As if Scud missiles were not enough. On Jan. 18, the Earth barely missed a direct hit from an asteroid, according to a report released this week by the International Astronomical Union.

Discovered by researcher David Rabinowitz of the University of Arizona, this asteroid appears to have set two cosmic records. Not only is it the smallest such object ever seen, Rabinowitz said, but its approach was the closest to Earth in modern times.

Designated 1991-BA by the astronomical union in Cambridge, Mass., it approached within .0011 astronomical units. An astronomical unit is about 93 million miles, the distance between Earth and the sun.

This means it passed within just over 100,000 miles of Earth, according to the astronomy circular. The moon is about twice that far from Earth.

Astronomers estimate that the asteroid measured between 16 1/2 feet and 33 feet in diameter, or the size of a three-story boulder.

"It was traveling much faster than normal. My first impression was that it was space debris," Rabinowitz said.

He and a colleague, James Scotti, tracked it for five hours. Safely past Earth, the asteroid is now outbound from the sun.

The closest previous sighting of an Earth-grazing asteroid was in March of 1989. It was 330 feet in diameter and passed at 10 times the distance to the moon.

Generally, an Earth-grazing asteroid is hard to anticipate, since the object is hurtling toward this planet and does not reflect sunlight well. Astronomers compare it to using a flashlight to watch a bullet travel in the dark. Rabinowitz used the Spacewatch telescope located at Kitt Peak near Tucson to track the asteroid.

From its velocity, astronomers can calculate the impact of this asteroid had it struck the Earth. Considering its speed of almost 12 miles per second, it might have had the explosive equivalent of 75 kilotons of TNT, according to Geoff Chester of the Smithsonian's National Air and Space Museum.

However, Rabinowitz said, if the hurtling boulder had hit Earth, it might have burned in the atmosphere.