We've all seen surreal photos and videos of astronauts slurping orange juice from jittering, suspended spheres, or seen a typical space traveler's meal spread. (Whether it's food or drink, it's probably in a pouch.) But here, in a video dug from the archives by Reddit, we see an uncapped cup, made from a folded piece of an astronaut's Flight Data File mission book, that seems to defy physics.
It's a fun experiment, and a catchy introduction to the concept of surface tension--that's what's causing the water to gather near the crease--but it's also a window into much larger engineering projects. The same principles that Don Petitt is using to slurp his morning brew figure into the vital systems of nearly every spacecraft that has ever flown.
On Earth, surface tension acts subtly. It changes the dynamics of spills, gives water droplets their shape and helps our liquid thermometers work, but its effects are tiny compare to the forces it acts with and against--namely, gravity. In the absence of gravity, surface tension isn't just a feature of fluid dynamics, it dominates it. Hence, the floating bubbles, and creeping coffee.
Just as astronauts must change their drinking habits, so do the craft in which they float. Bruno Pattan, in his book, Satellite Systems: Principles and Technology, sums up the problem:
Surface-tension propellant tanks are a must-have for any vehicle that operates in zero-g environments and can't depend entirely on solid-state fuel, which is to say just about every spacecraft. While Pettit's experiment may look like a party trick, he's actually showing us how satellites, shuttles and space stations manage the essential weirdness of zero-g fluid dynamics. Specifically, they use hardware like this:
This is a schematic of a surface-tension propellant take, or Propellant Management Device (PMD) designed by PMD Technology to be launched with a a geosynchronous satellite in 1988. The four vanes that run up the sides of the sphere are analogous to the crease of Petitt's coffee cup: where the crease guides coffee into its user's mouth, these vanes guide sloshing fuel into a sponge, which holds it close to the satellite's propellant intake.
The science isn't particularly new, but this demonstration serves as a reminder that the engineering challenges faced by the folks at NASA, as well as private firms like SpaceX, aren't just huge, they're incredibly, wonderfully strange.