Media and other Impacts

A meteorite enters the atmosphere over Russia and once more the media is caught with its pants down to the ankles. It is not at all obvious why a German newspaper would ask a German astronomer, if the meteorite was radioactive … except for the fact that we’re talking about a German newspaper, which could explain a thing or two.

Associated Press, ever diligent not to perform any sanity checks at all on their stories, released a completely incoherent piece of reporting that has already been cited on the associated Wikipedia article that merely adds to the confusion.

AP also reports that “Vladimir Chuprov of Greenpeace Russia noted that the area where the meteor exploded was 100 kilometers (60 miles) from the Mayak nuclear storage and disposal facility, which holds dozens of tons of weapons-grade plutonium. He said the Russian government has underestimated potential risks of the region.”Well, I guess you have to be a dues paying member of Greenpeace to believe that the likelihood of an asteroid impact depends on where on earth you build a nuclear facility. Surely, using the Greenpeace Trademark Brand of statistics, it is easy to prove that the Ural mountains attract meteorite impacts using the Chelyabinsk and Tunguska impacts as the only datapoints.

Furthermore, according to AP we are to believe the meteor travelled at 15km/s, Roscosmos claims in the WP article a speed of 30km/s. The latter seems to be more plausible to me. It would mean that its trajectory must have been roughly perpendicular to that of earth. (An angle of 60 degree instead of 90 still counts.) This indicates a highly elliptical orbit, which is what you would tend to get from a collision somewhere in the asteroid belt. Such as the one that was observed 3 years ago by the hubble telescope.
The break-up of the meteor is reported to be at an altitude of between 30 and 50km. The stated mass according to AP is 10 tons. The mass is almost certainly wrong. It takes a certain amount of energy to create a shock wave in thin air that is powerful enough to destroy windows at a distance of 30-50km.

I will take the Halifax Explosion as a reference point here. It released an energy of about 3.3kt of TNT equivalent (about 14 TJ), shattering windows up to about 15km away. The Hiroshima bomb (about 15kt) shattered windows at a similar distance. However, the air was much denser in both cases, allowing shockwaves to propagate better. Furthermore, having twice the distance to the blast further reduces the intensity of the blast to one eighth. To account for the greater distance and the much less concentrated release of energy (it wasn’t an explosion after all, but “aerobreaking” over a course of at least 20km), we must assume that a larger amount of energy must have been released as a shockwave.

Since the creation of a shockwave may be more efficient in a meteor strike, I will limit my estimate to that of the energy of the Hiroshima bomb.To get about 15kt of TNT equivalent, a mass between 500 and 2000 tons is necessary. My guess is that the culprit was an asteroid of about 10m size. (A sphere would be somewhat smaller, but asteroids are not perfect spheres.)

Of course this doesn’t mean that a single piece of 10m will be found or that all pieces put together would be of that size. A lot will simply have burned off during the entry process, especially after the breakup, which is followed by a total breakup of all the separate pieces. The smaller the piece, the faster it burns up at a given speed. When a meteor breaks up, it will trigger an avalance process that will rapidly release most of its kinetic energy. You can see this process on the various videos as the second or so, in which the meteor suddenly becomes very bright.

I will update this article when more reliable data is available.

The guardian linked to an article of nature that is a little more realistic in terms of the diameter of the asteroid:

“It was a very, very powerful event,” says Margaret Campbell-Brown, an astronomer at the University of Western Ontario in London, Canada, who has studied data from two infrasound stations near the impact site. Her calculations show that the meteoroid was approximately 15 metres across when it entered the atmosphere, and put its mass at around 40 tonnes. “That would make it the biggest object recorded to hit the Earth since Tunguska,” she says.

it seems that the “40 tonnes” refers to the mass of whatever actually reached the ground and not of what entered the atmosphere. A 15 meter sphere would weigh around 3000 tonnes, a more typical, irregular shape would put that at around 2000t. (Assuming a low density of 2000kg/m^3.) The energy released – at 30km/s – would be about 60kt of TNT equivalent, which is much more in line with the kind of damage that could be observed.

A day later, it seems that my estimates were as conservative as I thought they were. NASA now estimates the asteroid to have been 17 meters in size, with a mass of 10.000 tons and energy released to be 500kt TNT equivalent. The losses (in terms of shockwave strength) due to the thin atmosphere and gradual release of energy are rather large.

As asteroids get larger, their effects will be disproportionately worse. They will not only carry more energy, they will also break up faster and at lower altitude. The shallow angle of entry certainly also helped to minimize the damage. The lower the breakup, the more energy will be converted into the shockwave due to the denser atmosphere, the shockwave will also be more intense due to the faster breakup process. To make things worse, a lower altitude also means that there is less room to dissipate the shockwave before it hits the ground. A breakup at 20km altitude results in a 4-8 times stronger shockwave than a breakup at 40km.

The Chelyabinsk meteor has certainly set the upper bound in terms of relatively harmless effects resulting from an asteroid hitting the earth.

In any case, if there had still been any questions as to the the cause of the Tunguska event, they have been answered in a rather spectacular way. The size of the Tunguska asteroid may or may not be revised as a result though.

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