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       Two NASA-sponsored scientists have produced the first-ever detailed, three-dimensional reconstruction of one of the thousands of asteroids in our solar system whose orbits bring them extremely near to Earth.

       Dr. Scott Hudson of Washington State University in Pullman, Washington, and collaborator Dr. Steven Ostro of NASA's Jet Propulsion Laboratory created the computer model of the double lobed asteroid 4769 Castalia using radar data obtained in 1989 by Ostro and others, using the Arecibo Observatory in Puerto Rico. The asteroid was discovered by Eleanor Helin of JPL at the Palomar Observatory in 1989.

       Hudson and Ostro's computer model and the resulting pictures appeared in the February 18 issue of Science.

       "This computer model of Castalia represents the first detailed, three-dimensional reconstruction of a solar system body from radar data," Hudson said. "It provides our most conclusive evidence to date of a contact-binary asteroid." The effective resolution in this reconstruction is about 100 meters (300 feet).

       At a little under two kilometers across (about a mile), Castalia is smaller than any solar system object for which spacecraft images have been taken -- including the two asteroids, Gaspra and Ida, recently imaged by NASA's Galileo spacecraft.

       Ostro said that previously it was very difficult to interpret radar images of small, irregularly shaped bodies. But with the development of this new reconstruction technique, the scientific value of radar observations has been dramatically enhanced.

       "I hope that the Castalia model will enhance interest in a program of exploration of these small bodies, including both Earth-based observations and spacecraft missions," he said. "A radar-derived model of a target asteroid would make close maneuvering easier, and the mission easier and cheaper."      

       Ostro also noted that the Castalia model verifies the suspicion of many astronomers that the near-Earth asteroids would prove to be the most irregularly shaped worlds in the solar system.

       "Understanding the origins of those shapes, especially the detailed role of collisions, is an important theoretical challenge," he said. The scientists believe that the double lobed shape of Castalia shown by the model resulted from a gentle collision between two separate asteroids some time in the past.

       Nearly 300 near-Earth asteroids are currently known. It is thought that more than a thousand as large as Castalia, plus 100 million as large as a house, remain to be discovered. Most of them are thought to have been thrown into the inner solar system from the main asteroid belt between Mars and Jupiter by long periods of gravitational interaction with the planets.

       With unstable orbits, they might eventually be thrown out of the solar system by the same forces, or possibly collide with planets.

       The scientists believe that continuing improvements in radar telescopes, expanded optical programs to search for near-Earth asteroids and modeling techniques like this one will provide greatly increased knowledge of the properties and histories of these strange nearby worlds.

       The research was part of the Innovative Research Program, the Planetary Geology and Geophysics Program, and the Planetary Astronomy Program of NASA's Office of Space Science, Washington, D.C.


2/18/94 JHW

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