Friday, 15 February 2013

Size estimates for the Chelyabinsk event

There are several sets of video footage with audio. Just a few examples include:

The have audio blasts at various intervals. What is unknown, however, is their location with respect to the incident vector of the meteor. There is one super-loud explosion which is followed by various minor secondary noises. This is somewhat confused by echoes, local noise (e.g. falling debris), microphone noise, etc. These follow for many seconds after the main event.

The blast is not the meteor hitting objects, but rather the shock wave from the air burst. The meteor explodes during re-entry due to thermal stress. Essentially the surface of the meteor is super-heated due to friction against the atmosphere during re-entry. The interior, however, will still be extremely cold.

Relying only on reports, and in the absence of any confirmed finds of fragments, we can only estimate the composition. However, it is more likely to be a stony meteor based on the explosion and statistical likelihood, given the populations of this size of small object in the solar system.

There are no reliable reports of craters. The smoke trail burns out at high altitude, and while fragments may certainly have reached the ground, these will be at terminal velocity free fall (and also relatively cooler). The smoke trails indicate that the air-burst was occurring several kilometres up in the atmosphere.

The initial impact angle (on the atmosphere) will be around 50 degrees. This is based on the radiant location and the distance from zenith (earth travel direction). There is still a good degree of uncertainty in this, though, due to lack of precise information about the incoming orbit. There is a large error associated with this though and it remains a weak point in our findings. Impact velocity is probably 25 km/s. This is based on fact that the earth is moving at about 20 km/s and the impact is coming from the sun-side.

Based on these assessments, particularly audio shock information, and correlating damage reports, we can revise our estimate to make it larger than our first estimate.

Again, we must stress that the associate errors with these estimates remain large, but we are probably looking and initial object size of 10 to 20 metres across.

We will continue to provide updates and we continue to process the information that we have.

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