Boulder, Colorado — October 19, 2001 — An international team led by
scientists at the Southwest Research Institute (SwRI) has discovered some
curious properties of the largest asteroid, Ceres. The astronomers observed
Ceres with the Hubble Space Telescope (HST) at ultraviolet wavelengths using a
resolution higher than previously attained. The resulting images are the first
to resolve detail on the surface of Ceres and show features as small as 50
kilometers across.

Led by Principal Investigator Dr. Alan Stern of SwRI, the team detected a
dark spot on the surface of Ceres, which it nicknamed "Piazzi" in
honor of the discoverer of Ceres. "Although we can’t determine the nature
of the spot with these data, whether it is an area of different coloration or
possibly a crater from an impact by another asteroid, it is pretty big,"
says Dr. Joel Parker, also of SwRI, who led the team in the analysis of the
images. "The Piazzi feature has a diameter of about 250 kilometers, which
is more than a quarter the size of Ceres. If it resulted from an impact, the
object that hit Ceres would have been about 25 kilometers across. It must have
really shaken things up."

The high-resolution images allowed the team to refine measurements of Ceres.
Although Ceres is the largest known asteroid — estimated to contain more than
one-third of the total mass of all other asteroids combined — researchers still
dispute its size, even after 200 years of observations. The new HST measurements
indicate that the asteroid is slightly flattened, with a diameter ranging from
930 to 970 kilometers. Spinning objects can have a flattened or
"squashed" shape depending on how big they are, how fast they spin,
and what kind of material they are made of. However, the amount of flattening
seen on Ceres is more than expected and may indicate that the inner structure is
not as homogeneous as previously assumed.

"These results are very tantalizing," says Stern. "What we
need to be definitive are observations with better resolution and frequent
enough to follow Ceres through a nine-hour rotation period to track surface
features. This ‘movie’ would allow us to finally map the surface of Ceres and
figure out what the Piazzi feature is." The team has already proposed such
an experiment with a new instrument to be installed on HST next year.

The analysis of the Ceres images will be published in the January 2002 issue
of The Astronomical Journal. Authors include researchers from SwRI, the
Massachusetts Institute of Technology, Cornell University, the University of
Arizona, and the Observatoire Midi-Pyrénées in France.

In addition to being the largest asteroid, Ceres was also the first asteroid
to be discovered. In the latter part of the 18th century, astronomers noted a
regular spacing in the planets of the solar system, but with a gap between Mars
and Jupiter where they expected to find a planet. On January 1, 1801, the
Sicilian astronomer Giuseppe Piazzi at the Palermo Observatory discovered a
moving object in the region. Researchers at the time assumed that this object,
Ceres, was the missing planet. However, early observations indicated that Ceres
was too small to be a planet, and as more such objects were discovered in the
region, they became known as "asteroids" or "minor planets."
Ceres orbits the sun once every 4.6 years at a distance of 41 million
kilometers, and it spins on its axis once every nine hours.

EDITORS: The Ceres images are available for viewing and download at

SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,700 employees and an
annual research volume of more than $315 million.

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