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Radar echoes map asteroids

Bouncing radio beams give scientists a surprising glimpse of near-Earth asteroids.
Model of asteroid 4179 Toutaits
Asteroid 4179 Toutaits tumbles in its four-year-long orbit around the Sun. The object is about 3 miles (4.6 km) long. Radar studies have enabled scientists to create a 3D computer model.
Courtesy Steven Ostro
May 20, 2004
Uing powerful radar observations to produce strikingly detailed images, scientists are just beginning to reveal the menagerie of potentially hazardous near-Earth asteroids (NEAs). A team led by Steven Ostro of the Jet Propulsion Laboratory (JPL) in Pasadena, California, combined the radar muscle of the Arecibo telescope in Puerto Rico with NASA's Deep Space Network antenna in California. The asteroid echoes the group has captured reveal surface details as small as tens of meters. To obtain a similar resolution with optical telescopes would require mirrors hundreds of meters across.

Snapping a series of ghostly images over time allowed the researchers to determine the spin rate of each asteroid. This technique also enabled them to produce geologically detailed three-dimensional models of each object. Over 230 asteroids have been studied with radar so far, and the emerging picture shows a surprisingly diverse set of objects. Their sizes and rotation periods vary by a factor of 10,000, while their composition ranges from stony to metallic, and shapes run from "nearly featureless spheroids" to highly elongated bodies. At the smallest scales, say the scientists, NEAs can be smoother than the powdery surface of our Moon or rougher than the rockiest terrain on Mars.
Model of asteroid 6489 Golevka
Radar observations of asteroid 6489 Golevka reveal it to be unusually angular, as illustrated in this 3D model. The object measures about 1,650 feet (500 meters) across.
Courtesy Steven Ostro
The group's technique allows them to analyze how the radar echoes are distributed in range and frequency, instead of their angular distribution, as in normal optical pictures. As a consequence, in addition to generating detailed images, the JPL team also can measure the distance and velocity of the asteroids with high precision, thus allowing scientists to predict the objects' future orbits accurately. More precise than optical measurements alone, calculations of refined trajectories and collision-probability estimates are now possible, according to Ostro. "These radar-based astrometric measurements can offer us as much as one-third more warning time for any asteroid collision," he says. That could mean decades or even centuries of advance notice before any close approaches.

This detailed radar reconnaissance not only will be used as an early detection system for Earth-orbit crossers, but also will benefit directly any robotic or manned missions to asteroids planned for the near future. The team presented its findings at the 2004 Joint Assembly of the American and Canadian Geophysical Unions, held this week in Montreal, Canada.
Andrew Fazekas is an astronomy columnist based in Montreal, Canada, who frequently writes for magazines, newspapers, and the Canadian Space Agency. He currently does science news commentary for both radio and television, teaches backyard astronomy at Vanier College, and is an editor at the American Association for the Advancement of Science.

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