Perseid storm watch: waiting for the light.

On the night of Aug. 11, skywatchers in Asia, Europe, and eastern North America may witness one of the most spectacular displays of celestial fireworks ever recorded. That's because Earth seems poised to plow through a narrow, dense ribbon of the Perseid meteoroids -- dusty debris expelled by Comet Swift-Tuttle over the past several centuries.

A Perseid meteor shower graces our skies every August. Usually the streaks of light aren't as intense or frequent as expected this year because normally Earth passes through a broader, less concentrated stream of the meteoroids that lies farther from the orbit of Swift-Tuttle But last December marked the closest approach of the comet to Earth in its 130-year orbit around the sun (SN: 10/10/92, p.230). And on Aug. 11, Earth may travel through the densest part of the comet's dusty trail. Researchers therefore calculate that a short-lived "storm" of dust particles may enhance this year's Perseid shower, setting the sky ablaze for an hour or two as the meteoroids burn in Earth's atmosphere. The storm may begin around 8:45 EDT.

Meteor expert Bo Gustafson of the University of Florida in Gainesville cautions that such predictions have an inherent degree of uncertainity. Indeed, a team of British astronomers now suggests that although the Perseids may produce a riveting spectacle this year, Earth actually won't encounter the densest meteoroid swath until next August. Nonetheless, the fear that dusty debris could damage one of its spacecraft prompted NASA late last week to make an unprecedented decision. For the first time, the space agency delayed the launch of a craft - in this case the space shuttle Discovery -- to avoid a meteor storm.

The shuttle, which NASA planned to launch Aug. 4 on a nine-day mission, would have reentered the atmosphere at the height of the predicted meteor storm. NASA has now rescheduled the launch for Aug. 12, the day after the expected storm.

According to Donald J. Kessler, a senior scientist for orbital debris research at NASA's Johnson Space Center (JSC) in Houston, he and his staff used a computer program to calculate the impact of the meteor storm on critical parts of the shuttle, including its hydraulic lines and the thermal coating of its wings.

They based their calculations on the memorable Leonid meteor storm of Nov. 17, 1966, the most recent such event. Their analysis indicated that there might be twice as much debris in this storm as in the Leonid, an amount the shuttle could nevertheless withstand. NASA considered launching the shuttle on schedule but orienting it so that its tail, which is less vulnerable, would face the storm on reentry. But in the end, the scientists emphasized the uncertainties in their analysis, and NASA took a more cautious tack, says astronomer David Talent, a consultant at JSC.

"If it were my own personal plane, I might have decided to fly it," says Talent. "But this is a national asset. NASA decided it would rather be safe than sorry."