Every year, around mid-November, the Earth crosses the orbit of a comet called Tempel-Tuttle and passes through debris the comet has shed. This burns up in the upper atmosphere as a meteor shower. Every 32 to 33 years, the Earth runs into an especially dense cloud of debris, turning the shower into a storm. At the peak of the last storm, in 1966, the skies above North America were lit up by 5000 meteors in just 20 minutes.
Astronomers are now bracing themselves for the next Leonid storm, predicted to reach a peak around 17 November. Communications and other satellites could be threatened by the bombardment -- and both NASA and the Russian Space Agency have postponed launches until the danger has passed.
No one knows just how bad the damage will be. For example, astronomers can't predict with certainty exactly where the densest part of the debris cloud is. Now Duncan Steel, an astronomer with Spaceguard Australia in Adelaide, has thrown another variable into the equation. If his model of the chemical composition of the Leonid meteors is correct, attempts to observe the approaching meteors may detect only a few per cent of them.
Steel says that data gathered during the recent visits by comets Hale-Bopp and Hyakutake reveal that the dust these comets gave off was rich in volatile organic compounds. If the same is true of the cometary debris that forms the Leonids, most of the meteors may be invisible. This is because if they are made of highly volatile material, many will burn up at relatively low temperatures -- too low to leave behind glowing trails detectable from the ground. Cool-burning meteors will also emit relatively few electrons, and that will make them invisible to ground-based radars, which can only spot electron-dense trails.
"If small meteoroids in storms are largely composed of organics, then none of the data collected to date gives a realistic assessment of the hazard level," says Steel, whose conclusions are published this week in the journal Astronomy and Geophysics (vol 39, p 24).
Current estimates put the risk of a serious impact between a meteor and a large satellite at about one in a thousand. Steel says his study suggests that this "seriously underestimates" the hazard. "If I am right, the economic loss caused by the Leonids may be immense," he says.
Other astronomers agree that the reliability of the storm predictions depends crucially on the composition of the meteors. "Steel's paper is very interesting -- though whether it is actually correct is another matter," says Iwan Williams of Queen Mary and Westfield College, London. "We may know after the Leonids next month."
Steel's advice is not to rely too heavily on satellite communication and navigation systems in the coming month. "I would not depend for my life on the Global Positioning System being fully functional on 18 November," he says.
An expanded version of this article was published in New Scientist issue 3rd October 1998
Copyright: New Scientist