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Leonid meteors. Composite image of meteors during the Leonid meteor shower. This annual meteor shower occurs around the 17th November each year. Meteors are streaks of light in the sky caused by the burning up of tiny dust particles in the upper atmosphere as they fall to Earth. This shower is caused by the Earth passing through the stream of debris left in the orbit of the comet Tempel- Tuttle. The comet has a 33-year orbital period, and every 33 years the Leonids peak from their usual rate of around 12 meteors per hour to thousands. This image was taken in 1998, one year before the shower's peak in 1999. (KEYSTONE/SCIENCE PHOTO LIBRARY/TONY & DAPHNE HALLAS)