While the world prepares to welcome Halley's comet, solar-system scientists are keeping an eye on asteroids as well. Next year may bring the first close-up inspection of one of these little known bodies, in addition to probes of the famous comet.
Also, astronomers now suspect that there may not be as much difference as they had thought between asteroids, rocky and iron bodies believed to have formed along with the planets, and comets, largely icy bodies believed to have formed far beyond the orbit of Neptune.
As Dale Cruickshank of the University of Hawaii explained to the recent annual meeting of the Astronomical Society of the Pacific: ``Astronomers are used to thinking of asteroids as one thing and comets as another. It could well be that the comet population is a mixture of things formed from the asteroid belt out to . . . well beyond the outer planets. [Likewise] the asteroids represent things formed all the way into the outer solar system.''
Astronomers believe comets consist largely of dusty ice with perhaps solid, even asteroid-like cores. Comets often form spectacular tails when they approach the sun as their volatile materials evaporate. Dr. Cruickshank said that many asteroids may be comet-like bodies that have been baked free of volatiles because they remain in orbits relatively close to the sun at all times.
Indeed, some asteroids may be ``dead'' comets. Half a dozen asteroids, whose paths cross Earth's orbit, have been identified as traveling in comet-like orbits. Furthermore, Steven Ostro of the National Aeronautics and Space Administration's Jet Propulsion Laboratory reported radar studies that show the Earth-crossing asteroid Adonis has a residue of ice, as might be expected of a former comet.
T. Y. Brooke of the University of New York at Stony Brook reported observations made last Feb. 16 and 17 that suggest Comet Arend-Rigaux has an asteroidal character. Infrared (heat) radiation from that body suggests a composition with less than 30 percent ice, very low for a comet. Dr. Brooke said there was much similarity between Arend-Rigaux and outer solar-system asteroids.
With such similarities between asteroids and comets beginning to appear, B. Ray Hawke of the University of Hawaii noted that it is important to obtain close-up data on asteriods as well as comets to help settle questions of composition. He called the planned mission to inspect Ceres, the largest of the asteroids, in December 1986 ``the right mission'' at ``the right time'' to advance such knowledge.
NASA is scheduled to launch the Galileo mission to Mars next year. If the spacecraft is working well, the agency plans to have it pass close by Ceres and analyze that body's surface.
Meanwhile, measurements made of Ceres's shadow when the asteroid eclipsed a star last November have pinned down its size.
Robert Millis of the Lowell Observatory, reported that the expedition in which he took part measured an average diameter of 945 kilometers with an uncertainty of 6 kilometers. He said this number is unlikely to change significantly as other expeditions report their measurements and the results are pooled. Preliminary analysis by a team from the Massachusetts Institute of Technology and Florida Institute of Technology gives a diameter of 950 kilometers.