Scientists may get one of their last chances today to solve the stubborn puzzle of a tiny, unidentified object now more than a half-million miles from Earth.
Steve Ostro, an astronomer with NASA, will try to use radio telescopes in its Deep Space Network in Goldstone, Calif., to bounce radar waves off the 30-foot-long object, which is drifting behind the planet like a cork in the wake of an ocean liner.
Donald K. Yeomans, another scientist with the National Aeronautics and Space Administration, said Dr. Ostro has also arranged to try again Dec. 20, using the world's largest radio telescope in Arecibo, Puerto Rico.
Astronomers hope the observations will finally settle the question of whether the object -- which is circling the sun in an orbit slightly larger than that of the Earth -- is a peculiar asteroid or a piece of man-made space trash.
But they add that it may never again come close enough to be spotted by instruments on Earth. In fact, it may already be lost.
Since the object was first discovered Nov. 6 by James V. Scotti of the University of Arizona, some astronomers have insisted it must be part of a rocket that somehow reached solar orbit. Others, including Mr. Scotti, argue it is probably an asteroid, the first found with an orbit so closely mimicking this planet's.
Hoping to resolve the debate, astronomers trained powerful telescopes on the object Dec. 2, when Earth overtook the object, coming within 288,000 miles.
Astronomers at the observatory at Kitt Peak, Ariz., saw little change in the object's brightness over short periods of time, which is consistent with an asteroid. That convinced some at Kitt Peak and at the Jet Propulsion Laboratory in Pasadena, Calif., that it was a natural object.
But the same night, astronomers at the European Southern Observatory in La Silla, Chile, saw a rapidly blinking object, consistent with a shiny piece of spacecraft reflecting sunlight as tumbles rapidly.
NASA says Dr. Ostro hopes to be able to locate the object -- called 1991VG -- and, based on its radar image, decide how smooth it is. If it is very smooth, then it is probably man-made.
Brian G. Marsden, director of the International Astronomical Union's Central Bureau for Astronomical Telegrams in Massachusetts, said his best guess is that the object is the upper stage of a Centaur rocket that carried the Helios-A satellite into solar orbit in December 1974.
But Dr. Yeomans of the JPL said that if the object is man-made, it is more likely to be the upper stage of an Apollo spacecraft that was orbiting the Earth but somehow was catapulted into a solar orbit, perhaps by the moon's gravity.
o "I would argue that it's probably an asteroid because there are hundreds of thousands of asteroids up there and only about 100 spacecraft," he said.