Glowing clouds, courtesy of space shuttle

Atmospheric physicist Michael Kelley was long fascinated by records of a powerful glow that illuminated the skies for several nights in the summer of 1908, just after a mysterious explosion leveled thousands of trees in Tunguska, Siberia.

The glow was so strong that for several days, scientists as far as England could read their lab notes at midnight. "This phenomenon was really well known and has never happened since," said Kelley of Cornell University.

Except, maybe in a small way, when the space shuttle takes off.

When the shuttle launched in the summer of 2007, he said, glowing "noctilucent" clouds lit up the skies over Alaska. These icy, high-altitude clouds weren't bright enough to read by, but they created a similar effect, casting the dusky Alaskan midnight in electric blue.

The unusual clouds also appeared in the Arctic after another summer shuttle launch, and near the Antarctic following a winter launch. Kelley said noctilucent clouds are exceedingly rare because the layer of upper atmosphere where they form is usually bone dry.

He calculated that the amount of water vapor ejected from the shuttle's exhaust would be enough to form them, though some kind of wind current seems to propel the clouds toward the poles.

An even bigger injection of water vapor would have blasted into the stratosphere if, as many believe, the 1908 Siberian event was caused by a fragment of a comet exploding aboveground. Most scientists who have studied the event believe it was either a comet or a piece of an asteroid. A few have proposed more bizarre phenomena involving black holes or even aliens.

Kelley's findings back the comet theory. His work was published in last week's issue of Geophysical Letters.

He concedes his idea is unlikely to settle the long debate surrounding the Tunguska event, but he plans to keep strengthening his case.

He said his students will be watching the next shuttle launch scheduled for July, waiting to see if more glowing clouds appear.

- Faye Flam