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Tunguska Sky looking toward where the comet appeared.

On June 30, 1908 at 7:14 am the Earth, at Lake Chenko, Tunguska, Russia, was struck by something astronomical in origin. You may want to read this Wikipedia article before reading this post’s review, and proposal for my slightly new explanation of what happened. Being an astronomical phenomenon this event should be more properly timed, and located using astronomical terminology: Julian Day and decimal Earth coordinates. 1908ad June 30, 7:14:00am = 2418122.80139 and Lake Cheko which is somewhere in the vastness of Siberia is located inside of the blasted zone at 60.964 101.859. Cut and paste those numbers into a time system like Formilab, and a map system like Google Earth and you will at least be in a proper time, and space orientation to the problem. For a more realistic view in Google Earth, tilt the horizon down by pressing the arrow directly below the N. You can get this view of the impact site as seen from the south.

Tungusk blast zone

Lake Cheko, inside of the Tunguska blasted area.

The authors of the Scientific American article June, 2008 page 82-86, proclaim this Lake Cheko to be the crater formed by the strike of a several-meter-sized solid object which exited, unseen, from the exploding Tunguska fireball. They claim it is the only part of an approximately 10 megaton explosion which survived, and struck the surface. Although the lake looks a bit strangely shaped it doesn’t on first glance appear to be a crater, and there are some strange looking hollows, of approximately the same size, immediately north of the lake. Impact craters of this size are usually much more circular, and have a pronounced pile of rubble around them. The authors have a proposal that the unusual shape, and depth of the lake was created by a methane explosion, and venting created by the falling object piercing the permafrost.

Lonai Crater, India

Lonar Crater, India

A more typical impact crater is found at Lonai, India +19.976 +76.508 which is nearly perfectly circular. This crater is about 50,000 years old, and is roughly the same size and age as Meteor Crater, in Arizona, USA.

Meteor Crator

Meteor Crater, Arizona

Meteor Crater is located at +35.027 -111.022 near Flagstaff, Arizona. These two craters have pronounced rims made up of debris shoved up from the surface by the impact. Over a very long period of time the crater will fill in and the rim will erode away, but both of these 50,000 year old craters still have pronounced depths, and rims and the much younger 100 year old Lake Cheko does not. Thus there must be strong new evidence before it can be considered an impact crater for the Tunguska event. The lake does appear to be quite young, however, because the river carries sediment into the still lake water where it would settle out, and fill in the lake in a moderate number of years.

There are other strange phenomena which must be explained such as the unusual way the forest was blown down by the blast. It formed a ten kilometer wide butterfly shaped pattern of trees blasted down radially away from the center by a near vertical wind that spread out horizontally after striking the surface. This strong wind left trees lying like spokes of a giant wooden wheel radiating away from the axle of the blast — burnt, scorched, stripped of their limbs — but at the center of the blasted zone still standing.

Tunguska event trees blasted down

Tunguska’s trees still standing at the center of the blast.

Soviet experiments performed in the mid-1960s, with model forests (made of matches) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.

In a case like Tunguska it would progress like this:

First the bottom of a falling comet, which raked across the upper atmosphere, would form a brilliantly glowing sheet. Moments later the thicker cross-sectioned middle would strike further along its inertial path, and compress into a thicker, and wider sheet of rapidly decelerating incandescent gas, and finally the thin topmost portion would swiftly pass over this slowed middle portion, and strike the atmosphere. This topmost portion of the comet would strike furthest along the path, but being thinner, and having less mass this portion would be consumed more quickly, and stop glowing sooner. Thus the slowed, but thicker, central portion would still be glowing after the rapidly moving top had passed it by, and stopped glowing. This sequence of events would give the viewers below the seemingly inexplicable appearance of the swiftly moving fireball reversing its direction.

Tunguska comet\'s descent

The comet’s descent from right to left.

Second, a minute or so after the fireball had stopped glowing, the forest below appears to have been struck by a near vertical wind. I estimate the wind to have been about 200 mph because of the way the limbs were ripped from the trees. But, the wind didn’t last very long. If it had, the surrounding trees would have been picked up and hurled into a dense ring of jumbled lumber around the periphery of the blasted zone. From this estimated wind velocity, and the estimated velocity of the comet, the comet’s cross-sectional mass can be roughly calculated. To accelerate the air above the site to about 200 mph by mixing it with my postulated comet moving about 20,000 mph would require the comet’s cross-sectional mass to be roughly 1/100 that of the weight of the air above the site. That is equivalent in cross-sectional mass to about four inches of liquid water. Thus, I estimate the bulk of the Tunguska comet, and probably all comets, to be very tenuous, and less dense than air at sea level. They would be similar to space filled-crystals called aerogels. The Tunguska object itself had to be of a fairly uniform composition and contain no large solid or even liquid objects because nothing was reported to continue flying forward from its “fireball”.

The trees in the forest directly below the fireball were scorched by the incandescent heat of the fireball, but any resultant fire was quenched. Apparently this downward blast of air was saturated with water, and perhaps some carbon dioxide. The carbon dioxide may have helped extinguish the fires, and it may account for the people nearby being rendered temporarily unconscious without being injured. The foul smell may have been the comet itself or more likely that of the broiled forest. Any unconscious people were revived a few minutes later, before they died of suffocation, by the fresh air that would have blown in normally.

The above drawing and paragraphs are derived from Proba 95 which was published in November 1994.

This explanation of the Tunguska events differs only slightly from the British astronomer F.J.W. Whipple’s suggested explanation. The difference is that this scenario postulates a much less dense comet body. The less dense body permits more dragging along the bottom of the comet in the upper atmosphere to make a longer streaking, and the inexplicable reversal of direction at the end of the glowing part of the event which was reported by some observers. Sherlock Holmes always looked for the strange and difficult to explain phenomena, to find the keys to answering difficult problems. The reversal of direction of an object traveling at high velocity is impossible, and the observation is usually discounted as observational error, and not reported but it is the key to what happened, and to the composition of the comet.