by Brian Handwerk
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.
Tunguska "Afterglow"?
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.
Kelley was studying such clouds in Alaska when the space shuttle Endeavour launched on August 8, 2007.
"We had a huge [cloud] display on the 11th," Kelley said. "For me that really sealed it."
Similar clouds had been spotted in the days following previous shuttle launches, Kelley reports in a paper currently in press, which will appear in the journal Geophysical Research Letters.
A space shuttle's engine, which combines liquid oxygen and hydrogen, produces an exhaust plume of 300 metric tons of water vapor, which reaches 62 to 72 miles (100 to 115 kilometers) during each launch.
For Tunguska, Kelley theorizes that an entering comet shed its icy coating at a similar altitude, releasing similar amounts of water vapor and creating the clouds.
There is, however, a lingering mystery: "How do you get a water vapor plume from Florida to Alaska in a day and a half?" Kelley asked.
One possibility is that the plume got caught in giant, counter-rotating upper-atmospheric eddies that moved it at speeds of nearly 300 feet a second.
The same 2-D turbulence process was at work to create the 1908 British clouds.
Still Up for Debate
William Hartmann, an astronomer at the Planetary Science Institute in Tucson, Arizona, said he was intrigued by the study but noted that the distinction between comets and meteors can be somewhat semantic.
A wide range of bodies likely exists with various proportions of rock, metal, and ice, he said.
The space rock that caused Tunguska could have come from anywhere on such a spectrum and possibly still produced night-shining clouds.
"Noctilucent clouds can be caused by any fine particulate matter deposited at very high altitudes, so that they catch the sun well after sunset," he said.
"Ice crystals are one possibility. But if a weak [carbon-rich] meteoroid, or explosion of [some kind], injects a lot of dust at high altitude, that too could produce noctilucent clouds."
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