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Astronomers Image Double Asteroid Antiope and a New Moon Of Asteroid Pulcova

Boulder, Colorado – October 26, 2000:   Large telescopes with deformable optics are allowing astronomers to study distant asteroids with unprecedented clarity – two new double asteroids have been discovered with adaptive optics, adding to last year's discovery of the moon of asteroid Eugenia.
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  An international team of astronomers led by Dr. William Merline of the Boulder office of Southwest Research Institute® (SwRI®) released today the first-ever images of a large, double asteroid. Each asteroid in the pair is the size of a large city (about 50 miles across), separated by 100 miles, mutually orbiting the vacant point of interplanetary space that lies midway between them. The discovery was made using the W.M. Keck Observatory atop Mauna Kea, the tallest mountain in Hawaii. The asteroid pair was once assumed to be a single body, called Antiope, orbiting the sun in the outer parts of the asteroid belt between the orbits of Mars and Jupiter.

The team also released a picture of a small moon orbiting the large asteroid Pulcova. This moon was discovered in February 2000 using the Canada-France-Hawaii Telescope (CFHT), also on Mauna Kea. It is only the third asteroid discovered to have a small moon. Asteroid-moon pairs had not been seen until
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1993, when the Galileo spacecraft imaged the one-mile-wide moonlet Dactyl, as it rushed past the 19-mile-diameter asteroid Ida. The Merline team reported the second asteroidal moonlet a year ago, circling the 135-mile-sized asteroid Eugenia. The team named the companion Petit-Prince, officially accepted by the International Astronomical Union in August.
"It's getting to be kind of bewildering," says Dr. Christophe Dumas of the Jet Propulsion Laboratory (JPL), a team astronomer. "Asteroids were once thought to be single, mountain-like chunks of material, perhaps smashed into ‘flying rubble piles’ by occasional collisions among themselves."

Astronomers expect strange new configurations to provide still more surprises as the survey continues. "Every new asteroidal companion we discover seems to bring new configurations and new mysteries," says team member Dr. Clark R. Chapman, also of the SwRI Boulder office.

The team's approach uses a new technology, called adaptive optics, which enables telescopes to see asteroids and other small points of light in the heavens with the same clarity as the Hubble Space Telescope. Until recently, ground-based telescopes were hindered by distortions caused by Earth's atmosphere, in much the same way water distorts the view of an underwater object. The new technique passes light from the telescope through a specialized “correction box” to instantaneously analyze the distorted light and compute the amount of correction necessary to remove the blurring of the atmosphere. The correction information is then fed to deformable mirrors in the box that remove the distortion, providing a sharper image.

This week, Merline and his colleagues reported to an annual
Antiope This raw, unprocessed, adaptive optics image of asteroid 90 Antiope was obtained in August 2000 at the Keck Observatory. The sizes of the two separate components of Antiope are not as large, relative to the 100-mile distance between them as it appears. In a conventional telescopic photograph, the two objects would appear to be one big, blurry blob.
 Image:   W. Merline, SwRI 

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meeting of international scientists specializing in solar system studies on two years of asteroid surveys conducted at three observatories equipped with the new adaptive optics systems.

“In fact, large asteroidal satellites and twin companions are rather rare,” Merline told attendees of the 32nd annual meeting of the American Astronomical Society’s Division for Planetary Sciences, convened this week in Pasadena, California. "Preliminary study of about 200 asteroids has turned up only two asteroids with moons (Eugenia and Pulcova) and just one double (Antiope)," he explains. "It is possible that a few more moonlets might emerge from more sophisticated analysis of the data we have collected."

Pulcova is an asteroid about 90 miles in diameter. Its small satellite, roughly a 10th its size, orbits Pulcova every four days at a distance of about 500 miles.

Asteroidal companions provide vital information about asteroids that has been difficult to obtain. Until now, the best measurements of asteroid masses – their bulk densities, such as whether they are "light" like ice, "dense" like metal, or in between like rocks – came from deflections of spacecraft flying past an asteroid. Such spacecraft encounters are rare, and deflections of more distant
Pulcova Image of asteroid 762 Pulcova and its small moon, obtained on 22 February 2000 with the Canada-France-Hawaii Telescope.
 Image:   W. Merline, SwRI 
objects (other asteroids or planets) by an asteroid’s gravity are weak and difficult to measure. But an asteroidal satellite, or twin, is a body whose trajectory is so mightily deflected by the asteroid’s gravity that it is actually forced to orbit around it. The revolution time provides a measure of the body's mass, hence density. Using such techniques, Merline’s team find that Eugenia, Pulcova, and Antiope are all rather light bodies. They are much less dense than familiar rocks, more like ice, but their surfaces appear very dark, like rock. Interesting differences in the densities motivate further research on asteroids with satellites.
A fascinating demonstration of the new adaptive optics technology is in a movie of the asteroid Kleopatra, also released today, observed during a seven-hour period. 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. "Excellent agreement of both optical and radar pictures of Kleopatra's "dog-bone" shape provides added confidence in the reliability of adaptive optics images," says Menard.

"Radar works well for asteroids near the Earth, but adaptive optics is much more powerful for studying asteroids in the middle of the asteroid belt and beyond," says Dr. Laird Close of the European Southern Observatory and the University of Arizona.
Kleopatra This is a movie of the rotation of asteroid 216 Kleopatra in Nov 1999, when radar data were also beign acquired, that showed the weird 'dog-bone' shape of this asteroid. These data are from CFHT. The asteroid rotates once every about 5 hours.  
(Note that this is not the newly discovered double. It is showing the performance of the adaptive optics technique on a known asteroid, that looks like it is close to being a double, but actually is connected.)
 Image:   W. Merline, SwRI 


Previous News:  
Astronomers Discover Moon Orbiting Asteroid Eugenia, Oct 1999.

W.J. Merline, L.M. Close, C. Dumas, J.C. Shelton, F. Menard, C.R. Chapman and D.C. Slater,
"Discovery of Companions to Asteroids 762 Pulcova and 90 Antiope by Direct Imaging," American Astronomical Society, DPS Meeting 32, #13.06; Bulletin of the American Astronomical Society 32, 1017 (2000)
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