Water drops can exist inside the pressurized environment of the International Space Station, but send them outside the cabin into the vacuum of space, and they can be liquid no longer. Image credit: ESA/NASA, of Andre Kuipers.

Water in space: does it freeze or boil?

Where liquids are impossible, science gets really interesting!

“You can’t cross the sea merely by standing and staring at the water.”
–Rabindranath Tagore

If you brought liquid water into outer space, would it freeze or would it boil? The vacuum of space is awfully different from what we’re used to here on Earth. Where you stand now, surrounded by our atmosphere and relatively close to the Sun, the conditions are just right for liquid water to stably exist almost everywhere on our planet’s surface, whether it’s day or night.

The gravitational pull on the gases in our atmosphere cause a substantial surface pressure, giving rise to liquid oceans. Image credit: NASA Goddard Space Flight Center Image by Reto Stöckli, Terra Satellite / MODIS instrument.

But space is different in two extremely important ways: it’s cold (especially if you’re not in direct sunlight, or farther away from our star), and it’s the best pressureless vacuum we know of. While standard atmospheric pressure on Earth represents about 6 × 10²² hydrogen atoms pushing down on every square meter at Earth’s surface, and while the best terrestrial vacuum chambers can get down to about one trillionth of that, interstellar space has a pressure that’s millions or even billions of times smaller than that!