In a somewhat famous Star Trek: The Next Generation quote Data corrects Riker on the 'correct term' for a scenario where a hatch on a space capsule breaks open and all the crew are killed.

Riker: "You were right. Somebody blew out the hatch. They were all sucked out into space."

Data: "Correction, sir, that's blown out."

Riker: "Thank you, Data."

Data: "A common mistake, sir."

Is there a technical or scientific and objective correct answer? Was Data, from a technical perspective, correct in his assertion or are both statements equally valid?

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    $\begingroup$ Regardless of the suck/blow distinction, the air is definitely expanding. It just has a lot of volume to expand into. $\endgroup$ – Erin Anne Apr 19 '18 at 5:22
  • $\begingroup$ This question is on the edge of being "opinion-based". There are two possible non-opinion-based questions you might be asking: (a) is one of these terms in wider use in <some defined community> or (b) is one of these terms preferred in some official policy. Could you clarify the questin to ask one or other of these? $\endgroup$ – Steve Linton Apr 19 '18 at 10:09
  • $\begingroup$ A lot of activity on this situation suddenly. I wonder if it was on rerun last night. $\endgroup$ – 6587 Apr 19 '18 at 16:05
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    $\begingroup$ Maybe it is related to the Southwest accident: nytimes.com/2018/04/18/business/… $\endgroup$ – Rory Alsop Apr 19 '18 at 18:17
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    $\begingroup$ Thinking about this some more, this is not opinion-based. The answer is a result of physics: gases exert pressure (due to molecules bumping into each other). This is what causes a gas to expand to occupy all available volume. The only force at work is repellent, so the answer must be "blown out". $\endgroup$ – Hobbes Apr 20 '18 at 11:00

Both are correct, it just depends on who/what is doing the action (sucking/blowing).

Consider the following scenario. You have two large tanks, sitting in your garage (presumably on planet earth). One has a vacuum in it at 0.01 ATM, the other has some pressurized air, lets say at 10 ATM. The two tanks are connected by a sealed pipe with a valve in the middle.

Now lets say you open this valve so air now starts rushing from the pressurized tank into the vacuum tank. Is the air being sucked or blown? Well the answer is, both, just depends how you construct the sentence. You could say "The low pressure tank sucked the air out of the high pressure tank". Alternatively, you could also say "The high pressure tank blew the air into the low pressure tank". Both statements are true, the verb just changes depending on what is doing the action.

So back to your original Star Trek quote. Both are right. The people were sucked out by the vacuum of space. But it is also true that the people were blown out into space by the space station's atmosphere.

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    $\begingroup$ This answer is better than the highest voted one. $\endgroup$ – Magic Octopus Urn Apr 19 '18 at 14:53
  • $\begingroup$ @MagicOctopusUrn Thanks, I thought so too :) $\endgroup$ – Jeffrey Phillips Freeman Apr 20 '18 at 19:27


Thinking about this some more, the answer is a result of physics: gases exert pressure (due to molecules bumping into each other). This is what causes a gas to expand to occupy all available volume. The only force at work is a repellent one, so the answer must be "blown out".

In everyday language, there are some exceptions. We say a vacuum cleaner sucks. Physically, it pumps air from the vacuum chamber to the outside, lowering the pressure in the vacuum chamber and enabling air to rush in through the nozzle.

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  • $\begingroup$ Actually, it sounds like the distinction you are making is that a vacuum moves in toward a central location and a compressor moves out from the central location. So things are only ever "sucked in" or "blown out." "Sucked out" makes no sense. Is this correct? If so, that is the most sense I've ever heard of this. $\endgroup$ – 6587 Nov 15 '14 at 21:29
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    $\begingroup$ No. If you have a vacuum test chamber, the air is sucked out to create the vacuum. $\endgroup$ – Hobbes Nov 16 '14 at 9:49
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    $\begingroup$ But that is relevant to unnatural pressure differences from an air pump, which is actively moving air against the natural pressure differences (ie using energy). In a decompression scenario, the air passively moves due to the natural pressure differences that were maintained because of the impassable barrier (ie release of potential energy). $\endgroup$ – 6587 Nov 16 '14 at 17:49
  • $\begingroup$ @fredsbend: While the whole matter has a lot of blurred edge cases where the answer would be unclear, generally history of creation of the vacuum or overpressure matters: You have two identical chambers connected by a valve, chamber 1 is connected to compressor, chamber 2 to a vacuum pump. You create 1 bar pressure delta between them, then disconnect the compressor and vacuum pump. And while the flow caused by opening the valve is the same whether the delta was caused by pump or compressor, the nomenclature is not. It's silly like that. $\endgroup$ – SF. Apr 19 '18 at 15:48
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    $\begingroup$ Essentially, the side that has "atypical pressure" due to mechanical action (directly or indirectly, e.g. pressurized from a tank that was earlier pressurized by a compressor), this side "takes responsibility" - is blowing/sucking. Since void is "naturally empty" and ship is "artificially inflated", it's blowing out. (but take the prior example, pump air from tank 1 into tank 2, then open the valve between them... there's no correct answer because one equally sucks as the other blows.) $\endgroup$ – SF. Apr 19 '18 at 15:51

One simple rule: You follow the energy. Where the energy comes from defines the process.

So while an ice-cube on your hand does result in a cooling effect, it is your hand transferring energy to the ice-cube, melting it, not the ice-cube melting to draw heat from your hand.

We see the same with pressure gradients. A common turn of phrase in the winter might be:

"close the door, you are letting the cold in"

This is a poor description of the process that is occurring. In this case, the energetic warm air is providing the energy that leads to the convection and thus the transfer of air. What we should say is that we are

"letting the heat out"

In a venting spacecraft, the energy comes from the atmosphere inside. It's akin to holding your hand over a plug-hole - it may feel like it is being "sucked" onto the plug-hole, but it's really the force (in this case weight) of the water pushing your hand onto it.

This means that

"Blown out" is the correct terminology.

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  • $\begingroup$ Can you provide a reference, semantically or scientifically, that "where the energy comes from defines the process"? It seems very strange that under this definition, a refrigerator is a device that warms everything in your house except the food. Similarly, under this definition, there is no distinction between a vacuum cleaner and a leaf blower, since they both "blow" debris using relatively high pressure air. But in reality, they're very different - one generates negative pressure, the other positive, so one sucks and one blows. $\endgroup$ – Nuclear Hoagie Apr 21 at 14:22

Could it be the difference about on whom the vacuum acts?

When the hatch is opened, the vacuum's zero pressure causes all the air to rush out. However, the people, the hatch or anything else are blown out by the air which is rushing out to fill the vacuum.

Because without the air inside, there wouldn't be a "blow out"... (like when astronauts go out for space walk, they open the hatch after decompressing the chamber)

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If we think of the atoms and molecules of the gas in the airlock, the only thing that happens are collisons between two gas atoms or collisons of gas atoms with the wall.

There is no force pulling or sucking the gas atoms out into the vacuum. Atoms hitting the wall are reflected, atoms hitting another gas atom will bounce back. The possibility of interactions between gas atoms out in the vacuum is very low due to the large distance from atom to atom. Therefore these atoms keep moving in the same direction.

If a gas atom is moving by chance into the direction of the open hatch, it will leave the airlock. Only another collison with a gas atom before leaving the hatch may change the direction of the atom.

An astronaut in the airlock will be hit by many atoms pushing him into the direction of the hatch, but very, very few atoms pushing him away from the hatch. Unfortunately the resulting force on the astronaut is moving him out of the airlock.

So the gas is not sucked out, it flows out into space driven by the thermal movement of the gas atoms and molecules.

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    $\begingroup$ This is the correct answer -- too bad it was submitted late. When the hatch is present, it reflects the gas molecules. When the hatch is absent, objects in motion remain in that motion, and the gas escapes. $\endgroup$ – DrSheldon May 24 '19 at 13:17

Actually, Data is also wrong - possibly!

A pressure due to incident wind (blowing) and a pressure difference across an object due to a sudden pressure gradient are two separate things!

The air pressure pushes on the hatch, the hatch exerts a pressure back on the gas, and the hatch-latch in turn pushes on the hatch.

When the hatch-latch stops holding, the hatch pressure drops to zero, and the air pressure accelerates the hatch.

Now there is a pressure gradient in the air. The air closest to the opening can not push back on the air farther inside. The air deeper in the ship pushes and accelerates the air closer to the opening.

"Blowing" usually refers to being pushed by a wind or velocity. You might have used pressure to create the wind, but "wind pressure" is not the same as a pressure gradient. For example, wind pressure depends on the mass density of the fluid and the velocity of the fluid. A pressure gradient is just about pressure.

The air "Pushes" itself out, it doesn't really get "Blown" out.

Same for the people. While there will be a force associated with the moving air blowing on them, there is a much much BIGGER force associated with the pressure gradient.

So I think that Data is also wrong. They were "Pushed" out by the sudden, transient pressure gradient.

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    $\begingroup$ Yes, they are pushed out by the airflow, but something being pushed by a flow of air is usually referred to as being "blown". $\endgroup$ – Baldrickk Apr 19 '18 at 13:21
  • $\begingroup$ "A pressure due to incident wind (blowing) and a pressure difference across an object due to a sudden pressure gradient are two separate things!" I don't follow your reasoning on this. For there to be a pressure force on an object, there must be a differential pressure. $\endgroup$ – Organic Marble Jan 8 at 14:11
  • $\begingroup$ @OrganicMarble Wait! Where have I said that they are the same thing? Where have I said that there was no differential pressure? Think about hanging from a skyhook in the jet stream, or from a rope in a fast river. There is a strong wind or flow that continues for hundreds of kilometers, but the pressure difference between one point and another point one foot downstream is nearly zero. But when you insert a person, a pressure difference across them will be formed by the blockage. That pressure differential wasn't there before, it was formed by the person blocking the flow. $\endgroup$ – uhoh Jan 8 at 14:27
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    $\begingroup$ I wouldn't touch this one. $\endgroup$ – Organic Marble Jan 8 at 14:30
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    $\begingroup$ @OrganicMarble I'm gonna pursue this now, it's driving me crazy! I'll post a question or two in Physics SE soon and ping you with a link. $\endgroup$ – uhoh Jan 9 at 2:47

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