Fifteen megatons of TNT would set off a blast a thousand times more powerful than the weapon used on Hiroshima. 2000 square kilometers of flattened pine forest near the Podkamennaya Tunguska River in central Siberia bear witness to what such a blast can do. That explosion occurred on June 30, 1908 [original typo said ‘2008,’ an obvious mistake!], and we’re learning more about it. The Tunguska event seems to have been an air blast occurring at an altitude between five and ten kilometers. The presumed cause: A small asteroid slamming into our atmosphere at speeds in the range of 15-30 kilometers per second.

Tunguska damage

But just how big was the asteroid? I’ve seen estimates in the range of 50-100 meters in diameter, but we know surprisingly little about the object. No fragments exist. The effects of the fireball and blast wave are apparent (and there are eyewitness accounts of hot winds and shaking buildings), but there is no crater at the epicenter of the blast. We’re left to calculate the parameters of the object through its effect and its presumed composition and speed as it entered the atmosphere.

New work at Sandia National Laboratories suggests something that is worrisome indeed. I’m going to let Sandia’s Mark Boslough deliver the bad news:

“The asteroid that caused the extensive damage was much smaller than we had thought. That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider. Their smaller size indicates such collisions are not as improbable as we had believed… We should be making more efforts at detecting the smaller ones than we have till now.”

The Sandia work uses a supercomputer to match the known facts with their probable cause. The devastation delivered by the Tunguska object slammed into the ground in the form of a high-temperature jet of expanding gas. The fireball turns out to be more efficient than we realized, creating a blast wave and thermal radiation at the surface stronger than would have been predicted simply by modeling an explosion at the assumed altitude. New models of the fireball show that the energy it transported to the surface is consistent with only a three to five megaton blast, much smaller than originally thought.

So we’re looking at a smaller asteroid to account for Tunguska, which should set off a few alarm bells. For planetary security, think small. Smaller asteroids approach the Earth more frequently than the larger variety. Are we doing enough to track down not just the large potential impactors but those capable of Tunguska-like events? The question is obviously rhetorical at a time when we’re still wrangling over keeping planetary radars like Arecibo’s alive so that they can play their role in such detections. To avoid another Tunguska, we’d better get smart about not just maintaining but ramping up the asteroid search.