Asteroid Davida
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.
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.
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.