KAMUELA, Hawaii — A team of scientists from the W.M. Keck Observatory and
several other research institutions have made the first full-rotational,
ground-based observations of asteroid (511) Davida, a large, main-belt
asteroid that measures 320 km (200 miles) in diameter. These observations
are among the first high-resolution, ground-based pictures of large
asteroids, made possible only through the use of adaptive optics on large
telescopes. This research will help improve understanding of how asteroids
were formed and provide information about their compositions and structures.
Because the asteroids were formed and shaped by collisions, a process that
also affected the Earth, Moon, and planets, these studies will also help
astronomers understand the history and evolution of the solar system.

“Asteroid Davida was discovered 100 years ago, but this is the first time
anyone has been able to see this level of detail on this object,” said Dr.
Al Conrad, scientist at the W.M. Keck Observatory. “With adaptive optics,
we’re finally able to transform asteroids like Davida from a single, faint
point-source into an object of true geological study.”

Ground-based observations of large, main-belt asteroids are made possible
only through a powerful astronomical technique called adaptive optics, which
removes the blurring caused by Earth’s atmosphere. Without adaptive optics,
critical surface information and details about the asteroid’s shape are
lost. The techniques used at the W.M. Keck Observatory allow astronomers to
measure the distortion of light caused by the atmosphere and rapidly make
corrections, restoring the light to near-perfect quality. Such corrections
are most easily made to infrared light. In many cases, infrared observations
made with Keck adaptive optics are better than those obtained with
space-based telescopes.

The observations of asteroid (511) Davida were made with the 10-meter
(400-inch) Keck II telescope on December 26, 2002. Images were taken over a
full rotation period of about 5.1 hours, just a few days before its closest
approach to Earth. At that time, Davida’s angular diameter was less than
one-ten-thousandth of a degree, about the size of a quarter as seen from a
distance of 18 kilometers (11 miles). The high angular resolution allowed
astronomers to see surface details as small as 46 kilometers (30 miles),
about the size of the San Francisco Bay area. The next time Davida comes
this close to Earth will be in the year 2030.

At the time of the observations, Davida’s north pole faced Earth. While
scientists could see the asteroid spinning, only the northern hemisphere was
visible. Yet the profile of the asteroid is far from circular: At least two
flat facets can be seen on its surface. Although scientists knew previously
from light variations that Davida must have an oblong shape, details of that
shape were not available until now. Initial evaluation of the images reveal
some dark features, and scientists are still working to understand to what
extent these are surface markings, topographical features, or artifacts of
the image processing.

“Adaptive optics on large telescopes is allowing us to make detailed studies
from the ground that were previously impossible or prohibitively expensive,”
said Dr. William Merline, principal scientist with the Southwest Research
Institute, and a participant in this research. “We can now make observations
that once required either the scarce resources of space telescopes or
spacecraft missions to asteroids. While these space telescopes and space
missions are still needed for complete study of the asteroids, ground-based
observations such as these will help tremendously in planning the mission
observations and focusing the resources where they will be most effective.”

Asteroids are the collection of rocky objects orbiting between Mars and
Jupiter. They were likely prevented from forming into a planet, partly due
to Jupiter’s massive gravitational influence.

“Although the asteroids began their lives colliding gently, in a way that
would lead them eventually to form a planet, Jupiter’s gravity eventually
stirred up their orbits, and they began to collide at higher speeds,” added
participant Dr. Christophe Dumas, planetary astronomer with the Jet
Propulsion Laboratory.
“These collisions tended to cause them to break up rather than gently stick
together. The resulting fragments, numbering in the hundreds of thousands,
are the asteroids we see today. They collide with each other and have
impacted the Earth, Moon, and planets over time. One need only look at the
scarred surface of our Moon to see the cumulative result. Study of the
asteroid’s shape, size, and surface features helps us understand how these
collisions operate and thus how our planet was, and still is, being affected
by these impacts.”

Observations of the shapes of asteroids, such as those released today, can
tell us about the types and severity of impacts that occurred, and possibly
also give clues into the overall structure of an asteroid — for example,
whether it may be solid rock, or a jumble of smaller rocks. Surface features
can reveal a history of large impacts or variations in the composition that
should, in turn, further help us understand the asteroid’s history.

Asteroid (511) Davida was discovered by R. S. Dugan in 1903 in Heidelberg,
Germany. The (511) in Davida’s name means it was the 511th asteroid to be
discovered and included in the list of asteroids maintained by the
International Astronomical Union.

Team members responsible for the observations are Al Conrad, David Le
Mignant, Randy Campbell, Fred Chaffee, Robert Goodrich, Shui Kwok of the
W.M. Keck Observatory; Christophe Dumas, Jet Propulsion Laboratory; William
Merline, Southwest Research Institute; Heidi Hammel, Space Science
Institute; and Thierry Fusco, Onera, France.

The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy, a scientific partnership of the California Institute
of Technology, the University of California, and the National Aeronautics
and Space Administration.

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Images: http://www2.keck.hawaii.edu/news/asteroid.html

Available for Comment:

  • Dr. William J. Merline, Southwest Research Institute, Boulder, CO; 303-546-9670
  • Dr. Christophe Dumas, Jet Propulsion Laboratory, Pasadena, CA; 818-393-5327
  • Dr. Albert R. Conrad, W.M. Keck Observatory, Kamuela, HI; 808-881-3812