 miles (456 kilometers) across, which is
nearly equal to Vesta's 330-mile (530-kilometer) diameter. If Earth had
a crater of proportional size, it would fill the Pacific Ocean basin.
The impact broke off chunks of rock, producing more than 50 smaller
asteroids that astronomers have nicknamed "vestoids." The collision also
may have blasted through Vesta's crust. Vesta is about the size of
Arizona.

Previous Hubble images of Vesta's southern hemisphere were taken in 1994
and 1996 with the wide-field camera. In this new set of images, Hubble's
sharp "eye" can see features as small as about 37 miles (60 kilometers)
across. The image shows the difference in brightness and color on the
asteroid's surface. These characteristics hint at the large-scale
features that the Dawn spacecraft will see when it arrives at Vesta.

Hubble's view reveals extensive global features stretching
longitudinally from the northern hemisphere to the southern hemisphere.
The image also shows widespread differences in brightness in the east
and west, which probably reflect compositional changes. Both of these
characteristics could reveal volcanic activity throughout Vesta. The
size of these different regions varies. Some are hundreds of miles
across.

The brightness differences could be similar to the effect seen on the
Moon, where smooth, dark regions are more iron-rich than the brighter
highlands that contain minerals richer in calcium and aluminum. When
Vesta was forming 4.5 billion years ago, it was heated to the melting
temperatures of rock. This heating allowed heavier material to sink to
Vesta's center and lighter minerals to rise to the surface.

Astronomers combined images of Vesta in two colors to study the
variations in iron-bearing minerals. From these minerals, they hope to
learn more about Vesta's surface structure and composition. Astronomers
expect that Dawn will provide rich details about the asteroid's surface
and interior structure.

The Hubble image of Ceres on the left reveals bright and dark regions on
the asteroid's surface that could be topographic features, such as
craters and/or areas containing different surface material. Large
impacts may have caused some of these features and potentially added new
material to the landscape. The Texas-sized asteroid holds about 30 to 40
percent of the mass in the asteroid belt. Ceres' round shape suggests
that its interior is layered like those of terrestrial planets such as
Earth. The asteroid may have a rocky inner core, an icy mantle and a
thin, dusty outer crust. The asteroid may even have water locked beneath
its surface. It is approximately 590 miles (950 kilometers) across and
was the first asteroid discovered in 1801.

The observations were made in visible and ultraviolet light between
December 2003 and January 2004 with the Advanced Camera for Surveys. The
color variations in the image show either a difference in texture or
composition on Ceres' surface. Astronomers need the close-up views of
the Dawn spacecraft to determine the characteristics of these regional
differences.

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope Science
Institute conducts Hubble science operations. The institute is operated
for NASA by the Association of Universities for Research in Astronomy,
Inc., Washington.

Credits for Vesta: NASA; ESA; L. McFadden and J.Y. Li (University of
Maryland, College Park); M. Mutchler and Z. Levay (Space Telescope
Science Institute, Baltimore); P. Thomas (Cornell University); J. Parker
and E.F. Young (Southwest Research Institute); and C.T. Russell and B.
Schmidt (University of California, Los Angeles)

Credits for Ceres: NASA; ESA; J. Parker (Southwest Research Institute);
P. Thomas (Cornell University); L. McFadden (University of Maryland,
College Park); and M. Mutchler and Z. Levay (Space Telescope Science
Institute)

Space Telescope Science Institute
Baltimore, MD



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Amara Graps, PhD      www.amara.com
Associate Research Scientist, Planetary Science Institute (PSI), Tucson
INAF Istituto di Fisica dello Spazio Interplanetario (IFSI), Roma, Italia