Why is decompression explosive?

We’ve all seen it in movies - a breach in the hull of a spaceship or high altitude plane and the air inside literally explodes outwards. But I started thinking about it last night and I began to wonder why.

Consider it from an atomic/molecular level. You’ve got a bunch of molecules moving around. Their various masses exert gravitational attraction on each other. Some of them occasionally move too close together and are repelled from each other by electromagnetic force. Overall, there is a bunch of small particles randomly floating around.

Then a “hole” is created to an area where there are no particles (or at least significantly fewer particles). What exactly is the force that is either pulling particles into the lower density or pushing them out of the higher density. I understand a few are going to randomly move out through the hole because they were already heading in that direction. But what’s causing the sudden mass movement?

You’re forgetting about pressure. Gas wants to expand as much as possible, and thus exerts a uuniform amount of pressure on the inside walls of the spaceship. When the evil space villain shoots a hole in the wall, that creates a pressure differential between the vacuum of space and the pressurized inside of the craft, so the gas goes bye-bye.

Also, gravitational attraction between air molecules is negligible; at the tiny mass level of atoms and molecules, the strength of gravity is vanishingly small compared to the electromagnetic force. It doesn’t even enter the equation.

What friedo said.

And in fact, the random movement of air molecules is what creates pressure in the first place.

Also realize that the air molecules at room temperature are not just “floating around” as you put it. For example the root-mean-square velocity for oxygen molecules at room temperature is 482.1 m/s. As the link states, this is equivalent to “1726 km/h, much faster than a jetliner can fly and faster than most rifle bullets.”

If you then consider the sheer number of air molecules involved in this rapid random motion, there will be quite a few going in the right direction to escape.

I was going to add that the gravitational interaction between the air molecules is negligible, but on preview, I see that Q.E.D. has beaten me to the punch.

The excellent TV program ‘Mythbusters’ showed that a small hole in a plane (e.g. from a bullet) would not cause explosive decompression and that nobody would be sucked out the hole (sorry, Mr. Bond - ‘Goldfinger’ wasn’t scientific :eek: )
They did show that a large hole would cause much more damage.

I think you are mostly correct in your point.

You do have to remember about pressure. Air doesn’t just diffuse out of the hole, the molecules on the side with air bump against the molecules in the middle, which are NOT bumped by any molecules on the side of outerspace. The molecules are then bumped out of the hole. This is the essence of pressure.

However, you’ll mostly only have wind in the proximity of the hole. Standing on the other side of the room you won’t get blown around. Also depends a lot on the size of the hole. If the hole is big enough, you’ll have to be hugging a wall so that the air behind you doesn’t throw you forward. (again, because of pressure, because of being bumped by molecules on one side and not the other).

What exactly is happening when you punch a hole in your spaceship, or get tossed out the airlock without a suit?

Air is a bunch of gas molecules bouncing around. Imagine one of those lottery things where the balls fly around. Open a hole on the sphere and by chance some of the balls are going to bounce out of the sphere. After a while, all the balls bounce out of the sphere, and the inside is now a vacuum. Same thing with air, except the balls are really really tiny, and we can detect the movement of very very large numbers of air molecules as “wind”.

So when you put a hole in a spacesuit, the air inside blows out. How much pressure is this? Well, 1 atmosphere. If it’s a small hole the air isn’t going to explode out, but sloooowly leak out, like a bicycle tube with a hole. If the hole is larger the air leaks out faster. So movies where people get torn to pieces are totally bogus. This isn’t much force, your body can withstand many atmospheres of pressure difference.

You’ll burp and fart as the pressure decreases, but you aren’t going to be torn apart, because 1 atmosphere of pressure in your lungs or stomach isn’t enough to tear apart your chest cavity and besides, that air has a much easier way to escape, out your mouth, nose, and anus. Your ears will hurt just like when an airplane descends except a lot faster, but you can clear your eustachian tubes to avoid getting your eardrums burst. Any mucous membranes are going to dry out fast, your eyes are going to hurt. But what will kill you is lack of air, not the drop in pressure. You’ll suffocate, not explode. One atmosphere of pressure is not enough to tear a person apart. This is the same amount of force a balloon pumped up to two atmospheres experiences in 1 atmosphere air.

Although, actually, i think the term “explosive” might be appropriate. In an explosion you have some amount of matter turned into a whole bunch of pressurized gas. If there happens to be a hole in the container, the fluid dynamics will be in principle the same. The differences are in degree. Explosions have vastly more pressure (way more than one atmosphere), and they’re not usually vented through a small hole. If in your spaceship you suddenly open all bay doors, it’ll look more like an explosion. If in your explosion you only have one small hole for gas to escape, it’ll look like a leak.

As they showed on Mythbusters, decompression isn’t necessarily explosive – they put a bullet in ther air tank a la Jaws, and it didn’t rupture. It just jetted out gas really quickly.

Lemur’s right about the human body, too. Arthur C. Clarke has been banging this drum for a long time. Even before 2001, he’s had astronauts being released into vacuum without ill effect. It appears in his fiction from the 1950s – The Other Side of the Sky and Earthlight. He even wrote a science fact article about it to coincide with the releas of 2001 (You can find it in his book ** The View from Serendip**).
But explosive decompression occurs in movies (Outland, License to Kill, Total Recall because it looks cool. It’s even in books (Pierre Boule’s Garden on the Moon, Martin Caidin’s Four Came Back. And Caidin, author of “Marooned” and of what became “The Six Million Dollar Man”, was a pilot – he shoulda known better)

This is incorrect. You body can withstand a lot of atmospheres of pressure, but it can only withstand a very small pressure difference.
Any SCUBA diver knows this from learning about Arterial Gas Embolism
Also don’t foget about decompression sickness (The Bends) When you take the pressrue off too fast, the nitrogen in your blood bubbles just like a soda does when you pop the cap. Death can result.

This is true when you’re diving, because the pressure causes the nitrogen to go into solution–very deep divers don’t breathe any nitrogen at all, for this very reason. They normally use a mixture of inert gas like helium, with a small percentage (typically around 3%) oxygen. Starting from one atmosphere inside your spacesuit, your blood won’t have all that much N[sub]2[/sub] dissolved in it. The bends are a problem for divers, not astronauts.

There have been at least two cases where a human body (or a part thereof) has been accidentally exposed to a significant partial vacuum, that I’m aware of. In neither of these cases was there any permanent injury.

At one ATM you will have more N2 in your blood then it could hold at 0 ATM.
Needless to say at 0 pressure you have other problems, but it does not take much in the way of N2 bubbles to cause a problem.
You are correct about very deep divers using mixed gas, sport divers usually don’t. Decompression sickness can set in even in shallow diving if repetive dives are made. IIRC there can also be problems if a dive is made too close to when an airplane trip is scheduled. :eek:

I’m not sure that Outland and Total Recall support your argument: in both, is it not the case that a bullet or something shatters the large glass barrier between atmosphere and vacuum, only after which there is a massive outgassing? This seems much more reasonable.

To clarify: the cause of “the bends” (clinically known as DCI or decompression illness) is that nitrogen gas (making up roughly 78% of atompheric gases) will dissolve in blood. At normal pressures, this isn’t an issue; it dissolves out at the same rate. However, if you are exposed to much higher pressures for a period of time, there will be an excess amount of nitrogen dissolved in blood that cannot be readily removed. When you ascend too rapidly, or don’t make decompression stops to allow the nitrogen to be removed, it forms small bubbles that for some reason collect at the joints and tend to cause pain when the joint is extended (hence, the bends). A secondary problem with nitrogen at deep levels is called nitrogen narcosis, where people start acting as if intoxicated. The reason for this is poorly understood but it is a concern. Other issues are arterial embolisms (gas dissolving out and forming bubbles in the blood stream) and pneumothroax injuries (ruptured lung) and other barotraumas from the sudden change in pressure.

As Q.E.D. notes, technical divers who dive to long depths or do extended duration dives will use a mixture with other gases–usually inert helium–to reduce or eliminate nitrogen (tri-mix or Heliox). This still typically requires decompression stops (in fact the use of helium generally requires longer stops than a Nitrox mixture alone) but reduces the hazards associated with compressed plain air at great depths. The reason the proportion of oxygen is reduced is due to its toxicity at depth; below about 160 feet a 21% oxygen level is dangerous.

Regarding explosive decompression: it seems to be mostly a myth; certainly the kind of eyeball-protruding, head-explosing decompression you see in movies is pure Hollywood bullshit. A rapid transition from 1 bar of pressure to 0 bar might rupture sinuses and cause surface capillaries in the eyes, nose, and perhaps lungs to break, but there isn’t going to be any kind of explosing mass of flesh and blood that made Event Horizon so gory. And I doubt you’d be subject to the bends; in any case, you’d have other things to worry about, like freezing to death and asphyxiation. If you’re really lucky, a ship with Improbability Drive might swing by and pick you up 29 seconds after exposure.


In both those movies the people practically explode. I’m saying that that doesn’t happen with people. If you put a hole or crack in something as fragile as glass, I have no doubt that it will shatter under the pressure behind it, but that’s not what I’m talking about.

Near the end, Cohaagen does explode. It made me roll my eyes, too.

I don’t think he does – they show the tongue protruding and the eyeballs sticking out on stalks with what is too obviously (for as big a budget film as this) a model, but my recollection is that they don’t show it blowing up. You do get to see or hear (or experience the aftereffects of) exploding people in License to Kill, Outland], and, I gather Event Horizon (I avoided that one like the plague, and have only stumbled across bits of it).

The trouble people have is thinking that vaccuums suck. Vaccuums don’t suck, air blows. So your eyes can’t be sucked out of their sockets, they have to be blown out of their sockets by air pressure behind your eyes. Trouble is, there’s practically no air behind your eyeballs. And that tiny amount of air can only exert a tiny outward force on your eyeballs…not nearly enough to blow them out of their sockets.

Same thing for your lungs or your head. The small amounts of 1 atmosphere air in your body just don’t have enough energy to rip your skin apart, let alone bones. And body parts that have large amounts of air also have holes to let that air escape. The only thing I’d be worried about are your eardrums, which are very thin membranes with a relatively large reservoir or air behind them and a mostly closed tube to equalize pressure.

So, I dragged out my DVD and skipped to the scene in question, and you’re right. I thought he actually blowed up, but nope. Sure looked like he was about to, though.

[SCTV]He sure almost blowed up good, though, din’t he?[/SCTV]