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An asteroid hunt finds mysterious object.

It took astronomer James V. Scotti several nights before he detected a puzzling pattern in a series of images taken three weeks ago with a telescope atop Arizona's Kitt Peak. These images reveal the motion of a mysterious object that in astronomical terms will come a hair's breadth from Earth on Dec. 5 -- passing within 468,000 kilometers, or a little more than the distance between the Earth and the moon.

Could the object, no more than 10 meters across, represent a nearby asteroid? Leftover debris from a space mission? An alien spacecraft? As recently as last week, researchers had ruled out only the last possibility. This week, some say the mystery appears solved.

Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., traced back in time the apparent orbit of the unknown object -- dubbed 1991 VG. His calculations indicate that this body probably constitutes a remnant of the rocket that in December 1974 launched Helios-A, a spacecraft that studied the sun.

Scotti calls the conjecture "circumstantial evidence" that will likely remain incoclusive unless researchers can analyze how the body reflects sunlight--no mean feat since the tiny object, though nearby, appears as only a faint speck of light. Irregularly shaped spacecraft debris should tumble faster than an asteroid, he notes, thus exhibiting greater variations in the amount of sunlight it reflects. A natural body would also absorb specific wavelengths of light and thus be distinguishable from metallic debris when viewed through colored filters.

Such analyses may require a larger telescope than the 0.9-meter Kitt Peak instrument called Spacewatch, used since 1990 by Scotti, Tom Gehrels, David Rabinowitz and their colleagues at the University of Arizona in Tucson to automatically scan the sky for fast-moving objects. Indeed, Marsden notes, the European Southern Observatory in Garching, Germany, and several other research groups with large telescopes plan to monitor the body in coming weeks.

Meanwhile, in hopes of identifying the last known orbit for the Centaur rocket that launched Helios-A, NASA has begun delving into its archives. Researchers speculate that the rocket's upper stage might have escaped Earth's tug and begun orbiting the sun. Says Richard Rast, a Lockhead Corp. scientist based at NASA's Johnson Space Center in Houston, "People pay all their attention to the payload and they just totally forget about what happened to the rocket."

Marsden observes that his tentative identification of the mystery object as space debris may seem "boring" compared with the prospect of discovring a new near-Earth asteroid. "But I think it's brought out the question that we aren't adequately cataloging the artificial junk that's being sent into heliocentric orbit. When something has gone around the sun . . . and has come back to haunt us, I think it's something to think about."

Even if astronomers confirm an earthly origin for 1991 VG, first seen Nov. 6, Spacewatch has already proved its value as an asteroid detector. Last January, astronomers used the telescope, built in 1919, to detect the closest known approach of an asteroid that hasn't crashed through Earth's atmosphere. Known as 1991 BA, that body came within 171,000 kilometers of Earth, or less than half the distance to the moon. Moreover, it measures just 9 meters across, one-tenth the size of any previously detected asteroid. The observation marked the first time that researchers used a sensitive light detector known as a charged-coupled device, routinely employed to image such exotic objects as quasars, to find an asteroid. Rabinowitz and his colleagues report the asteroid's discovery in the Nov. 28 NATURE.
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Author:Cowen, Ron
Publication:Science News
Date:Nov 30, 1991
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