Two Japanese amateur astronomers later noticed the object, and on the strength of the combined measurements, the asteroid was added to a list of 108 known ''potentially hazardous objects,'' or ''P.H.O.'s''

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 500,000 miles -- close enough to cause concern.

But on March 3 and 4, Dr. Peter Shelus of the McDonald Observatory in West Texas brought a 30-inch telescope to bear on the asteroid; and by tracking it, Dr. Shelus greatly improved calculations of its orbit. On the basis of overall tracking time of 88 days, the asteroid was pronounced likely to approach to within barely 30,000 miles from the center of Earth at a speed of up to 45,000 miles an hour.

Dr. Marsden, the observatory official, said in his announcement: ''There is still some uncertainty to the computation. 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, the object could come significantly closer than the Moon.''

The asteroid takes about 21 months to circle the Sun and may have been photographed during previous approaches toward Earth, particularly in 1957. Dr. Marsden said an early priority would be to search photographic plates for evidence of the object.

''I would say that this thing probably won't hit us,'' he said. ''But if its orbit continues to approach us, we should have a better idea before long. In the year 2000, 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.''

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