Two astronomers have calculated that Earth will cross paths with a dense trail of dust, producing an unexpected meteor shower in the early hours of Friday, December 22.

Peter Jenniskens of the SETI Institute, working at NASA Ames Research Center, and colleague Esko Lyytinen, from Helsinki, Finland, have used skills honed during the 1999 Leonid meteor storm to make the prediction. The shower is expected to hit Earth at 2:29 a.m. Eastern Standard Time (07:29 GMT) on December 22 (11:29 p.m., December 21, PST). Our planet will encounter a dense trail of dust that will make for a good show out of the normally rather inconspicuous Ursids shower.

The Ursid meteors appear to fan out from the constellation of Ursa Minor (the Little Bear) close by the pole star, which accounts for their name. The meteors are dust particles ejected from Comet 8P/Tuttle that slam into our atmosphere and burn up. In its 13.6-year orbit around the Sun, Comet 8P/Tuttle never ventures inside Earths orbit. As a result, its associated meteor shower is usually a poor performer, and would elicit little attention if not for these intense outbursts.

In a paper submitted to WGN, the journal of the International Meteor Organization, Jenniskens and Lyytinen explained the unusual occurrences of the Ursid showers. They found that once the meteoroids are ejected into space, it takes up to 600 years before their orbits are sufficiently perturbed to hit Earth. As the meteor paths change, the particles slowly fall behind the orbiting comet that produced them. After six centuries that lag amounts to six years.

Jenniskens and Lyytinen are now able to identify intense Ursid showers with a particular passage of Comet 8P/Tuttle. The 1945 outburst was caused by dust shed in the year 1392, while the 1986 shower was the result of dust from 1378.

The upcoming shower will be produced by dust ejected in 1405, before the birth of Columbus. The shower is expected to last two to three hours, possibly reaching rates of one meteor per minute. Many of these will be faint meteors, so people are encouraged to go to a dark location for best viewing. The Moon will not be in the sky to interfere with observations.

This is an unexpected bonanza for astrobiologists. The shower will enable researchers to probe the composition and morphology of grains from a comet not previously sampled. The meteorites can be precisely dated just as for the Leonid meteors but they have spent six times longer in the solar system environment and plow into Earths atmosphere with half the speed of the Leonids.