Space is largely a near-perfect vacuum, meaning it does not contain free, breathable oxygen as found in Earth’s atmosphere. While oxygen is one of the most abundant elements in the universe, it rarely exists in a readily breathable form. Oxygen atoms are typically bound within molecules or frozen onto dust grains, making them inaccessible for human respiration. However, oxygen does exist in space in these bound forms within various celestial objects and cosmic structures.
Understanding the Vacuum of Space
Space as a vacuum means the extreme absence of matter. Unlike Earth’s atmosphere, which is dense with gases, the space between planets and stars is incredibly sparse. A typical cubic centimeter of air at Earth’s sea level contains about 2.5 x 10^19 molecules. In contrast, interstellar space, the region between stars, may contain only about one atom per cubic centimeter. Even sparser is intergalactic space, the vast areas between galaxies, where particle densities can be 100 times lower.
This low density means oxygen atoms are too far apart to form a breathable atmosphere. Gravity plays a significant role in this distribution, causing matter to clump together into stars and planets, leaving the vast stretches between them largely empty. Space lacks the concentration of gas molecules necessary to support human life without technological intervention.
Oxygen Beyond Earth’s Atmosphere
Oxygen is present throughout the cosmos, though not typically in the molecular form (O2) that humans breathe. It is the third most abundant element in the universe, formed during stellar processes within massive stars. Oxygen is highly reactive and readily forms chemical bonds with other elements.
Much of the oxygen in space exists as part of compounds, such as water ice (H2O) or carbon dioxide (CO2). Water ice is found abundantly on comets, within nebulae, and on various moons and planets, including Mars and Earth’s Moon. For example, the Moon’s regolith is estimated to be about 45% oxygen by weight, chemically bound within minerals. Mars’ atmosphere is primarily carbon dioxide, with oxygen atoms bound within these CO2 molecules.
Oxygen is also found in nebulae, vast clouds of gas and dust where stars are born. While molecular oxygen (O2) has been detected in some nebulae, it is generally dispersed and not in breathable concentrations. Oxygen atoms also combine with dust grains, forming part of the interstellar medium. Scientists are exploring methods to extract this bound oxygen from lunar soil or Martian atmospheric carbon dioxide for future human missions.
Providing Oxygen for Space Travel
Since space does not offer breathable oxygen, humans rely on technology to create and maintain artificial atmospheres for space travel. Spacecraft, such as the International Space Station (ISS), are sealed environments designed to contain a breathable atmosphere similar to Earth’s. Life support systems on these spacecraft actively generate and recycle oxygen.
The primary method for oxygen generation on the ISS is electrolysis, using electricity to split water (H2O) into hydrogen and oxygen. The Oxygen Generation System (OGS) and the Russian Elektron system are examples of such electrolyzers. Water for this process is recycled from various sources, including astronaut urine and condensation. For shorter missions or as a backup, spacecraft also carry oxygen in pressurized tanks or utilize solid fuel oxygen generators.
Astronauts conducting spacewalks rely on space suits, which function as personal, self-contained environments. These suits provide a pure oxygen atmosphere, supplying breathable air and removing carbon dioxide. The oxygen is typically carried in tanks within a backpack-like Portable Life Support System (PLSS) integrated into the suit. This ensures a continuous supply of breathable oxygen, always brought from Earth or generated onboard, never collected from space.