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In the lastest meteor event in Russia there was a big BOOM heard long after the meteor had passed. In this youtube video we can see at 4:30 the Meteor passing.. then hearing the BOOM at 7:00 , 150 seconds later, so what cause the boom? Sonic Boom from the meteor entering the atmosphere? or the big explosion that happens a bit later on its decent?

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There's good coverage at Bad Astronomy, the blog of the astronomer Phil Plait. Note this "UPDATE (23:00 UTC) After looking through more footage, it's become clear that the multiple booms heard were in fact explosions, and not just shock waves from the meteoroid's passing through the air. In some videos, you can see multiple flashes of light inside the contrail, which are clearly from the rock breaking up and then burning up very rapidly and with intense energy—the very definition of an explosion." –  Michael Brown Feb 16 '13 at 6:47
    
Here are direct links to the Bad Astronomy articles slate.com/blogs/bad_astronomy/2013/02/16/… slate.com/blogs/bad_astronomy/2013/02/15/… –  raindrop Feb 17 '13 at 4:00
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The flash seen in this video may not be the glow of initial entry since the camera angle is angled somewhat along the path of the meteor; it is likely the active burning after the meteor has already entered the atmosphere. The mesosphere begins about 50 km above the earth's surface. However, the speed of sound in this region is a bit lower than the speed of sound on earth's surface, ranging between $280\text{m/s}$ and $330 \text{m/s}$. Coupled with the fact that we continue to hear less intense popping sounds for more than two full minutes afterwards suggests that the large boom heard is the shock wave, while the following sounds are actually from earlier events.

Meteors tend to enter the atmosphere at between 20 and 50 km/s, although they may be slower or considerably faster than this. This means that if their journey were straight down through the atmosphere, rather than across it, the trip would take 2 seconds or less. The trajectory of the meteor across the sky illustrates that it is a more glancing impact. The fact that the first sound takes 140 seconds to reach the camera indicates that the meteor is about 45 km from the camera. This is not reflective of the height of the meteor, but the absolute distance of the meteor path from the camera. There are estimates that the breakup altitude is around 20km. The following two minutes of reports reflect a linear distance of about 40 km of travel ($120\text{sec}\times300 \text{m/s}=40 \text{km}$), which probably took the meteor about 2 seconds to cover. So what we hear afterward is like a slow motion recording of the meteors breakup history, slowed down by a factor of somewhere between 40 and 100 times. If we could have heard these breakups in a frame comoving with the meteor, it would have sounded nearly like a single, continuous explosion. The first and loudest boom we hear is likely not from initial entry, but rather the sonic boom from the meteor's closest approach to the camera.

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When this meteorite entered the atmosphere, it was traveling at approx 36000 miles per hour. Now consider this fact; I have a roof rack that creates a howling sound when I speed up to 100 kph. This howling sound is caused by air moving past the roof rack. I can hear this howling sound with my car windows closed. If there were sensors mounted on the vehicle to sense vibrations, there would be vibrations that could be recorded from the air flowing past the roof rack. Now consider an object entering the atmosphere at an extreme velocity like the meteorite. The atmospheric abrasion not only causes the meteorite to vaporize and glow brightly but as it enters the more and more dense atmosphere, the vibrations set up by the friction of the air would not sound like a howl as my roof rack does, it would be more like standing behind a jet engine totally unbearable, and that's only mild in sound. Now as the object enters further into the dense atmosphere, The sonic vibrations, if you can call them that, become so violent, that the rock cannot take those stresses and as a result shatters. Then you see a bright fireball flash as all the pieces are exposed to the air and quickly vaporize and burn. Being much smaller now and quickly slowed down by the air resistance, you would only find a few little pieces that would cool and rain down. That's why after the bright explosion, you could only see a few wisps vapor trails following the cooling debris. The sonic boom heard by this object flying at this velocity and height would be vey very very loud. Just think if a jet making a sonic boom sounds like a loud thunder clap and not only shakes windows but can shatter them, then try to multiply this by several thousand times and you get what this meteorite did. Now sometimes there are meteorites that don't break up as they enter the atmosphere. That depends much on the composition, often rock, other times pure nickel and iron; which can stand tremendous vibrational stresses, speed angle of entry and most likely other factors that we can only speculate when dealing with speeds such as these traveling through the air. Now just consider this last thought if a piece of the moon, which is, I hear, made of blue cheese, broke off and came hurling into our atmosphere, what that would be like. I'd hate to speculate...just a thought to consider for the future.

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At 4:30, light from the burning meteor reached the video camera. At that time the meteor was still in the air and it did not generate the boom. After 150 seconds the sound waves reached the video which means that the point where the boom were generated was at a distance of 333 m/s x 150 s = 49.95 Km from the spot where the video was shot. The reason for the boom could be either because the meteor exploded in air due to heat caused by air friction or because the meteor hit the earth as one major chunk of rock (along with other tiny pieces) and we are hearing the sound due to impact. My guess is that since we hear one big explosion followed by multiple smaller explosion, the meteor has just exploded into several pieces just before hitting earth. The first explosion is due to the exploding meteor and the smaller explosion are due the pieces hitting the earth.

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protected by Qmechanic Feb 20 '13 at 19:58

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