[Intro]

An important problem with space exploration that you're, no doubt, familiar with, even if you've never been up there is gravity. Or the relative lack of it. As we've mentioned before, it's not like there's no gravity in space. If there was no gravity, planets and stars would just fly off in different directions instead of moving in orbits.

The same goes for astronauts and spacecraft orbiting earth. They're always being pulled toward the planet at the same time they're moving horizontally really fast far above its surface. So, in effect, they're falling around the planet instead of straight toward the ground.

This constant state of free-fall creates the sensation that we often call weightlessness. More accurately, it's microgravity. And it changes a lot of what we take for granted. Astronauts on the International Space Station have to pee into a vacuum hose, for instance, which ... we don't.

But, anyway, before we send people up there, we have to make sure that both they and their equipment can work in microgravity. So, how do you do that without actually going all the way to space? There are two main ways that the pros use: neutral buoyancy and parabolic flight.

The neutral buoyancy method uses giant tanks of water. We're talking up to millions of liters to create a space-like environment. Water can be a fairly good substitute for space because, under the right conditions, you can have the same freedom of movement in all directions that you do in space.

Now, normally, objects in liquid will sink if they're denser than the liquid and float if they're less dense. That's why an ice cube will float in water and sink in alcohol. Neutral buoyancy means that an objects density is effectively equal to that of the liquid. So it neither floats nor sinks.

Robotic arms are pretty dense, as you can imagine. So, to test them under water, NASA engineers cover them with big, Styrofoam floats. Meanwhile, anything that's less dense than the water is attached to weight, so it won't float to the surface. Once everything is made neutrally buoyant, these objects can move in all directions, like they would in the microgravity of space. So, astronauts can experience a sort of, pseudo near-weightlessness.

Let's say a new part has to be installed on the outside of the International Space Station. The astronaut is going to need to do a spacewalk in order to install it, but she's going to need to practice. She might also need help on her mission from one of the station's resident robotic arms, so she'll need to figure out how to use that most efficiently. In neutral buoyancy testing, engineers will build a mock-up of the part of the space station that the astronaut will work on and also a water-optimized version of the robotic arm she'll be using. She can then have as much time as she needs to figure out an exact procedure for the tasks ahead of time, instead of trying to wing it while free-falling out in space.

But even though neutral buoyancy is a fairly good model for microgravity, it's not exactly like being in orbit. For one thing, the water's resistance makes it harder to move, but easier to remain still than it is in space. It's also not that practical to attach giant floats to a small tool. Imagine trying to use a drill covered in Styrofoam. There are ways to partially fix these problems, like using fake plastic tools, which are often used in astronaut training, but if that isn't enough, there's another way to simulate microgravity: parabolic flight.

You know that feeling when the elevator starts going down really fast and your stomach drops? That swooping sensation mean that you're feeling lighter than usual, a very faint and temporary version of a free-fall. In parabolic flight, airplanes create the free-fall effect by flying at steep angles, up and down, thirty seconds at a time, one hundred times in a row.

There's a reason these flights are fondly known as "vomit comets."

Parabolic flights create the effect of microgravity when the acceleration of the plane cancels out the acceleration of the stuff inside it that's falling due to gravity. Basically, if the aircraft and its occupants are falling together, the aircraft can't exert any force on the people and the objects inside, so the occupants feel weightless.

Despite what you might think, though, this effect doesn't just happen on the way down when the astronauts are in free-fall. It actually starts at the plane is flying up but begins to slow. At that point, the people in the plane will continue moving upward as the plane falls away beneath them.

Parabolic flight can be useful for things like instrument testing because it takes place in air rather than water, but it also has some obvious drawbacks. Namely, its 30-second time limit and its expense, mean that its not suited for training astronauts for long tasks, like simulated space walks. But for any potential astronauts, I'm sure it's a gut check in the most literal sense of the term.

Thanks for joining me for SciShow Space. If you want to keep helping us explore the universe, just to go Subbable.com/SciShow. And don't forget to go to YouTube.com/SciShowSpace and subscribe.

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