Comet At Tunguska

One of the enduring mysteries of the twentieth century concerns a horrific explosion over Tunguska in 1908 that leveled 2,000 square kilometers of Siberian forest, but left no crater. Had the explosion occurred over Europe, for instance, the loss of human lives would've been staggering. Had it happened sixty years later, it could easily have triggered nuclear war.

Scientists have narrowed the causes of Tunguska to either a comet or an asteroid exploding in Earth's atmosphere. New evidence may tilt that towards a comet.

Noctilucent clouds are the highest clouds produced in Earth's atmosphere, and physicists are just now beginning to get a handle on them. They consist of water vapor, and are very bright visually. Space shuttle launches, in fact, can trigger formation of these clouds because 97 percent of the exhaust from shuttle engines is water vapor, and some of it is released at very high altitude.

After the Tunguska event, northern Europe experienced several "bright nights"-- nights that didn't get entirely dark. Some scientists now suggest that could have been caused by bright noctilucent clouds high in the atmosphere that were fed by water vapor from whatever exploded. If that were the case-- a comet carries more water than an asteroid.