Dust from long ago made Leonid fireballs.

Last Nov. 16, the night came cool and clear on La Palma, an island off northern Africa stippled with hills--and telescopes. Frustrated by problems in setting up a camera to monitor the 1998 Leonid meteor shower, Alan Fitzsimmons of Queen's University in Belfast, Northern Ireland, exited the domed observatory where he was working and looked up. What he saw dazzled him.

More than 100 times brighter than the brightest stars, hundreds of fireballs streaked across the sky. These brilliant meteors--fiery trails generated when dust particles shed by a comet burn up in Earth's atmosphere--arrived more than 16 hours before the shower's expected peak. Researchers now calculate that the fireballs observed that night came from centimeter-size dust grains expelled by Comet 55P/Tempel-Tuttle when it passed by the sun 665 years ago--in September 1333.

The Leonid shower takes place every November when Earth passes through debris spread out along the orbit of Comet Tempel-Tuttle. About every 33 years, when the comet passes nearest Earth, observers see a much heavier shower. Last year, observers did see a large number of meteors but the fireballs were the standout. Their intensity and duration indicate that Earth must have passed through a dense, narrow strand of dust.

The strand has stayed intact for nearly 700 years because of a special relationship between the orbit of the grains and that of Jupiter, assert David J. Asher and Mark E. Bailey of Armagh Observatory in Northern Ireland and Vacheslav V. Emel'yanenko of South Ural University in Chelyabinsk, Russia. They describe their findings in the April 2 MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY.

Tempel-Tuttle is one of several comets whose orbit about the sun is in synch with Jupiter's. For every 14 times that Jupiter swings around the sun, Tempel-Tuttle makes 5 passes. This relationship, called a resonance, also holds true for large dust grains thrown off by the comet and allows Jupiter's gravity to corral the large grains ejected at different times into distinct, narrow strands. Smaller grains disperse because they are pushed around by the pressure exerted by sunlight.

Asher and his colleagues calculated the motion of dust grains ejected by the comet during its last 42 passes by the sun. They found that an arc of dust from 1333 crossed Earth's orbit at the hour last November when the fireball number peaked.

Dust trails associated with other comets in a resonance with Jupiter may show similar fireball activity, Bailey says. Moreover, some strands of dust might stay intact for up to 50,000 years, long after Jupiter has lured the parent comet into another orbit. Indeed, he notes, the solar system may contain numerous "orphan strands" capable of generating fireballs without evidence of any comet.

Donald K. Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, Calif., says the researchers have demonstrated "that a resonance with Jupiter can preserve the ... strand of large particles over hundreds of years." He adds, however, that their argument "would be more compelling" if they could calculate the intensity of fireball activity for previous approaches of the comet.