Leonids: the coming storm.

Earth is poised to plunge headlong into the most intense interplanetary dust storm it has encountered in 33 years.

A souped-up version of the annual Leonid meteor shower, this storm won't harm Earthlings when it strikes in late 1998 and again in 1999. However, satellites could suffer significant, possibly fatal, electrical damage, says Peter Brown of the University of Western Ontario in London, Ontario.

Leonid meteor showers occur each November, when Earth plows through a broad, tenuous band of dusty debris, or meteoroids, expelled by Comet Temple-Tuttle during centuries of passages near the sun. As the debris burns up in Earth's atmosphere, some 100 meteors an hour may grace the skies.

A Leonid storm occurs once every 33 years, when Earth passes through the meteoroid stream shortly after Temple-Tuttle has neared the sun and spewed fresh particles. On Nov. 17, 1998, Earth will hit the Leonid stream just 9 months after the comet has passed closest to the sun. In that short interval, the torrent of new meteoroids won't have had time to spread out. Our planet will encounter a dense swath of debris, creating a veritable tempest.

At the storm's peak, which may last for just a few hours, observers in the right location--the Far East--should be treated to a terrific light show, with as many as 100,000 meteors an hour streaking through the sky. One year later, an encore performance should thrill skywatchers in Western Europe.

The dust particles are tiny, so chance collisions with spacecraft aren't the prime worry of scientists. Rather, researchers express concern about the potential of these particles to create localized clouds of electric charge, or plasma, that can penetrate satellites and short-circuit equipment.

The high speed of a Leonid meteoroid--about 72 kilometers per second, more than three times that of an average meteoroid--favors the production of clouds of charged material, notes Brown. These can generate lightninglike discharges inside satellites, zapping fragile electric components.

Another meteor storm, this one associated with a swath of cometary debris known as the Perseids, is credited with taking a satellite out of commission in 1993 (SN: 10/2/93, p. 217). However, the potential for damage is highly uncertain, notes Brown. Come 1998, "everyone is going to go through this test, whether they like it or not."

To study the Leonid storm and its parent comet at wavelengths including the ultraviolet, which are undetectable from the ground, Peter Jenniskens of NASA's Ames Research Center in Mountain View, Calif., is proposing to fly a pair of spacecraft during the 1998 and 1999 events. By flying in tandem, the craft should obtain the complete, three-dimensional orbit of the debris. They may also provide a view of a special class of clouds, predicted to condense high in the atmosphere around electrified dust grains from the storm.

Set to take place on a moonless night, the 1998 storm is likely to generate a spectacular light show. The 1999 event, however, promises to be a windfall for lunar astronomers. Encountering Earth during a full moon, the storm will provide researchers with a unique opportunity to observe what happens when meteoroids penetrate the thin lunar atmosphere and pummel the moon's surface. Astronomers have conjectured that meteoroids striking the lunar surface generate the sodium in the moon's atmosphere.