The trunks were pointing to ground zero - the tips were pointed out.
It’s counterintuitive, but lab experiments with much smaller explosions have produced the same result.
OK, what makes it explode?
I’m not sure I’ve got it exactly right, but as I understand it, a meteor/commet burning up in the atmosphere is initially traveling at several dozen kilometers per second, and thus has a huge amount of kinetic energy. Since it is moving so fast, it penetrates our atmosphere very rapidly, and if it is not big enough or dense enough to make it all the way to the ground (as the Tunguska object apparently was not), all that kinetic energy is converted to heat very rapidly as the object vaporizes. Plus, before the object vaporizes, as it plunges into our atmosphere it pushes a lot of air in front of it, creating a large column (I’m not sure if it’s actually a column, but that was the best term I could think of) of compressed, heated gas in front of it. So on the one hand, you have a gigantic release of energy (converted from kinetic energy to heat) very quickly, and on the other hand, there’s the highly compressed and heated air column produced by the object before it disintegrates. Lots of heat and a violent outward expansion of compressed gases are the main characteristics of any explosion, right? So, while the meteor/comet isn’t a “real” bomb, it produces an explosion like one.
This makes sense to me now, chorpler; thanks.
The Tungus Effect? The Tungus EFFECT?!?!? What was the OP’s problem? Oh yeah, maybe it was 5:30 AM and he was really tired.
Sorry about that. :o
Jackelope: Asteroids are made of rock and/or metal, while comets are made mostly of ice with some dust/very fine dirt.
I think Carl Sagan’s explanation (in Cosmos, maybe?) was that it was probably a small chunk of comet-type material. The understanding I got when I read it was that the rapid heating (due to friction) as it fell through the atmosphere caused the ice to evaporate suddenly and violently, making it explode before it even hit the surface. An asteroid, a solid object, just burns away its outer layers as it falls.
Anyways, I came away thinking of a comet in our atmosphere as being like a kernel of popcorn-- there’s material in it that reacts violently to being heated, so once it’s been heated enough that material evaporates and it goes POP! But I was seven when I read that, so maybe I should read it again.
I have watched the Perseid shower quite a few times. So far, I have not seen any of the meteors reach the ground. They all extinguish somewhere in the atmosphere above the earth. Never is there an explosion at the end of their visible life…one presumes that all their matter has become vaporized, and they simply exist no more, as coherent objects.
The “What explosion?” question is a great one, I think. Regular meteors do no explode.
“Regular” meteors ablade away. They have to be large enough to survive long enough to be heated hot enough to explode.
What about the theory that a huge methane pocket somehow leaked into the low atmosphere and then, further somehow, was ignited by some kind of spark? I heard some drunk guy talking about that theory at the bar one day. Yeah.
Hmmm … well, presumably a meteor has to be of a certain size to create enough of a column of heated air in front of it to qualify its destruction as an “explosion.” Without that, I assume the meteor will just boil away, producing the characteristic streak of light that we see.
If anybody has more (or more accurate) information, I’d love to read more.
It’s not just that the Tunguska object was too small to reach the surface: There have been plenty of meteors to reach the surface in recent history, and none of them caused anywhere near the devastation of Tunguska (although one did destroy the trunk of a parked car in New York). Whatever this thing was, it was plenty big. If it was a meteor, there had to be something special about it, either in composition or trajectory, to cause it to explode that way.
And Achernar, he must have refined that lecture some since I went through. Our class definitely heard about the reindeer, but no mention of Christmas.
Since DRomm mentioned it, has anyone done any simulations of the effects of a Tunguska-event located over any of the major cities of the day? If it were to happen now over a city?
Hmmm… lots of almost-but-not-quite-right thoughts going on here. What follows is an amalgam of things I have picked up here and there, and is a working theory used by scientists as far as I know.
A rocky asteroid in the 100 meter size range enters the atmosphere. As it violently compresses the air in front of it, the air heats up (Boyle’s law, if memory serves). A shock wave forms a few asteroid radii in front of the incoming rock (called a standoff shock). The radiant heat from the air heats the rock. The surface melts, and the relatively slower air in contact with the rock blows off the melted material (ablates it). Friction is not a big factor in heating, since the air behind the shock that actually touches the rock is moving only a few hundred kilometers per hour.
However, there is still a huge amount of dynamic pressure on the rock as it comes in. This pressure distorts the rock, flattening it. This increases the pressure, and it breaks up into smaller pieces. This increases the surface area to volume ratio, and the pieces heat even more rapidly. They break up, increasing the ratio further, until they are small enough to vaporize rather suddenly. This releases most of the kinetic energy of the rock into heat, and you get an explosion.
The blast wave slams into the air and ground, knocking down trees, frying reindeer, etc. The asteroid itself is mostly gone, because the rock has disintegrated.
An iron asteroid would probably survive intact. Witness Barringer crater in Arizona; there is still a sizable chunk of iron down there, even though a lot of it vaporized and littered the countryside in iron dust.
The explosion of the bigger asteroids can scatter smaller pieces all over the place. Iron meteorites under a few centimeters across are common; they break off of bigger chunks. That’s what happened in Peekskill, NY a few years back. Amazing stuff; check out that link.
Smaller meteors don’t explode (well, usually) because, I believe, the amount of heat released isn’t as sudden or as large. I am not so sure about this, however (this isn’t really my field, though I’m familiar with some of it). It may also have to do with smaller meteors burning up much higher in the atmosphere. I don’t know.
And if I may disagree with DRomm:
I think the biggest problems with the UFO theory are that it’s silly and unneeded. Asteroids (or comets) do exactly what was seen over Tunguska, and we know they exist. Yeah, I know, you were not being exactly serious, but I’ve been stomping on more than my share of pseudoscience today, and I still find myself a bit hungry.
A question for the particle physicists.
I read an article in Wired last month about WIMPs (weakly interacting massive particles) and dark matter and other mysterious high energy particles around. Yeah I know Wired isn’t the best scientific source, but I have read about these things in Science and Nature too. Things like the “Oh My God” particle – a single proton with the kinetic energy of a dropped brick. There are these other huge particles hitting the earth apparently pretty commonly. The Wired article mentions a bunch of physicists using seismographs to detect strange particles hitting the earth.
Well, I suppose it is 15 orders of magnitude of so away from what caused the Tunguska event. But has anyone considered it could have been something like this hitting the earth?
I had a lot of stuff to post, but the Bad Astronomer has already beaten me to most of what I was gonna say. 
However, I think I can still add a couple of little things.
An exploding meteor is called a bolide. I have seen these myself; they don’t seem common during the Perseids, for whatever reason, but the Geminids do have a fair number of them. (The Geminids definitely have a different composition, though – I think they are much more beautiful than the Perseids or Leonids because they are distinctly blue in color.) I’ve never seen a really dramatic one, but I’ve seen ones that clearly flare abruptly before burning out. It’s quite a sight. Like most meteors, the ones I’ve seen have been extremely small so they didn’t produce much of a shockwave for me to hear or feel on the ground.
Meteors entering the atmosphere can do a variety of things depending on the angle of entry, the speed of entry, the direction they are facing, whether or not they are tumbling, what they are made of, and what kind of structure they have. Consider the way Columbia broke up on reentry, or the way deliberately-deorbited spacecraft such as the Mir have broken up. Now consider how much faster a meteor is travelling when it enters the atmosphere! If a meteor has a very uneven structure (some are downright spongy), it will not heat up evenly as it passes through the atmosphere. This uneven heating leads to uneven expansion and sooner or later, something has to give. The meteor will fracture, and then the smaller pieces will burn much more rapidly than the one big meteor did, simply because they are smaller. This can be a violent process, and it can be described as an explosion, although that’s not really a very accurate word for it. (Columbia didn’t really explode either, incidentally.)
[hijack]
My mother and I were among the many eyewitnesses of the Peekskill meteor, and the first thing we noticed about it was its color: It was a very clear, intense green. But I’ve never seen this mentioned in any of the web pages or articles about the event. I can understand that the color might have washed out in photographs or video stills, but didn’t any other eyewitness notice the color?
[/hijack]
Maybe it was supposed to be 13 KILOmeters?
Ed?
The Bad Astronomer: Please read the whole paragraph of my comment. No, I don’t believe the UFO theory has any credibility. Yes, I was being silly. In fact, I still am:
UFO theories are desperately needed, the sillier the better. Idiotic movies about giant ants helped prevent a real nuclear war by getting people scared. They were scared of the right things for the wrong reasons, but they WERE scared of the misuse of radiation and nuclear war and that was a good thing.
Silly UFO theories will pave the way for acceptance during First Contact. It hasn’t happened yet, and may never, but it probably will. And when we meet aliens, I want us to have some sort of reference to them in the popular culture. I LIKE the idea that people are thinking about other life forms, even if they have to go on wild paranoid streaks to do it.
Besides, it makes for such great jokes, such as the short bit in Babylon 5 where Earthlings are suing the aliens who look like the standard big-headed aliens of the National Enquirer…
I also like people talking about Tesla, a really interesting guy who doesn’t get nearly the recognition he should. He, after decades in court, wound up with the patent on radio, but Marconi continues to get the credit.
The video in the link I posted above shows it as green. I’ve seen other videos where it was green as well.