Water, essential for life on Earth, behaves differently in outer space. Its familiar liquid, solid, and gaseous forms are shaped by Earth’s atmospheric pressure and gravity. Beyond Earth’s protective atmosphere, the extreme environment of space dramatically alters water’s properties, especially in a vacuum and microgravity.
Water’s Phase Transitions in Vacuum
In the vacuum of space, water behaves uniquely due to the absence of significant atmospheric pressure. If exposed directly to space, liquid water would simultaneously boil and freeze. Low pressure causes water molecules to escape rapidly as vapor, even at low temperatures. As energetic molecules leave, the remaining water loses heat and cools rapidly, leading to freezing.
This dual phenomenon is governed by the relationship between temperature and pressure, particularly around the triple point of water. The triple point is the unique temperature and pressure where water can coexist as a solid, liquid, and gas in thermodynamic equilibrium. For water, this occurs at 0.01 °C (32.02 °F) and a pressure of 611.657 pascals (approximately 0.006 atmospheres). Below this pressure, liquid water cannot exist; ice transitions directly into vapor through sublimation without melting.
Water’s Behavior in Microgravity
Within a spacecraft like the International Space Station (ISS), water is contained and not exposed to vacuum, but behaves uniquely due to microgravity. In this near-weightless environment, surface tension becomes the dominant force. Water molecules are strongly attracted to each other (cohesion) and to other surfaces (adhesion). Without gravity, these molecular forces cause water to form spherical droplets, as a sphere minimizes surface area for a given volume.
Pouring water as on Earth is impossible; instead, it forms blobs that float away. Astronauts drink from sealed pouches with straws or use specially designed cups that employ capillary action. Water droplets can cling to surfaces or float freely, requiring careful fluid management.
Water on Other Celestial Bodies
Beyond Earth, water exists in various forms throughout the solar system and interstellar space. The Moon harbors significant amounts of water ice, particularly in permanently shadowed regions within polar craters. Estimates suggest hundreds of billions of kilograms of water ice are present, a valuable resource for future human missions.
Mars also possesses substantial water, mainly as ice in its polar caps and as subsurface permafrost. While liquid water is rare on the surface today, evidence suggests large quantities existed in the past. Recent findings indicate liquid water might still exist deep underground, 10 to 20 kilometers below the surface.
Further out, water ice is abundant on comets and asteroids, which may have delivered water to early Earth. Icy moons like Europa and Enceladus are believed to harbor vast subsurface oceans of liquid water. In interstellar space, water is found as icy mantles on microscopic dust grains within cold molecular clouds.
Water’s Importance in Space Exploration
Water is an indispensable resource for human space exploration, far beyond simple hydration. For astronauts, it is necessary for drinking, food rehydration, and personal hygiene. Given the high cost of transporting supplies from Earth—thousands of dollars per pound—space missions rely heavily on recycling water.
The International Space Station (ISS) utilizes advanced Water Recovery Systems that reclaim and purify nearly every drop of moisture. These systems process urine, sweat, and humidity from the cabin air, achieving up to 98% efficiency in converting wastewater into clean, potable water.
Beyond supporting life, water can be a source of fuel; electrolysis can split water into hydrogen and oxygen for rocket propellants. Water also offers potential as a shielding material against harmful space radiation. The ability to find and utilize water on other celestial bodies, known as in-situ resource utilization, is a significant focus for future long-duration missions to the Moon and Mars.