Starts With A Bang!

Liquids in Space

My biggest fan in the world, Zrinka from Croatia, wants to know how liquids work in space. After all, most of us are used to water (and other liquids) on Earth, where we know they do two major things:

1. They take the shape of whatever container they’re put in.
2. They flow downwards, towards lower elevations and fall towards the center of the Earth.

So they wind up doing very familiar things, like filling up glasses:

But something as simple as a liquid is completely different in space! First off, once you stop accelerating, there’s no external gravity! This causes water, or any liquid, to become a sphere:

There are certainly some neat things you can do to it, but I’d like to tell you how liquids become spheres first. Unlike a gas, the molecular bonds between a material in a liquid state are very strong. The more polar a molecule is (negatively charged on one side and positively charged on the other), the stronger the bonds between the molecules are. Water is an example of a very polar molecule, and that helps explains why it so easily becomes a liquid:

And that also explains the presence of surface tension, or the force that forms an outer surface on the liquid. The same phenomenon that forms a surface to water on Earth causes random water in space to form into spheres. We see this even on Earth, kind of. Take a look at this hi-resolution photo of water emerging from a faucet:

It’s starting to break up into spheres, but they’re deformed on Earth, whereas they aren’t in space. Why? Because there’s gravity and air on Earth. The gravity accelerates the sphere in one direction, and the air provides an uneven drag force on the sphere, causing it to flatten and deform. Here’s an actual photograph of a falling raindrop:

But what if we did this experiment someplace where there was no atmosphere, like on Mercury or the Moon? Would the sphere just fall, perfectly, without deforming? The answer is yes, but there’s one caveat. Because there’s no atmosphere, that means there’s no pressure on the surface of the liquid. There’s still surface tension, sure, but those of you who remember your chemistry will remember that if there’s no pressure on a liquid, it becomes a gas! So any liquid in the vacuum of space will form a sphere, will fall towards whatever gravitational body pulls on it while remaining perfectly spherical, and perfectly undeformed, but the outer layers of it will either freeze (if the temperature is cold enough) or boil (if the pressure is low enough) or both.

Check out the phase diagram if you don’t believe me — you get the pressure low enough, and everything becomes a gas:

Of course, some people have done far more advanced things with water, bubbles, etc. in space than I have the resources to do. But I do have the resources to show you their wonderful video; check it out!