Monday, June 09, 2008

The biggest historic blast, a century later

kw: observations, musings, science, meteors

In 1908, January 30th was a Thursday. This year it will be a Monday, the centennial of the mighty blast that flattened more than 2,000 square km of a Siberian forest north of Tunguska.

The air blast, at a height of (roughly) 8km, was most likely made by a body of low competence (a comet or a loose aggregate of rocky material) about the size of a suburban house. Had the Richter scale been in use, and calibrated seismometers, it would have registered as about a Richter 5 event.

A study in 1961 (reported here) found only magnetite-bearing particles scattered over the area and to the northwest. No larger items have been found. Lake Cheko, 7 km to the northwest, may be a crater from the impact of a portion of the exploding body. If so, it could harbor a meter-sized chunk, which would be well worth recovering.

A recent article in Scientific American kindly provided a sidebar with the coordinates of the blast's epicenter (60°55'N, 101°57'E), and of Lake Cheko (60°57'50"N, 101°51'36"E [corrected]). The yellow dot (a Google Earth pushpin) on the globe in this image shows the epicenter's location.

I've wondered why there was no crater. This is apparently a problem for the scientists also, as rocky bodies in the several-meter size range have plenty of stuff left to make a crater with after passing through the atmosphere, while icy comets tend to explode at heights in the 30-40 km range.

I have seen several bolides, large fireballs that come in at a low angle. Two of them exploded, though one exploded only partially, only to continue for another second or so before exploding again, and completely. The others faded out, and apparently skipped right out of the atmosphere again. All were greenish-blue, and so were most likely cometary. I've seen the same color in the haloes of gassy comets such as Hyakutake. The gas tail of dusty comets is more bluish and very faint.

Regardless, the body is considered anomalous; too hard for a comet, and too soft for an asteroid.

In this image, seen as if from an elevation of 12 km, Lake Cheko is in the upper left corner, and the epicenter is just south of the comma-shaped feature near the opposite corner (The pushpin was objectionably large at this scale, so I left it out; look a little left of Google's Copyright notice). Sometime in July or August, workers who visited Lake Cheko last year plan to visit again to make a detailed study and attempt to recover any impactor. I sure hope they can do so!

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