I just read this article
http://science.nasa.gov/headlines/y2001/ast30nov_1.htm?list643088
about observations of Leonid meteoroids exploding on the surface of the moon (rather than burning up, as they do on Earth, because there’s no lunar atmosphere).
Then I got confused–how do they explode if there’s no air to support the explosion? I’m not sure if I’m wording this correctly, but doesn’t an explosion mean that something is burning up?
I’m recognizing that I was wrong to assume that air would need to be present to support this, but I haven’t quite got the process cleared up in my head. Help.
Did you read the entire article? I think this quote explains it pretty well:
The heat of the impact generates light even in the absence of oxygen.
My guess is that you’re thinking of something along the lines of a chemical explosion?
This isn’t an explosion because of oxidization like TNT or, uh, for some strange reason the only other example I can come up with is when dust particles from grain get too concentrated and spontaneously blow up corn silos . . .
But, anyway, the detonation of an explosive causes the release of chemical energy. The impact of a meteor on a planet causes an explosion because of the meteor’s kinetic energy.
Your typical garden-variety meteor travels through space at a speed of several to tens of kilometers per second. Think about that. Imagine a distance of ten kilometers. Now imagine something going so fast that it can cover that distance in one second!
Something going so fast has a great deal of kinetic energy. When it’s stopped its kinetic energy goes to zero–but energy is conserved. Were does the energy go? It goes mainly into heat. This vaporizes the meteorite, and a good chunk of what it has slammed into. As my main man Jay Melosh says, the hot, vaporized stuff expands rapidly and gives off a lot of light (in the same way that a heated iron bar glows red.)
Incidentally, meteors that “burn up” in the atmosphere are not on fire. They are simply shedding their kinetic energy. The glowing trail you see is not due to a chemical fire. It’s due to the tremendous heat caused by friction between the meteorite and the atmosphere. Meteorites are made of metal and rock–not stuff that will readily burn. Rather, they melt and/or vaporize.
You don’t need air for any kind of explosion. If a chemical reaction depends on the oxygen in the atmosphere, then it generally won’t explode, because the reaction will be limited by how quickly it can get that oxygen. For example, a fuel tanker won’t explode like in the movies, but will burst into flames, the fuel burning only as quickly as it can get oxygen.
Things that explode (chemically) already have all the chemicals they need for the reaction built-in. With the silo explosion phenomena that Pokadyne mentioned, you have a silo full of dust and air, so it too has all the chemicals availabe right there.
Minor nitpick.
Although friction plays a part, the main source of heat is caused by the compression of the air in front of the meteor. This is the same mechanism that causes a bicycle pump to get hot.
Meteors fall at supersonic speeds, so I don’t see how compression can happen in front of the meteor. It’s more of a collision.
A blunt object at supersonic speed has a bow shock in front of it. The temperature and pressure of the air go up dramatically across that shock (by definition, since the shock is defined as the position of the pressure change).
Nope. It means that something is flying apart violently.
The heating of meteors is fascinating, and not well understood by the public. The supersonic compression of air in front of the meteoroid (the actual bit of rock, ice, metal, whatever) heats it tremendously. This hot air then heats the meteoroid by radiating on it, like the way you warm your hands in front of a fire. The bow shock wave stays a few meteoroid-diameters in front of the actual meteoroid. Between them is a layer of relatively slow moving air, which flows past the meteoroid at a few hundred kilometers per hour. This wind blows off the outer meteoroid layers that have melted from heating. This process is called “ablation”. This is the only instance of friction really playing a part. The weird scoop-like depressions in meteorites are formed during this process, and are called “regmaglipts” check out this way cool image, for example).
This heating phase is pretty short, lasting a few seconds. The fantastic amount of energy lost to compressing the air slows the meteoroid tremendously, and it quickly reaches terminal velocity, which is a few hundred kph. It then simply falls the rest of the way down, assuming it doesn’t ablate away completely.
Small meteoroids (smaller than a meter across or so) therefore hit the ground at subsonic speeds, and do not create huge explosions and craters, despite every movie ever made showing impacts. 
Matter of fact, the heating of the meteoroid is so short in duration that many meteorites (what they are called after impact) are not only not hot when they hit, but are actually cold, and sometimes covered in frost! Remember, the ambient temperature of space near the Earth is below the freezing point of water, so the inside of the meteoroid is quite cold.
I highly recommend the book “Rain of Iron and Ice” by John Lewis, an expert on these things. It’s a fascinating and somewhat terrifying read.
Just wondering: If the hot air is radiating upon the meteroid as you describe, why should the trails change color? Even if not burning, surely the meteor must become incandscent to see the indication of copper and iron composition, respectively, right?
I’ll have to check out the book you recommend, too…
- Jinx
I knew it wouldn’t take long to get some good answers to my question. Thanks to all of you. Bad Astro especiallly gave me the factoid for the meteroids that I was craving. Nice image, too.
I’m also glad to hear that “the heating of meteors is . . . not well understood by the general public”, also to learn a new word: ablation. Now if I can figure out how to work that into casual conversation. And forget about my using “regmaglipts”–no way could I use that one without choking a bit on it.
I used it just the other day… but I was talking about meteors anyway.
Last I heard, the different trail (sometimes called “train”) colors are due to chemical differences in the meteoroids. Different chemicals burn with different colors.
I rsead too that by itself, the meteorid and the shock wave in front are not bright enough to account for how bright meteors are. Turns out there is a chemical in the air that is created by the passage that happens to glow very efficiently when heated. I can’t remember what it is now, dadgummit, my brain is getting old, but it’s something like sulfur dioxide or thereabouts.