Whenever you see a movie or even a serious documentary about the prospect of a meteor striking the Earth and ruining our day, it always depicts a fiery explosion resulting. Why?
I don’t doubt the severity of the impact, the force of the blast, the resulting tidal wave, etc., but the movies always depict it as very similar to a nuclear explosion.
Why would it blow up like a bomb? Even if the material heats and burns on entering the atmosphere, I don’t see why it would explode.
Or is that nothing more than Hollywood hooey?
– Greg, Atlanta
The quick, two-paragraph answer:
In an impact of the kind we’re used to in everyday life (eggs hitting floor, ball hitting bat, etc.), the impact speed is much slower than the speed of sound. Meteorites hit the Earth at speeds much larger than the speed of sound in rock or metal. Thus, a shock wave is produced at the point of impact, which is very efficient at heating and dispersing material. So, Hollywood actually gets it right. At least a bit.
The realization that high-speed impacts are very much like explosions was made in the 1920s (I think) by an astronomer studying Meteor Crater and trying to figure out if any of the pure iron impactor was still around, offering mining opportunities. The answer was no.
Simple 4 word answer: Lots of energy produced.
Simple 22-word follow-up:
I guess I’m asking whether “lots of energy produced” is always in the form of fire, which is what is always depicted.
– Greg, Atlanta
The meteor has a lot of kinetetic energy. A lot of it turns into heat. If you put penny on the sidewalk and repeatedly smack it with a hammer. It will heat up pretty quickly. larger impacts generate enough heat to eject a shower of melted earth, but it would look more like spray from a drop of water then a nuclear type mushroom cloud.
No, it would look like a mushroom cloud. The physics of supersonic impacts are different from hitting a penny with a hammer. The amount of energy involved with shocks is really quite large. I mean, really big. So fabulously huge that… well, you get the idea.
Nuclear craters are very good analogs to impact craters…
Read “Lucifer’s Hammer” by Larry Niven and Jerry Pournelle.
They used modeling and computer programs run at JPL in pasadena. If anyone has information on high speed impacts, JPL does.
The descriptions of the impacts in the book are very realistic. (not to mention scary as hell)
FixedBack
FixedBack
“Moderate strength is shown in violence, supreme strength is shown in levity.”~~G.K.Chesterton 1908
This was actualy not my oppinion, I was on TLC friday night “Crater of death” or some such. They were talking about the crater a Chixalub(sp?) causing the iridium anomoly at the kt boundry. Not that these infotainment shows are 100% acturate, but the were runing tests with a hypersonic gun.
Its kinetic energy turning to heat. So its a million ton hammer moving at mach 30 that generates enough heat to melt inself, the penny and millions of tons of earth.
Nuclear ground bursts are such a bad idea, I’ve never actually seen a publicized picture of one. Usualy nuclear test try to avoid having the fireball even touching the ground. The Tungusta blast of 1909 was aparently a comet that expoded in midair and didn’t leave a crater
I didn’t mean to come on as forcefully as i did. Do forgive me.
Nevertheless, I must disagree.
I just went down to my buddy who wrote her thesis on the Chixulub impact and talked to her. A large impact creates a high-temperature, low-pressure column behind it. This column rises without mixing (pretty much) until it reaches a level at which its pressure is at the ambient pressure (more or less). At this point, the could expands. Hence, mushroom cloud. Early enough in the impact, this hasn’t had time to happen yet-- this may be what the TV show showed.
There’s a bit more confusion about the hammer on penny as an analog for impacts. The physics is different, somewhat. The simple fact that all the energy in an impact is deposited practically instantaneously (from the viewpoint of the materials involved) is a big difference from continually pounding on something a little bit. Yes, kinetic energy being turned to heat is basically what’s going on in both at bottom, but the efficiency of the process among other things are rather different.
Nuclear craters from above-ground tests are good analogs for impact craters. Geological field trips here at the University go to the Nevada Test Site to study nuclear craters as impact analogs. Below-ground tests are a different beast, as you pointed out. Airbursts, of the Tunguska and Hiroshima sort, again are somewhat different, though here impact airbursts are reasonable analogs for nuclear airbursts (I think).
-Andy
I should have read your post a bit more
carefully. 
I should also learn how to
quote previous posters…
A million ton hammer moving at mach 30 is
going supersonically. It can do things that
a 30 ton hammer (or even a 1000 ton hammer) moving at 30 mph can’t do.
To zeroth order, the back of a body moving at the speed of sound or faster does not “know” that it should stop when the front hits something. The back of a body moving less than the speed of sound does.
Of course, it is a continuum of results. But having or not having a shock wave is something of a big deal.
You guys managed to lose me. I thought the simple explaination was that a very large mass (meteor) moving at many times supersonic speed hitting another large object (earth) was so powerful that it actually crushed (split) the earth atoms and you end up with a very large nuclear explosion. Tell me it’s not that simple.
It’s not that simple (it’s also not nuclear).
Stephen
Stephen’s Website
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When we visited Meteor Crater on the Great American Vacation in my youth, a had a little piece of the meteorite sold to me mounted on a card. Was there just not enough left to mine?
Isn’t there a theoretical weapons system called the “Sword of Damocles”.
IIRC it was basicly an orbital platorm that would accelerate several Kilos of mass to near light speed. The impact was supposed to produce an effect like a nuclear blast, with out the radiation.
Guys, it’s really simple. Nuclear bombs make a big boom because they release a lot of energy very quickly. Large meteorites make a large boom because they release a lot of energy very quickly. The bomb releases energy from the fission of atoms. The meteorite releases energy from the conversion of kinetic energy. The mechanisms are different, but the results are very similar.
Does anyone remember when Shoemaker-Levy 9 hit Jupiter? You could clearly see the explosions from a spacecraft (Galileo?) which we had a few million miles away. When the impact locations became visible from Earth hours later, you could still see the hot spots where the fragments impacted.
I kept wondering how the speed of sound had anything to do with this, and now I think I hear what you’re saying. In this case, it’s not the speed of sound in air that matters, but the speed of sound in the material that the meteorite is made of. It does seem that this would be more violent than a meteor moving at just under the speed of sound in iron or whatever.
But I’m still not convinced that that matters. It still seems to me that what matters is not the speed relative to the speed of sound in that material, but just the total kinetic energy that has to find a place to go very suddenly. In other words, if the speed of sound in the material is 10 km/s, a 100 Mg object travelling at 11 km/s would have a smaller explosion than a 200 Mg object travelling at 9 km/s, because of the greater kinetic energy. Is this correct?
FYI:
Mushroom clouds are NOT the result of nuclear explosions. They are the result of ANY sufficiently large release of energy in a very short period of time.
The US deployed the largest convetional bomb in its arsenal against Iraq (the bomb is actually pushed out the back of a cargo plane as it’s too big to fit in any of our bombers). The explosion of this CONVENTIONAL bomb created a mushroom cloud so many Iraqi’s assumed we were nuking them.
Everyone associates mushroom clouds with nukes since generally they are the only things photographed blowing-up that creates one. A sufficiently large impact from a meteor (or any extraterrestrial object) could also produce this effect.
Why not? If they’re going to make an explosion, why not make it as dramatic as they can get away with? How many people have actually seen a huge meteor striking the Earth, anyway? Who would know if it was right or not?
It is too clear, and so it is hard to see.
I think meteor craters are made to look like traditional “mushroom clouds” in the movies 'cuz that’s what Hollywood knows. After all, there haven’t been any Texas-sized asteroids slamming into our planet for the past century, right? How would Hollywood know what a meteor impact REALLY looks like without having seen one in real life? So they don’t bother… they just make a really big explosion. As long as it looks like it’ll create a lot of destruction, it’s all right.