for National Geographic News
Space shuttles blasting off from Earth may have helped solve the mystery of what came careening down from space to explode over Russia in June 1908.
The so-called Tunguska event leveled 770 square miles (2,000 square kilometers) of forest in a remote area of Siberia.
What caused the blast has puzzled scientists, because only a handful of people saw the explosion and it left no easily recognizable debris.
The leading theory has been a mid-air explosion of either a rocky meteor or an icy comet that rocked the region with the force of several hundred atomic bombs.
Now studies of so-called night-shining clouds sometimes linked to space shuttle launches suggest that it was, in fact, a comet that caused Tunguska.
Atmospheric scientist Michael Kelley of Cornell University first noticed a potential link between the Tunguska event and night-shining clouds decades ago as he was combing through historic scientific papers.
"Several British scientists commented that three days [after Tunguska] they could read a newspaper at midnight in England," Kelley said.
Around the same time Kelley had begun studying night-shining clouds, and it occurred to him that such features could have caused the strange afterglow.
Night-shining, or noctilucent, clouds form only in the high, cold skies on the edge of space, when water vapor condenses around dust particles and freezes into tiny ice crystals.
The clouds shine because they are high enough to be lit by the sun from below while an observer stands in twilight.
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