For the fifth time in two years, astronomers have discovered an asteroid hurtling through space that might collide with Earth.

The likelihood of collision is considered slim: one chance in a million. While the projected date of any impact is 2022, astronomers say additional observations are needed to calculate the orbit of the asteroid better and to rule out a collision.

The asteroid, 2000 BF19, is about half a mile wide, relatively small by cosmic standards, and if it struck Earth could do tremendous damage to part of the planet but would probably not cause planetwide destruction.

It was discovered by three astronomers led by Dr. James V. Scotti using the Spacewatch Telescope on Kitt Peak in Arizona. They spotted the object on Jan. 28, and it was tracked until Feb. 3, when it disappeared. On the same day, the International Astronomical Union by the Minor Planet Center of Smithsonian Astrophysical Observatory in Cambridge, Mass., announced the discovery of the asteroid. The announcement did not mention a possible collision but suggested that the asteroid should be watched.

The chance of a collision was raised yesterday by Dr. Benny J. Peiser, who runs the Cambridge-Conference Network, or CCNET, an Internet newsletter on astronomy.

''The current impact probability of one in a million can easily and quickly go up or down,'' Dr. Peiser wrote. In all likelihood, the asteroid will turn out to be of no risk, he said.

''Only further observations can provide us with the information to assess the potential danger,'' he added.

The calculations of the current risk and the projected impact date were done by Dr. Andrea Milani, an astronomer at Pisa University in Italy, Dr. Peiser said. The asteroid was expected to pass near Earth every 11 years, next in 2011.

''There are a wide variety of orbits that can fit the observations,'' Gareth V. Williams, an astronomer at the Smithsonian Astrophysical Observatory, said in explaining the collision uncertainty. Only three groups, including an amateur astronomer, have been able to track the asteroid.

''The only people who should be concerned are astronomers who should be trying to get additional observations,'' Dr. Williams said. ''Since the arc of observation is so short, it is highly possible that any additional observations will be sufficient to reject the 2022 impact solution.''

Dr. Williams said that in other instances, follow-up observations have allowed astronomers to rule out collisions. In one case observations were too limited to let them do so before the object disappeared.

For 2000 BF19, he said, with the asteroid fading fast, a clarification might take years rather than weeks.

''It's a hefty object,'' Dr. Williams said. ''We would not like an object of this size to hit us.''