This is an article, now updated, that I originally posted in 2009 during my first year of blogging. Since then more research has been done on the famous 1908 Tunguska Event and we even had a remarkably similar type of thing occur in February 2013 over the Chelyabinsk area, not too far from Tunguska, but even today, the 108th anniversary of the event, scientists aren’t in agreement over what it was that violently exploded over the boggy forests of Siberia—asteroid or comet.

Long the subject of debate, with various theories ranging from meteorite impact to a comet to a flying saucer’s sudden engine meltdown, there’s actually strong evidence that the 1908 “Tunguska Event” was likely caused by the explosion of a comet in the upper atmosphere.

This idea has been suggested before, and is supported by Cornell University engineering professor Michael Kelly’s study of – strangely enough – the space shuttle’s exhaust plumes and their effect on high-atmosphere cloud formation.

To those not familiar with the Tunguska Event, something exploded or impacted in the remote area of Russia on June 30, 1908, flattening the forest in an 830-square-mile area, but leaving no visible crater or other obvious sign of what caused the event. Nearby residents reported the sound of a massive explosion, but that’s about it. Whatever it was, the devastation it caused was extensive and undeniable.

It has been estimated that the blast was equivalent in power to 1,000 Hiroshima-sized detonations.

Michael Kelly noted that high-altitude clouds, called noctilucent clouds (NLCs) commonly form after the entry of the space shuttle into the atmosphere. These clouds tend to reflect light from the sun long into the night, and were particularly visible on July 1, 1908, one day after the Tunguska event. In fact it was noted that many cities across Europe experienced unusually bright nights. The reflective clouds are made up of ice particles, and may have been caused by massive amounts of water vapor scattered into the cold upper atmosphere from the vapor trail and explosion of a comet, which are typically made of rock and water ice.

See images of NLCs made by the shuttle Atlantis’ launch in 2007 here.

Made up of ice formed on the surface of exhaust and dust particles, noctilucent clouds occur only at high altitudes. The researchers believe that water vapor discharged into the atmosphere by a comet’s icy center was caught up in a vortex of immense energy in a process called two-dimensional turbulence, traveling at speeds of nearly 300 feet per second—which explains why the clouds formed thousands of miles away. “It’s like solving a murder mystery,” says Kelley. “We were finishing a puzzle. The pieces were there, but we put them together.” (Source: Cornell Alumni Magazine)

The comet theory is also supported by the obvious lack of an impact crater…an icy comet would have exploded high in the atmosphere, its ice rapidly boiling and expanding upon entry, and although the shockwave would have been – was – extensive and devastating, there was little left to strike the ground. Anything that was left likely disappeared into the soft peat bogs of the Siberian tundra.

But despite the association with NLCs it’s still not known exactly what to blame for the 1908 event—comet or asteroid. Although continuing research over the years has unearthed evidence for both (with some scientists quite strongly asserting their own findings as conclusive) nothing as yet has proven to be a true “smoking gun” for either option and every finding seems to quickly get its fair share of detractors.

Regardless of an origin of comet or asteroid—or something else entirely—the Tunguska Event has been a staple of pop science enigma for decades and probably will be for many more to come.