# Impacts

Ok, that was close, and I don't mean the zip by of li'l old 2012 DA14 this evening.

The Chelyabinsk meteor looks to have been on the high end of the range of quick and dirty estimates, with impact energy of maybe 300 kTon equivalent. This is a size impact we'd expect every few decades, maybe, quite a bit smaller than Tunguska, but larger than anything we know of in the last 20 years.

Meteors come in with speeds ranging from 10-70 km/sec.
This one appears to have had a speed of ~ 20 km/sec, which gives it kinetic energy of 2*108 J/kg
As you know, the handy unit here is the ton-TNT equivalent at 4*109 J, so at this speed, a 1 kg rock has energy of 50 kg-TNT equivalent.

Rocky meteors have densities of about 2, so assuming a cubical meteor, because no one needs the factors of π, we find that a meteor which is 1 m on a side, has a mass of 2,000 kg, and impact energy of 100 ton-TNT equivalent.

Mass goes like size cube, so a 2 m rock comes in at a respectable kiloton-TNT equivalent, and at 12 m, we are at about 200 kiloton-TNT equivalent with a mass of about 3,500 ton.
12m is about the length of a normal US school bus.

Ok, meteors hit the atmosphere and start braking due to friction, which heats the meteor, the air and, since we are coming in at about 60 times the speed of sound, starts a shock wave propagating.

Typically a meteor will breakup and release the bulk of its kinetic energy abruptly, if, either internal pressure shatters it, eg due to ice vapourizing, or, if mostly solid, when the column of air swept up by the meteor has sectional density comparable to the meteor.

Air has density of about 10-3, at ground level decreasing approximately exponentially with scale height, and the scale height is of order 10 km. Therefore meteors tend to break up at 10-20 km altitude if they don't make it to the ground.
A 1 m rock needs to sweep a path of over 2 km through the air to stop effectively, a 12 m rock needs about 24 km of air to stop.
So a rock that big coming straight down will likely hit the ground.

The Chelyabinsk meteor came in at a shallow angle, and so traversed a column of air long enough to brake it and break it.
This is very fortunate, or we'd have had a ground detonation of a few hundred kilotons and likely mass casualties.

Most of the injuries seem to have been from broken glass, consistent with reports of other large explosions.

Glass breaks from overpressure of about 1/4 PSI - and as the bomb damage calculator (below) shows, that overpressure goes out to about 20 km radius (for ground detonations which this was not).
Here we had an air detonation (worse) but with the energy spread out over a linear track, not deposited instantaneously at a point (both better and worse).

Hence the damage was consistent along the track and for tens of km either side of it, but nowhere was there a point or line of extreme destruction.
A little bit higher energy impact, steeper impact angle, faster speed or bigger rock, and there would have been a zone of severe damage surrounded by an elongated annulus of the moderate damage actually seen, and there would have been many deaths.

The initial news reports of the event were very confused.
Some reports claimed the rock was intercepted - this was confusion from videos of the first sudden breakup of the rock, as it breaks the fragment surface area increases sharply and there is increased energy deposition.
There was a lot of emphasis on radiation levels - this is because standard, decade old, ER protocols are for responders to check for radiation when there is a big bang, especially one in the sky - for 30-40 years the most likely source of a 300 kT explosion over Chelyabinsk would have been a nuclear warhead on a missile.

This rock had nothing to do with the 2012 DA14 which passed by friday night, it had a completely different trajectory. The latter is also quite a bit bigger and would do serious damage if it hit in a populated region.

Most impacts are over water, about 2/3 are over ocean, mostly the Pacific.
Yes. Russia gets hit disproportionately often, that is because it is large.

We should be worried, impacts are more frequent and more energetic than people appreciate. As population spreads, the odds of a given impact leading to a mass casualty event increases.

We currently do not have near complete lists of possible impactors.
We never will, because there is always a risk of long period comet impacts, but the inner system objects can, with modest effort, be characterised arbitarily well.
We can do everything down to 1 km easily and will have done so in the relatively near future.

To get every inner system rock down to 100m is hard work and requires a modest level of funding.
The ATLAS project will give some warning of approaching impactors of 50m or larger, for \$5M, but will not catalogue them.
That'd take at least 10 times more money.

To get all objects down to 10m is possible, with current technology, eventually, but would be a long hard project costing billions of \$.

If impactors are detected they can be deflected, in principle, they can also be captured and mined, in principle, and there are some semi-serious proposals to start getting on with that (though most of the proponents need to read some good '70-90s SF by Bova and Flynn).
Deflection requires long lead times (longer the better) and good data.

There are several techniques that can work, none have been tested (duh) and some may not work in practise depending on annoying details like the frangibility of real meteors.
We will be doing these things, eventually, possibly even soon enough.

Recent Meteorite Impacts - Norway 2006, Greenland 1997

Estimating small impact rates

NORAD list of impacts 1994-2008 - via Peter Brown

Airburst over Washington DC in 1995 - note this was ~ 1011 J or about 40 ton of TNT equivalent

Overpressure calculator for detonations (ground level)

Rate of Impacts of Different Sizes

BBC summary of the news

Starts with a Bang overview

Torino Scale for Impact Hazard

Meteorite Impact near Indonesia in 2009

Reconstructing he Chelyabinsk meteor’s path

Ok, Pretty Pictures - from American Meteorological Society

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By Phoenix Woman (not verified) on 16 Feb 2013 #permalink

This was an excellent post. Very detailed, and interesting. Thanks!