Silverpit crater the Chicxulub impact near the northwest corner of the Yucatan Peninsula, which occurred 66 million years ago and probably played a major role in the extinction of the dinosaurs. Several other large impact craters of around the same age have been discovered, all between latitudes 20°N and 70°N, leading to the speculative hypothesis that the Chicxulub impact may have been only one of several impacts that happened all at the same time. The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 proved that gravitational interactions can fragment a comet, giving rise to many impacts over a period of a
David H. Levy biography of Pluto-discoverer Clyde Tombaugh in 2006, and his tribute to Gene Shoemaker in "Shoemaker by Levy". He has provided periodic articles for "Sky and Telescope" magazine, as well as "Parade Magazine", "Sky News" and, most recently, "Astronomy Magazine". Periodic comets that Levy co-discovered include 118P/Shoemaker–Levy, 129P/Shoemaker–Levy, 135P/Shoemaker–Levy, 137P/Shoemaker–Levy, 138P/Shoemaker–Levy, 145P/Shoemaker–Levy, and 181P/Shoemaker–Levy. In addition, Levy is the sole discoverer of two periodic comets: 255P/Levy and P/1991 L3. On February 28, 2010, Levy was awarded a Ph. D. from the Hebrew University of Jerusalem for his successful completion of his thesis "The Sky in Early Modern English Literature: A Study