Exactly 101 years ago today, the most mysterious explosion in history occurred over the remote Tunguska region of Siberia. It was so powerful it reportedly knocked people and horses off their feet as far as 100 to 150 miles away. Theories to explain it have included: a giant rocky meteor, a piece of an icy comet, the destruction of a nuclear-powered alien spaceship, the testing of a Tesla particle-beam weapon gone wrong, a piece of anti-matter, a miniature black hole.

But now, a press release from Cornell University announces some new evidence that may finally explain the awesome "Tunguska Event."

The mystery explosion of 1908: At about 7 a.m. on that fateful day, observers over a large region of Siberia saw an object moving across the sky that was so bright, it appeared to be a piece of the sun. The object exploded about 5 miles above ground level, producing a 12-mile-high flame directly visible from as far as about 250 miles away. An estimated 80 million trees felled radially outward for about 30 miles.

The explosion was about equal to that of a 15-megaton nuclear bomb but was more effective in some of its energy output. It was heard more than 600 miles away, and its pressure wave was still detectable by seismographs after the wave circled the world twice.

Meteoroid or comet? The first idea scientists had was that the object that exploded was a huge rocky body from space - perhaps a very large meteoroid or a very small asteroid. But they were puzzled as to why such a large object did not reach the ground (no crater has been found) or at least leave large quantities of fragments (no debris field of meteorites has been identified).

A second idea, one I have always favored, was that the Tunguska object was a sizable chunk of an icy comet nucleus. The comet piece would have approached Earth from the direction of the sun, so it would not have been visible until it blazed in Earth's atmosphere. The initial speed may have been more than 100,000 mph.

The explosion of this giant mass of ice may have been a kind of enormous steam explosion, leaving little rocky debris. But some experts have disputed that a comet piece coming in at this angle and speed could have produced the results observed.

Mystery solved? Now, the Cornell-based team of scientists has provided new evidence that supports the comet hypothesis. Remarkably, the evidence involves Earth's highest clouds and the exhaust plumes produced by a space shuttle launch in 2007.

I'll have the conclusion of this amazing story here next Tuesday.

This week's sights: On Friday night, a big bright moon shines not too far to the right of orange Antares, the brightest star of Scorpius the Scorpion. And outrageously early - at 4 a.m. to 4:30 a.m. - Saturday and Sunday, a lovely triangle of brilliant Venus, greatly dimmer Mars and the tiny dipper-shaped Pleiades star cluster is visible low in the east.

Fred Schaaf is a local author and astronomer. He can be reached at: