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Comet likely culprit in Tunguska blast

Clouds over London following the event are similar to those formed from today’s shuttle plumes

Night-shining clouds created after space shuttle launches may offer clues into the cause of the Tunguska event, a mysterious blast which rocked southern Siberia more than a century ago.

Thin clouds have appeared at abnormally high altitudes over polar regions following space shuttle launches on several occasions in the past decade. These noctilucent, or night-shining, clouds typically occur in summer and lie at altitudes of about 85 kilometers, in a layer of the atmosphere called the thermosphere, says Michael C. Kelley, an atmospheric physicist at Cornell University. Kelley and his colleagues suggest in the July 28 Geophysical Research Letters that data gleaned from analyses of these high-flying clouds, as well as knowledge about the speed at which shuttle exhaust wafted to polar regions, now hint that the Tunguska blast of June 1908 (SN: 6/21/08, p. 5) resulted from a comet slamming into Earth’s atmosphere.

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