Spacecraft decompression after the hatch is opened
A spacecraft’s outer hatch cannot simply be yanked open by accident. If it were somehow opened while the cabin was at normal pressure, however, the air would rush out, in severe scenarios within seconds. An unprotected person would have roughly 10 to 15 seconds before the brain became critically short of oxygen. Contrary to what films often show, the body would not explode, the blood would not boil violently, and the person would not freeze in an instant. The real threat is the sudden loss of breathable air.
What you need to know
- NASA safety requirements call for pressure to be equalized on both sides of a hatch before it is opened, and unlocking it requires a sequence of steps.
- In one well-known NASA vacuum test, a person remained conscious for about 14 seconds.
- Holding your breath during rapid decompression raises the risk of lung injury, because the air in the lungs expands as outside pressure drops.
- Lack of oxygen would kill a person before the cold of space could, because in a vacuum the body loses heat mainly through relatively slow radiation.
A spacecraft hatch is not a door you can just throw open
A spacecraft hatch is nothing like a balcony door with a handle. Modern crewed vehicles use locks, pressure monitoring and multiple required actions to prevent accidental opening. According to NASA safety requirements, the pressure on both sides of a hatch must be equalized before it is opened, and unlocking it takes a series of sequential steps.
The reason is straightforward. Inside the spacecraft there is an atmosphere, and outside there is almost a vacuum. Opening a hatch across a large pressure difference is extremely dangerous. That is why spacewalks start from an airlock. Astronauts enter a small separate compartment, close the inner hatch and lower the pressure inside. Only then do they open the outer hatch.
The air leaves first
Now imagine an emergency in which a large hatch is opened directly from a crew compartment. The air would surge outward. It is not quite accurate to say the vacuum “sucks” people out. Instead, air in the high-pressure area pushes everything toward the low-pressure area.
During a serious decompression, a spacecraft’s atmosphere can escape very quickly, and in severe scenarios this really is a matter of seconds. Light objects would fly toward the opening, and the flow could also drag along a person nearby.
Air and objects escaping through an open hatch
The first moments would also be quite loud. As the pressure fell, there would be less and less air to carry sound, and near-total silence would follow.
The spacecraft itself would not necessarily explode or break apart. The main problems would be the loss of atmosphere, flying objects, and people who suddenly had nothing to breathe.
Without a spacesuit, survival is measured in seconds
On this point, films can actually be too optimistic. A person does not die instantly, but there is very little time. During one well-known NASA vacuum test, a person remained conscious for about 14 seconds. That is not a universal timer, but it is a useful guide. After roughly 10 to 15 seconds, the brain begins to run critically short of oxygen.
A person without a spacesuit next to an open airlock
Holding one’s breath would also be a mistake. The air in the lungs expands as outside pressure drops, so a heroic attempt to take a deep breath and hold it would only increase the risk of lung damage. The body would also swell slightly, and problems could develop in the ears and other cavities that still contain gas. Still, a person does not inflate like a balloon or burst.
Blood will not boil, and the body will not turn to ice
Several persistent myths surround exposure to a vacuum. Here is how they compare with the physics.
The body will not explode. Skin and tissues are strong enough to hold the body together even when the outside pressure is practically zero.
The blood will not boil violently. Pressure inside the blood vessels prevents this. However, water on the tongue, on mucous membranes and in some tissues can begin to evaporate and form bubbles at very low pressure.
A person will not freeze in a couple of seconds. A vacuum contains no cold air to draw heat away quickly. Heat is lost mainly through radiation, which is far slower. Temperature in open space therefore works very differently from air temperature on a winter day. In other words, a lack of oxygen would kill a person before the cold of space could.
A person without a spacesuit in open space
Could the hatch be closed again?
While air is rushing out, fighting the flow is very difficult. NASA standards even cite a 1997 incident aboard the Mir space station, when the airflow during a decompression hindered the crew’s efforts to close a hatch. Once the pressure inside matches the pressure outside, the flow stops. In theory, the hatch could then be closed again, provided its design allows it. For an unprotected person, though, the situation would already be critical by that point.
A pressurized spacesuit is a completely different matter. It creates a small atmosphere of its own around the body and protects the wearer from a sudden drop in pressure. This is also why Alexei Leonov’s first spacewalk proved so difficult: the pressure inside his suit literally changed its shape.
Before a routine spacewalk, astronauts also breathe oxygen in advance. During an abrupt shift from high to low pressure, nitrogen dissolved in the body can form bubbles, much as it can in divers who surface too quickly. For this reason, efforts to reduce the risk of decompression sickness begin before the outer hatch is ever opened.
