Meteor watchers have been anticipating the Leonids for a year, after the 1999 Leonid shower turned out to be a meteor storm for observers in southern Europe and northern Africa. In most years this is a minor shower, but every 33 years or so, the Leonids are capable of creating a spectacular meteor storm. A meteor shower is considered to be a storm when it reaches a rate of one meteor per second, or 3,600 meteors per hour. But in 1999, for the first time in history, a team of two astronomers successfully predicted the exact time of the Leonid storm, missing by only two minutes instead of by hours as previous estimates always had done. They did it by tracing specific paths of debris left behind in space during different passes of the parent comet, Tempel-Tuttle, from which the Leonid meteors come.
Most meteor showers happen when Earth crosses the orbit of a comet.
The meteors are caused by tiny particles of dust and other debris that were released from the comet's nucleus and left behind in space as the comet got closer to the sun. As Earth plows through this stream of particles, each one collides with our upper atmosphere at tremendous speed and burns up almost instantly from friction with air molecules. Understanding of the Leonid meteor shower has greatly improved during the past two years, and the astronomers who so accurately predicted last year's Leonid storm have made new predictions regarding the 2000 and 2001 Leonids. This year they forecast only a modest shower during early morning darkness on Nov. 18, since they have calculated that Earth will pass very close to two trails of cometary debris left behind during the 1733 and 1866 passages of Comet Tempel-Tuttle. But in 2001, they say, Earth will pass right through the position of both the 1733 and 1699 trails, and the moon will be out of the sky, providing perfect viewing conditions for an even greater meteor storm than last year's.