One of the projects I lead put a pair of Microsoft HoloLens mixed reality devices on the International Space Station. Before we did that, we had to make sure that the devices were actually going to work in orbit. The issue is that the HoloLens relies on a bunch of sensors to keep track of where it is at all times, and one of those sensors pays attention to the direction of gravity. So the problem is not that there's no gravity on the space station (there's about 80% of what there is on Earth), but that the space station is constantly falling. That's what orbit is-- falling all the time and never hitting the ground because you're moving forward so quickly that you miss it! We had to make sure that the HoloLens wouldn't lose track of which way it was pointing if it couldn't figure out which way was "down".
Our collaborators at Microsoft did a ton of simulations that all looked very encouraging, but the bottom line was that we didn't feel comfortable launching the HoloLens to the space station until we actually tested how it would perform in microgravity. There's really only one way to test how a piece of equipment reacts to an (apparent) lack of gravity, and it looks like this.
NASA calls it the "Weightless Wonder" but I'm guessing that's just because they decided that the more popular name of the "Vomit Comet" was less publicly acceptable. And just to get it out of the way, yes, that's an accurate name but no, I personally did not get sick.
Over the space of two days I went on three flights, each consisting of 40 parabolic flight maneuvers. During each parabola, the pilots first point the nose of the plane at the sky to climb to around 32,000 feet, then cut the engines and let the plane fall like a rock for about 20 seconds, then point the nose of the plane at the ground and gun the engines to pull back into a new climb. So, every minute is divided roughly in half, with half the time consisting of hypergravity (a bit under 2 gees) and the other half consisting of microgravity (around 0 gees). This rather unique oscillation is what causes some participants in reduced gravity experiments to part with the contents of their stomach. It probably also has something to do with the fact that you are gaining and losing 8,000 feet of altitude in the space of about a minute, over and over again.
I know this sounds like the world's largest roller coaster (not an entirely inaccurate description), but the whole point of all of this insanity is those brief spans of microgravity freefall that are quite similar to what the space station experiences constantly. Unfortunately, all microgravity experiments on the plane have to be designed to fit entirely within those 20-second blips, so we had to move quickly.
Every minute looked about like this:
0:00 - 0:20: Lie flat on your back while the pilot pulls ~2 gees. You can hear the engines roaring and it feels like someone is sitting on your chest. Lifting or turning your head during this time is a bad idea.
0:20 - 0:40: The engines of the plane suddenly go quiet and every part of your body starts to feel very light. You hear the flight director shout "Push!" You push up from the floor a bit, but not too hard! If you do anything that's close to a jump you'll fly up and smack into the ceiling! Quickly work through the steps of your test procedure as you float lazily through the air.
0:40-0:60: "Feet DOWN!" shouts the flight director as you fall to the floor (sometimes bouncing right back up in the air for a second). You have just a couple of seconds to get on your back and in your assigned spot before the 2 gees are back on your chest.
Repeat 120 times.
Our experiments were a complete success. The HoloLens handled microgravity just fine, and we were even able to test out some of the applications that expect to run onboard once we launch this winter.
Pictures don't do it justice, so here's a video of us that shows what it was like: