Night-Shining Clouds
Tracking plumes from space shuttle launches provided researchers with one of the strongest pieces of evidence that a comet crash was responsible for flattening a Siberian forest in 1908.

The crash, which leveled trees for hundreds of miles in Siberia, was followed by the appearance of extremely bright clouds, visible by night.

Similar clouds triggered by the flights of space shuttles through atmosphere were found over the planet's poles two days after a launch from Florida, research published in last week's Geophysical Research Letters shows.

The creation of so-called noctilucent, or night-shining clouds from water vapor in shuttle rocket plumes buttresses the theory that the clouds spotted after the 1908 impact were triggered by similar atmospheric dynamics, with the water vapor coming from a comet, lead researcher Michael Kelley told Discovery News.

"The shuttles put 300 metric tons of water vapor at the same region that a comet would," said Kelley, a professor at Cornell University.

The cause of what has been referred to as the "Great Siberian Impact Event," or the "Tunguska Event," has been debated for decades. No meteorite remains have ever been found and the appearance of the mysterious night-shining clouds led many scientists to suspect a comet was responsible.

"It's kind of important that the world got hit by a comet, since Jupiter just got hit again," Kelley said.

Astronomers have been watching an Earth-sized gouge in Jupiter's atmosphere believed to be caused by a comet impact on July 19.

Noctilucent clouds, which form at the edge of space 62 to 68 miles above the planet, are about 10 million times brighter than ordinary clouds. They contain electrically charged ice.

"It's a very difficult region to access," said Clemson University's Miguel Larsen.

The shuttle plumes, which are laced with iron atoms and iron ions, provided researches with a way to track how water vapor is transported by winds and other dynamics of the upper atmosphere.

Noctilucent clouds, which have been increasing in appearance, also can serve to gauge if efforts to mitigate global climate changes are successful, Kelley added.

"It's thought that these clouds are like a miner's canary of global change," he said. "The atmosphere there is so tenuous its responsiveness to a mitigation technology would be easy to detect."