PREDICTING how spectacular the annual Leonid meteor shower will be, and when it will hit its peak intensity, is fraught with difficulty. Now astronomers think they know why: Jupiter鈥檚 gravitational pull stirs up the dust clouds that cause the showers.
The Leonid meteor showers happen every year in mid-November, sometimes producing spectacular meteor 鈥渟torms鈥� of more than a thousand meteors an hour. The showers are caused by rock and dust that is continuously shed from a comet called 55P/Tempel-Tuttle. This trail of debris is known as the Leonid meteoroid stream.
Every 33 years, the comet makes its closest approach to the Sun, which heats it up, so it sheds more dense clouds of debris into the stream. The result is a series of huge, dense patches of particles dotted throughout the stream.
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As Earth moves through these dense patches of debris, the particles burn up, producing the meteor storms. But because the planet passes through a different region of the stream each year, it hits different clouds at different times (see Graphic). This year, the planet will move through two clouds 鈥� one left by the comet鈥檚 solar pass in 1767 and the other from 1866.
Now Marco Langbroek, an archaeologist at the University of Leiden in the Netherlands and a member of the Dutch Meteor Society (DMS), thinks he knows why the Leonid showers are so hard to predict. For the past 3 years, he and other members of the DMS have dispersed to Spain, China and the US in November to record the number of meteors visible from each country. Because the observers were effectively witnessing meteors from different parts of debris clouds, they could estimate roughly how large and dense each cloud was.
Langbroek found that the debris cloud left in 1767 was much wider than any left in the 1800s. This unusual spreading out coincides with a well-documented shift in the orbit of the comet. During the early 18th century, the comet passed close to Jupiter, whose gravity shunted it into a different orbit. Langbroek suspects that as Jupiter came close to the comet, its gravitational field also pulled the dust cloud apart, spreading it out far more than astronomers expected, throwing out their calculations of when meteor storms would happen.
Other shifts in the comet鈥檚 orbit have been attributed to Jupiter, suggesting that the planet might also have pulled apart other dust clouds in the Leonid meteoroid stream. Taking the effect into account could help astronomers predict the showers much more accurately. 鈥淚t sounds right, but it鈥檚 not yet proven,鈥漵ays Alan Fitzsimmons, an astrophysicist at Queens University, Belfast.