Tunguksa Blast Mystery Solved by Space Shuttle?

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(See a picture of night-shining clouds over Hungary.)

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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