Atmospheric pressure on Earth, approximately 14.7 pounds per square inch (psi) at sea level, is created by the weight of the air column above us. While space is not absolute zero, the pressure is so incredibly low that it is best described as a near-perfect vacuum. Understanding this extreme difference explains why human and spacecraft survival depends entirely on engineered environments.
Defining Pressure and Vacuum
Pressure is a fundamental physical concept defined as the force exerted by matter over a specific unit area. This force results from the constant movement and collision of molecules within a gas or liquid against a boundary surface. For instance, the pressure inside a balloon is the cumulative effect of countless air molecules striking the inner walls.
A vacuum, in scientific terms, is an enclosed volume entirely devoid of matter. A perfect vacuum—a space with absolutely zero pressure—is a theoretical ideal that is virtually impossible to achieve. Any volume where the pressure is significantly lower than standard atmospheric pressure is considered a practical or partial vacuum.
The Near-Perfect Vacuum of Interstellar Space
The pressure in deep interstellar space drops to values so low they are often measured in fractions of a Pascal, far below \(10^{-17}\) atmospheres. This profound lack of molecules is what defines space as a vacuum.
Even in what appears to be empty space, a few particles, primarily hydrogen atoms, still exist, meaning the pressure is not technically zero. In intergalactic space, the density of matter can be less than one atom per cubic meter. This extreme scarcity means particles are too dispersed to exert any meaningful, collective force defined as pressure.
The Physical Effects of Low Pressure on the Human Body
Sudden exposure to the near-zero pressure of space has immediate and dramatic effects on the unprotected human body. The most rapid and life-threatening danger is an immediate lack of oxygen, known as hypoxia. Within seconds of exposure, the lungs can no longer exchange gases, causing a loss of consciousness in as little as 9 to 12 seconds.
Another severe consequence is ebullism, which is the boiling of bodily fluids at normal body temperature. This phenomenon occurs because the external pressure is no longer sufficient to keep water in a liquid state at 98.6°F, causing tears, saliva, and the moisture in lung tissue to vaporize. Ebullism begins at the Armstrong limit, where the pressure drops below a critical point of about 6.3 kilopascals (kPa). Though the body will swell due to the expanding gases, the skin is elastic enough to prevent the body from rupturing instantly, a common misconception.
Maintaining Internal Pressure in Spacecraft and Suits
To survive and function in the vacuum of space, humans must carry their own sealed, pressurized environments. Spacecraft like the International Space Station maintain a comfortable, full-pressure environment, closely mimicking the 14.7 psi, nitrogen-oxygen atmosphere of Earth. This eliminates the need for specialized breathing procedures and supports a long-term living environment.
In contrast, spacesuits, such as the Extravehicular Mobility Unit (EMU), operate at a much lower pressure, typically around 4.3 psi. This reduced pressure prevents the suit from becoming too rigid, which would make movement during a spacewalk nearly impossible. To compensate for the low total pressure, spacesuits use a pure oxygen environment, ensuring the astronaut still receives enough breathable oxygen to avoid hypoxia and ebullism. However, the transition from the spacecraft’s full atmosphere to the suit’s low-pressure, pure-oxygen environment requires a pre-breathe procedure to safely remove dissolved nitrogen from the astronaut’s bloodstream, preventing decompression sickness.