Space is defined by a nearly perfect vacuum, an environment where the pressure of gas molecules approaches zero. This extreme condition presents two immediate challenges to human survival: the absence of breathable air and the lack of atmospheric pressure. On Earth, the atmosphere presses down with a force of 14.7 pounds per square inch (psi) at sea level, which is necessary to keep bodily fluids stable. Without this external counter-pressure, the fundamental biological processes that rely on dissolved gases fail instantly. Consequently, breathing in space is impossible, and an unprotected human body faces rapid physical catastrophe.
Physiological Effects of Vacuum Exposure
The most immediate danger upon exposure to a vacuum is not the freezing cold, but a rapid, severe lack of oxygen called hypoxia. Without external pressure, the partial pressure of oxygen in the lungs drops to near zero, causing gases to rapidly escape from the bloodstream into the vacuum. This process quickly depletes the oxygen supply in the blood, leading to unconsciousness in as little as 10 to 15 seconds as the deoxygenated blood reaches the brain.
A secondary, yet equally devastating, effect is ebullism, which is the spontaneous boiling of bodily fluids due to the extremely low pressure. Ebullism occurs when the ambient pressure drops below the vapor pressure of water at body temperature, around 47 millimeters of mercury (mmHg), which is equivalent to an altitude of approximately 63,000 feet. This boiling starts in the moist tissues of the mouth, eyes, and lungs, and then progresses to the soft tissues and venous blood.
The formation of water vapor and other gases in the tissues causes the body to swell dramatically, potentially doubling its volume, although the skin and underlying tissues are resilient enough to prevent rupture. If the exposure is a rapid decompression, an additional risk is pulmonary barotrauma, or lung rupture, which occurs if the victim attempts to hold their breath. The rapid expansion of air trapped in the lungs creates a fatal pressure difference, making it safer to exhale immediately upon exposure.
The Necessity of External Pressurization
To counteract the vacuum, any environment supporting human life must act as a pressure vessel, whether it is a spacecraft hull or a spacesuit. This requirement for external pressure is far more important than the composition of the breathing gas itself. The sealed structure applies physical force against the body, preventing the onset of ebullism and keeping gases dissolved in the blood.
Spacesuits, which are essentially personalized spacecraft, maintain an internal pressure, typically around 4.3 pounds per square inch absolute (psia). This pressure is significantly lower than Earth’s sea-level pressure of 14.7 psia, which is a compromise to allow the suit to remain flexible enough for the astronaut to move. The suit’s pressure is contained by an inner bladder layer made of gas-retaining material, while outer restraint layers prevent the suit from ballooning excessively.
Because the spacesuit pressure is lower than Earth’s, astronauts must pre-breathe pure oxygen for several hours before a spacewalk, a process called denitrogenation. This flushes inert nitrogen from their bloodstream and tissues, preventing decompression sickness, commonly known as “the bends,” when transitioning to the lower-pressure suit environment.
Life Support and Atmospheric Regulation
Maintaining a life-sustaining environment requires a closed-loop system of atmosphere management called the Environmental Control and Life Support System (ECLSS). Within a spacesuit, this system is condensed into the Primary Life Support System (PLSS), worn as a backpack during spacewalks. The PLSS provides a continuous supply of breathable oxygen to the suit and regulates the total pressure.
The breathing mixture in US spacesuits is typically 100% oxygen at the reduced pressure of 4.3 psia. This pure oxygen environment ensures the partial pressure of oxygen is high enough to sustain life, despite the low total suit pressure. A continuous flow of oxygen is circulated through the suit to provide ventilation and remove contaminants.
A primary function of the PLSS is carbon dioxide (\(\text{CO}_2\)) scrubbing, which removes the exhaled waste gas from the circulating atmosphere. High \(\text{CO}_2\) levels can cause hypercapnia, leading to headaches and impaired performance. This removal is often accomplished chemically using cartridges containing lithium hydroxide (LiOH) or advanced regenerative sorbents.
Temperature and Humidity Control
The PLSS also manages temperature and humidity. This is achieved using a liquid cooling garment worn by the astronaut and a sublimator that rejects heat into space via water evaporation.