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An international team of astronomers led
by Dr. William Merline of the Boulder office of Southwest Research Institute
(SwRI) has discovered a "moonlet" orbiting around asteroid Pulcova, only
the third such discovery of an asteroid moon. The observations were made
on the Canada-France-Hawaii Telescope. The discovery was announced today
at the annual meeting of the Division for Planetary Sciences of the American
Astronomical Society in Pasadena California. The research team is reporting
on its two-year study of over 200 asteroids, the largest study of asteroids
to ever be undertaken.
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Asteroid Pulcova was observed in February
2000 at the 3.6 m Canada-France-Hawaii Telescope,
located atop Mauna Kea on the Big Island of Hawaii. To make these observations
the telescope was equipped with its adaptive optics instrument, PUEO.
The instrument allows astronomers to compensate for blurring caused by
atmospheric turbulence above the telescope. Using adaptive optics, astronomers
can see detail that would normally appear as a blur of light. The unique
and exquisite image quality achieved by CFHT using adaptive optics allowed
the team to detect a tiny moon - only 15 km across - orbiting the asteroid.
Asteroid Pulcova itself has a diameter of 150 km, and the moonlet orbits
Pulcova at a distance of 800 km.
This is only the third time an asteroidal satellite
has been detected. The first discovery was made during a flyby by the spacecraft
Galileo in 1993. Dactyl, the moonlet, was then seen orbiting around asteroid
Ida. The second discovery was made at CFHT in 1999 when the same team of
astronomers discovered a moonlet orbiting asteroid Eugenia. Since then,
the International Astronomical Union has officially named Eugenia's moonlet
"PetitPrince".
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Image of asteroid 762 Pulcova and its small moon,
obtained on 22 February 2000
with the Canada-France-Hawaii Telescope.
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These discoveries of asteroidal moons from
ground-based telescopes provide astronomers with critical information on
the composition of asteroids. Unlike orbiting telescopes, ground-based
telescopes can be used to observe a single object over consecutive nights.
With these consecutive observations astronomers can measure the orbit of
the moonlet, and from this derive the gravitational pull of the main body.
Once the gravitational pull is know the density of the asteroid can
be calculated. The measurements made on Pulcova confirm that it, like Eugenia,
is a low density object, with a density just barely greater than water.
Both asteroids are therefore likely to be flying "rubble-piles" rather
than solid rock.
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Movie of the asteroid Kleopatra, observed
during a seven-hour period with the CFHT Adaptative Optics Bonnette (AOB).
Here is a fixed view of
all the images used in the movie...
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Discovering asteroid moons is not the only
exciting thing Adaptive Optics can do for the astronomers: It allows to
look at the shape of the asteroids when they are not far from the Earth.
Earlier this year, Steve Ostro of JPL published
reconstructions
of Kleopatra's shape based on radar reflections obtained when that
asteroid was fairly close to the Earth in November 1999. During the
same month, team member Dr. Francois Menard, currently a visiting scientist
at CFHT, obtained adaptive optics images covering a seven-hour period.
Kleopatra measures about 217 kilometers (135 miles) long and about 94 kilometers
(58 miles) wide, and was at around 170 million kilometers from Earth...
The excellent agreement of both optical and radar
pictures of Kleopatra's 'dog-bone' shape provides added confidence in the
reliability of adaptive optics images.
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The team members are:
William Merline, Clark R. Chapman, David Slater-
from the Boulder office of the Southwest Research Intitute (Colorado, USA)
Francois Menard- CFHT
Christophe Dumas- CALTECH/Jet Propulsion Laboratory,
USA
Laird Close- Univ. of Arizona & European
Southern Observatory
Chris Shelton- Mount Wilson Observatory, USA
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PUEO,
the adaptive Optics system, was built in collaboration by the Canada-France-Hawaii
Telescope Corporation, the Dominion Astrophysical Observatory in Canada,
Cilas Corporation in France and Observatoire de Paris, France.
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