Antiope (top) is a double asteroid, in contrast to Kleopatra (bottom) which is a single connected body. Photo: SwRI

Large telescopes with deformable optics are allowing astronomers to study distant asteroids with unprecedented clarity -- leading to the discovery of new shapes and configurations and presenting scientists with new puzzles to solve.

An international team of astronomers led by Dr. William Merline of the Boulder office of Southwest Research Institute (SwRI) released this week 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 about 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 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."

Image of asteroid 762 Pulcova and its small moon, obtained on 22 February 2000 with the Canada-France-Hawaii Telescope. Photo: SwRI

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.

A fascinating demonstration of the new telescope 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.

This week, Merline and his colleagues reported to an annual 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.

Double asteroid 90 Antiope. Photo: SwRI

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 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.

NASA and the National Science Foundation are funding this research. Observations are being conducted at the Keck Observatory and the CFHT (operated by the National Research Council of Canada, the French Centre National de la Recherche Scientifique, and the University of Hawaii). Other team members are Dr. J. Chris Shelton (Mt. Wilson Observatory) and Dr. David Slater (SwRI, San Antonio).