A New Companion to Asteroid Pulcova

An international team¹ (including a CNRS researcher) has just discovered
a moonlet orbiting around asteroid Pulcova. The observations were made on
the Canada-France-Hawaii Telescope (CNRS-CNRC-University of Hawaii) thanks
to the Adaptive Optics instrument PUEO² . The announcement will be made
on October 26, 2000 at the meeting of the Division for Planetary Sciences
of the American Astronomical Society in Pasadena, California.

Asteroid Pulcova was observed in February at the 3,6 m Canada-France-Hawaii
Telescope (CFHT), located atop Mauna Kea, Hawaii. This telescope was equipped
with the adaptive optics instrument PUEO which compensates the atmospheric
turbulence that blurs images obtained though ground-based telescopes. Thanks
to the excellent quality of the images, astronomers were able to detect
a moon near the asteroid. The asteroid has a diameter of 150 km and its
moonlet, whose diameter is only 15 km, 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, the first made from a ground observatory, was made at CFHT in
1999 with the same adaptive optics instrument (see CNRS-INFO n°379,
12/99), when the astronomers discovered a moonlet orbiting around asteroid
Eugenia; the International Astronomical Union has officially named Eugenia’s
moonlet "Petit Prince."

These discoveries from ground-based telescopes are crucial, since the latter
can be used to repeat observations over several consecutive days. This makes
it possible to measure the orbit of the moonlet, derive the gravitational
pull between the two bodies and calculate the mass and density of the asteroid.
Thus, recent measurements made by the Canada-France-Hawaii Telescope confirm
that Pulcova, like Eugenia, is a low density object, with a density just
barely greater than water. Thus, it is quite likely that both asteroids
are flying "rubble-piles" rather than solid rock.

1 – The international team includes:
– François Ménard, CNRS, astronomer at the Canada-France-Hawaii
Telescope,
– William Merline, Clark R. Chapman, David Slater from the Boulder Office
of the Southwest Research Institute (Colorado, USA),
– Christophe Dumas of the Jet Propulsion Laboratory (USA),
– Laird Close of the University of Arizona (USA) and the European Southern
Observatory,
– Chris Shelton of the Mount Wilson Observatory (USA).

2 – PUEO, the adaptive optics system, was built in collaboration by the
Canada-France-Hawaii Telescope Corporation, Cilas Corporation in France,
the Dominion Astrophysical Observatory in Canada and the Observatoire de
Paris.

Researcher
contact:
François Ménard
Canada-France-Hawaii Telescope
Tel: (+ 1) 808 885 3158 (until Friday October 27)
After Monday October 30, from 3 PM to 7 AM at the Observatoire du Pic-du-Midi:
tel: + 33 5 62 53 31 20
E-mail: menard@cfht.hawaii.edu