On Earth, water stably exists as a liquid under our atmosphere and proximity to the Sun. But in the vacuum of space, extreme temperature drops and near-zero pressure create dramatically different conditions. So, does water freeze or boil there?
Space is profoundly cold, especially far from stars, with the cosmic microwave background radiation holding steady at -270.45°C. Atmospheric pressure on Earth equates to roughly 6×1022 hydrogen atoms per square meter, while space's pressure is millions or billions of times lower.
Beyond Earth's atmosphere, both temperature and pressure plummet. At cosmic background temperatures well below water's freezing point, you'd expect instant freezing. However, pressure plays a crucial role too.
Reduce pressure around liquid water, and it boils—even at room temperature. This is why water boils below 100°C at high altitudes with thinner air, or vigorously in a vacuum chamber.
A video demonstrating water boiling in a vacuum chamber illustrates this perfectly.
Water remains liquid only within a precise range of temperature and pressure. Drop the pressure at a given temperature, and it boils; lower the temperature at a given pressure, and it freezes.
Related: Does cold water freeze faster than hot water?
In space's low temperature and pressure, water does both: it boils first, then freezes.
Astronauts on spacewalks from the International Space Station have observed this with expelled urine: it boils violently, the vapor deposits directly into ice crystals via deposition, forming a cloud of frozen particles.
This sequence stems from water's high heat capacity and surface tension, which forms droplets into spheres in microgravity, slowing heat loss. If isolated molecules faced vacuum instantly, freezing would be immediate—but clustered liquid boils first due to low pressure.
The resulting vapor, spread thin at near-zero pressure, cools rapidly. Below -63.5°C, it transitions from gas to solid ice, bypassing liquid, yielding delicate ice crystals.
