Monday, June 30, 2008

Tunguska

Monday is the 100th anniversary of the Tunguska event. On June 30, 1908 a mysterious visitor from Space entered Earth's atmosphere and exploded in the skies above remote Siberia. 100 years later there are still competing theories what it was: meteorite, comet, even a tiny Black Hole. Humans are still woefully unprepared to deflect such objects. In the case of Black Holes, most scientists don't admit they could exist in our Solar System. Concerning asteroids, humans will venture to them asteroids or they will venture into us.

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5 Comments:

Blogger Kea said...

Is it true that Cassini has found evidence for hydrocarbons on Saturn? That would be quite a kick in the face of Old Round Earth theorists.

5:56 PM  
Blogger nige said...

This comment has been removed by the author.

6:25 AM  
Blogger nige said...

It was most likely a stony asteroid. The effects, including the blow-down of all forest trees to a mean radius of 18 km and moderate damage out to 26 km, and the lack of a characteristic explosion crater at the hypocentre, suggest a 10 megaton of TNT-equivalent explosion air burst at about 10 km altitude. See Christopher F. Chyba, et al., "The 1908 Tunguska explosion: atmospheric disruption of a stony asteroid", Nature, v. 361 (1993), pp. 40-44.

Stony asteroids are massive boulders which heat up and explode in the atmosphere before they can reach the ground. The earth orbits the sun at 29 km/s so it's easy for a 1 metric ton meteoroid to deliver an impact explosion energy of E = (1/2)mv^2 = 0.5*1000*29000^2 = 4.2*10^11 Joules, i.e. the equivalent of 100 metric tons of TNT (0.1 kt of TNT explosion energy = 10^12 calories = 4.2*10^12 Joules).

Hence, since the Tunguska explosion was 10 Mt TNT equivalent in yield, if it involved a 29 km/s impact, then the object had a mass of 10 Mt / 0.1 kt = 100,000 metric tons, which for a density of 2000 kg/m^3 implies a asteroid radius of 126 metres. There are some variations, however. The smallest impact velocity likely is the earth's escape velocity of 11.2 km/s (a body falling towards the earth from a great distance in space in the absence of other effects acquires a velocity equal to the escape velocity, because of its change in gravitational potential energy). The biggest impact velocity is likely to be 42.1 km/s which is the speed of a body falling into the solar system towards radially inwards the sun from a great distance, and chancing to strike the earth. So the margin of uncertainty in impact velocities is not massive, just about a factor of two or so either way.

Iron meteorites have high thermal conductivity so they don't explode, and can reach the ground intact apart from minor surface ablation.

Comets, containing a lot of water, tend to explode at much higher altitudes and would therefore have affected a larger area but with lower overpresure damage than actually occurred at Tunguska.

The paper I cited above contains simulations of impacts showing the amount of energy deposited in the explosion of different kinds of impactor. If it had been an icy comet, most of the explosion energy would have been released occurred at greater altitudes, causing only minor blast effects at ground level but extending them (due to the Mach stem reflection effect) over a much wider area. If it had been an iron meteorite, the atmosphere would not have caused it to break up into fragments and be stopped in the atmosphere. So it wouldn't have air burst, and would instead have hit the ground, causing a classic explosion crater with a lip surrounded by an ejecta zone. So from the effects observed, the Tunguska impactor seems to be an air burst at moderate altitude, which implies a stony asteroid.

This seems to me to be the simplest hypothesis which fits the solidly known facts. From the known rate of impacts of different sizes, it's been estimated that the Tunguska 126 metre radius stony asteroid is likely to hit planet once each 300 years on the average.

Is there any mechanism showing why a black hole might explode with the stated energy at the known air burst altitude?

6:27 AM  
Blogger nige said...

The line above giving the conversion should read: (1 kt of TNT explosion energy = 10^12 calories = 4.2*10^12 Joules).

11:31 PM  
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5:28 AM  

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