For nine hours last month, a small band of astronomers got an unexpected scare.

Their calculations indicated that a newly discovered asteroid was on a collision course with Earth, and that it had a 1 in 4 chance of striking the planet, somewhere in the Northern Hemisphere, in less than two days' time.

It did not happen, of course. Further observations in the wee hours of Jan. 14 put the asteroid in a completely different orbit, and no threat to Earth.

But the episode, the latest in a series of false alarms, pointed up the disquieting prospect of an asteroid's showing up suddenly on Earth's doorstep with no time for Hollywood heroics. No guidelines exist for who should have been informed and when and what emergency measures should have been taken, if the threat had been real.

At least one planetary scientist said he was ready to recommend a public warning. "I would not have been comfortable with being quiet through the next morning," said the scientist, Clark R. Chapman of the Southwest Research Institute in Boulder Colo., who was involved in discussions of the meteor that night. "I think the public should be informed of that high a probability of that big an event occurring."

Chapman presented a paper on Monday at the Planetary Defense Conference in Garden Grove, Calif., recapping the events in the evening of Jan. 13 through the morning of Jan. 14.

The asteroid, now designated 2004 AS1, was not a planet killer, like the one that caused the extinction of the dinosaurs, or even a city killer. Believed at the time to be about 100 feet across, 2004 AS1 would most likely have exploded with the force of a one-megaton bomb several miles up in the atmosphere. The shock waves would have set off hurricane-force winds that could have damaged buildings below.

"It's right at that boundary line, so we don't know how much damage it would have done," Chapman said. An object half the size would explode harmlessly. One twice as wide would be catastrophic, he said.

Donald Yeomans, head of NASA's Near Earth Objects program office, said he would not have raised an alarm until a second set of observations confirmed the collision path. He said he hoped the episode would prompt guidelines for future warnings.

For several years, NASA has spent $3.5 million a year on surveys to find and map asteroids in Earth's neighborhood, but the program, called Spaceguard, focuses on the larger asteroids, and the presumption that a potential impact would be years away.

"The Spaceguard Survey is not designed to detect objects on their final approach to Earth," said David Morrison, a NASA space scientist.