A team of
astronomers led by F. Marchis, PI, at the SETI Institute and at UC-Berkeley,
and P. Descamps from Paris Observatory announced recently the discovery of two
moons around an intriguing asteroid. The main-belt asteroid 216 Kleopatra has
two companions.
When
Marchis observed this asteroid for the first time in October 1999 with the 3.6m
telescope at ESO-La
Silla in Chile, he did not know that he was starting a lengthy quest. The
first data recorded with an adaptive optics system, which improves the angular
resolution of the image on ground-based telescopes, reveal that the asteroid
was made of two components. One year later, Steve Ostro, astronomer at the Jet
Propulsion Laboratory, published an article in Science that summarized the
analysis of radar observations that revealed that the mysterious shape of this
M-type, main-belt asteroid. Since then, Kleopatra has been called the
"dog-bone" asteroid. Its weird shape is probably the outcome of an
impact event. The two lobes could be fragments resulting from the disruption of
a parent asteroid that later gently collided to form a dumbbell-shaped body
with overall dimension of 135 miles by 58 miles by 43 miles (217 km by 94 km by
81 km).
To
reexamine this interesting scenario and better determine the size and shape of
this fascinating asteroid, Marchis led a team that obtained telescope time to
observe the asteroid in September 2008. The observing schedule allowed the team
to take advantage of the asteroid's position when it was close to Earth at 1.2
AU. They used the Keck-II telescope,
the largest optical telescope in the world located on the top of Mauna Kea, in Hawaii. The Keck Adaptive Optics system was recently improved and its large 10-m aperture
produces images with an incomparable quality in sharpness (resolution up to
0.035"). Using the time-zone difference, Marchis' collaborator, Pascal
Descamps, located in Paris, France, participated remotely in the observations.
Shortly after the first images were recorded and processed, they realized that
the dog-bone shape model obtained by radar inversion agreed with the direct
images recorded at the telescope. Descamps also pointed out that a tiny 3.1
mile- (5 km-) sized moon was seen on the first images of Kleopatra. Additional
data taken during this eventful night revealed a second fainter satellite (2.9
mile- or 3 km-sized) that was closer to the primary.
Because of
its elongated and bilobated shape, the team expected to detect companions
around 216 Kleopatra. They predicted that the rubble-pile structure of the
primary, linked with its fast rotation (~5 h), could result in ejection of
fragments from the primary after an oblique impact that formed satellites. It
is also possible that these moons are remnant of the catastrophic disruption of
the parent asteroid, which were subsequently captured.
In 2005 the
same team discovered in the asteroid belt the first triple system composed of
two moons around
87 Sylvia. Two years later, a second triple system (45 Eugenia) was
discovered in the main-belt also using an adaptive optics system. These three
systems are strikingly similar since all of them are composed of a large
primary (diameter larger than 54 miles or 100 km) and their km-sized moons
orbit very close to the primary. Without the improvement in image quality
provided by the adaptive optics systems installed on Keck's telescopes, these
multiple asteroid systems would have remained unnoticed.
The team
announced its discovery in an IAU circular 8980 on September 24. If more data
are collected over the next few weeks, it will be possible to estimate the
mutual orbits of the satellites and then infer the bulk density of this
interesting M-type asteroid.
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