ASTEROID A THREAT IN 2028.

Byline: Malcolm W. Browne The New York Times

An asteroid is likely to pass within 30,000 miles of Earth on Thursday, Oct. 26, 2028, and might even hit Earth, the international astronomical agency that tallies the orbits of asteroids and comets announced Wednesday.

Brian G. Marsden, director of the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory in Cambridge, Mass., cautioned in an interview that calculations of the asteroid's progress are approximate and there is no immediate cause for alarm. It is impossible at this time to calculate the odds of an impact, he said. But he appealed to astronomers with large telescopes to begin measurements of the asteroid's brightness and size, estimated to be as large as a mile in diameter, and to refine measurements of its orbit.

There is ample evidence that Earth has been frequently bombarded by asteroids and comets, some of which might have contributed to mass extinctions.

Many scientists believe that the impact of an asteroid or comet some six miles in diameter on the coast of the Yucatan Peninsula 65 million years ago (releasing some 5 billion times more destructive energy than the atomic bomb that leveled Hiroshima) contributed to the extinction of the dinosaurs.

Global effects

The impact of an asteroid one mile in diameter would have devastating global effects, including tidal waves, continent-size fires and an eruption of dust that could cause global cooling and long-term disruption of agriculture. But Marsden said such an asteroid impact would not necessarily be severe enough to wipe out the human race.

The scale of devastation could be gauged from the effects of a more recent impact. On June 30, 1908, a stony meteorite hit near the Tunguska River in Siberia. That object, later estimated as less than 100 yards across, exploded six miles above Earth. It flattened trees over nearly 900 square miles, ignited forest fires, and caused damage equivalent to that of a 15-megaton hydrogen bomb.

The latest threat is believed to be much larger than the Tunguska object.

Asteroid 1997 XF11, as the object is designated, was discovered Dec. 6 by James V. Scotti as part of the Spacewatch program at the University of Arizona. Participating scientists use a 36-inch-diameter telescope equipped with special instruments atop Kitt Peak, Ariz., to maintain a watch for all small objects in the solar system, especially asteroids and comets that approach Earth dangerously close.

Two Japanese amateur astronomers later picked up the object, and on the strength of the combined measurements, the asteroid was added to a list of 108 known ``potentially hazardous objects,'' or ``PHOs.''

Orbit measured

Thereafter, astronomers in several countries refined measurements of the orbit and concluded that the asteroid would come particularly close to Earth in 2028. Their estimate was that it would approach to within a half-million miles - close enough to cause concern.

But on March 3 and 4, Peter Shelus of the McDonald Observatory in western Texas brought a 30-inch telescope to bear on the asteroid, and by tracking it he was able to greatly improve calculations of its orbit. On the basis of overall tracking for 88 days, the asteroid was pronounced likely to approach to within barely 30,000 miles of the center of Earth.

``There is still some uncertainty to the computation,'' Marsden said in his announcement. ``On the one hand, it is possible that 1997 XF11 will come scarcely closer than the moon.'' (The moon is about 240,000 miles from Earth.) ``On the other hand,'' he said, ``the object could come significantly closer than the moon.''

Old photos

The asteroid takes about 21 months to circle the sun and it might have been photographed during previous approaches to the vicinity of Earth, particularly in 1957. Marsden said an early priority will be to search photographic plates for evidence of the object.

``I would say that this thing probably won't hit us. But if its orbit continues to approach us, we should have a better idea before long. In the year 2,000 it will be back, allowing us to improve measurements, and with its next appearance in 2002, we should know how great the danger may be.

``All we know right now is that the asteroid will come interestingly close in 2028 - probably much closer than any asteroid recorded by astronomers in the past.

``We have plenty of time - 30 years, in fact - to improve our knowledge of this thing and take steps, if necessary,'' Marsden said.

The steps to which he alluded might involve detonation of a nuclear device near the asteroid to throw it off course - a solution advocated by Edward Teller, sometimes called the father of the hydrogen bomb. Other scientists believe a relatively small chemical explosion might divert an asteroid safely, provided it was detonated long enough in advance.

CAPTION(S):

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Map: How close is close?

The New York Times