### Falling free

There was a rather neat paper on arxiv.org today: The Flow Of Granular Matter Under Reduced-Gravity Conditions, by Hofmeister et al. They point out that while we understand the movement of, say, sand fairly well under terrestrial gravity, objects like the Moon and Mars are covered with fine powdery regolith, and we don't really understand how it moves under the lower gravity there. This includes, for example, the steepest stable slope, obviously important for understanding the dunes on Mars. So they did some experiments in reduced gravity - which is much harder than it sounds.

Gravity is tricky stuff. It's such a weak force, you can't manpiulate it by moving sources around, like you can with electricity and magnetism. You basically need to use whole planets to make any appreciable amount of gravity. But you can fake it with accelerations - and in fact it turns out that "fake" isn't the right word, since accelerations are indistinguishable from gravity. So if you want to do some experiment in higher than normal gravity, you can put it in a centrifuge. But if you want less than normal gravity, that's not so easy.

There are basically four ways to get reduced gravity. The first is to fake it: many experiments (wind tunnel tests for example) are done on scale models. The scaling requires cleverness, because (for example) the surface-to-volume ratio of a toy airplane is very different from that of a real airplane. So you also need to scale things like air speed and air density appropriately. So if you're studying some gravitational phenomenon, fluid flow perhaps, you may be able to build a model scaled so that the needed gravity for the model is one Earth gravity. This only works for systems with pretty simple physics, though, for which you understand the scaling.

The second way to get reduced gravity is to do your experiment on the ISS. Unfortunately, it costs hundreds of thousands of dollars per kilogram to get anything up there, so this is rather a last resort.

There is also the famous "Vomit Comet", which flies along parabolic trajectories to provide about half a minute of microgravity to those aboard. This is great for training astronauts (and wealthy space nerds) but it's very difficult to keep residual accelerations small enough for precision experiments.

This brings us to the system Hofmeister et al. used: drop towers. In principle they're very simple: you put your experiment in a box, then drop it from a tall tower. For as long as it takes to fall, you have microgravity. If you want to double the time, you can even fling it up from the bottom. Of course, the devil is in the details; for example you want to evacuate the tower so there aren't residual accelerations from air drag, and you need to plan on the capsule being pushed a few centimeters to the side by the Coriolis force. Plus, of course, you need to stop it non-destructively when it reaches the bottom. But all these problems are tractable, and in Bremen there is a drop tower that can give up to nine seconds of free-fall.

Hofmeister et al. used the Bremen drop tower for their experiment. Perhaps perversely, in order to get the reduced but non-zero gravity they needed, they put their experiments in a centrifuge inside the capsule. They hooked up a high-speed camera, and were able to track each grain of sand as it slid down the slopes. They found that the steepest stable angles and velocity patterns of the flows were rather poorly described by existing theory. So it looks like the theorists have some work ahead of them before they can understand the Martian dune fields.