Optical image of the periodic comet Swift-Tuttle (bottom left) taken on November 19th 1992. It has been rediscovered exactly where the astronomer B. Marsden had predicted in 1973. The comet, the debris of which causes the Perseid meteor shower, was telescopically discovered in 1862. The first calculations indicated a period of about 120 years. Marsden recalculated the orbit considering the perturbations caused by the 'rocket effect' of gas escaping from the comet. He proposed a new period of about 130 years and predicted (rightly) its next appearance in 1992. It has been suggested that the Swift-Tuttle comet could hit the Earth in its next return in 2126.

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