How do astronauts train for weightlessness?

Space is a dangerous place, so you need to be prepared before you go there. In particular, you’ll need to be ready for an environment where you can’t rely on your own weight, especially if you’re likely to leave your vessel at any point.

On Earth, you’ll never experience the problem of getting trapped in mid-air in a room, because gravity means you’ll always be able to reach the floor. In space, though, if you’re not already moving and you can’t reach anything to pull on or push off, you can’t get anywhere.

It’s not easy to practise being weightless on Earth, though, so how are you supposed to get that experience?

In this post, we take a look at microgravity and how astronauts prepare for it.

Why do astronauts float on the International Space Station?

The further you move from Earth, the weaker Earth’s gravity becomes. However, the International Space Station (ISS) is relatively close to Earth.

The ISS is in low Earth orbit, around 400 km above Earth’s surface. At this altitude, the force of Earth’s gravity is roughly 90% of the force on the planet’s surface. This means that, if you were 400 km above the Earth, you would still weigh 90% of your weight on Earth.

Being at 90% of one’s usual weight is a long way from being weightless, so why do astronauts on the ISS float?

Essentially, they’re not really floating; they’re falling.

The ISS, as an Earth satellite, is orbiting the Earth. This means it’s constantly being pulled towards Earth, but, as it does so, it’s moving forward very fast, meaning it goes past the curve of the planet rather than being pulled down to the ground. Basically, as the ISS travels over the Earth, the curve of the Earth means the ground falls away at the same rate as the ISS falling towards it. Without the Earth’s gravity causing it to fall, the ISS wouldn’t have anything to keep it near Earth and would shoot off into space.

Everything inside the ISS, including its inhabitants, is falling at the same pace as the ISS. Because of this, they seem to float. As the ‘floor’ is always moving away exactly as fast as they move towards it, the astronauts will never reach it unless they propel themselves.

You might have heard the experience of weightlessness in orbit called ‘zero gravity’. Because gravity is still involved, though, astronauts and space agencies often call it ‘microgravity’ instead.

Astronauts and parabolic flights

Parabolic flights are one method used to train astronauts for weightlessness. These flights let astronauts experience the weightlessness of freefall for brief periods without leaving Earth’s atmosphere.

In a parabolic flight, an aeroplane speeds upwards at a steep angle, slows down at the top of its altitude, then travels downwards again. It does this multiple times.

At the top of each arc, the people aboard the flight experience weightlessness for about twenty seconds, as they’re falling at the same rate as the plane around them. You may have felt this yourself on a rollercoaster; at certain high points, as the rollercoaster begins to drop, it can feel as if you’re almost beginning to float off your seat.

These brief periods of weightlessness are used to prepare astronauts for the experience of being in space. Parabolic flights are also sometimes used to perform scientific experiments in simulated low gravity without needing to leave Earth. If your experiment takes more than a few seconds to complete, though, you might have to travel to space to get it done.

Astronauts and neutral buoyancy pools

Neutral buoyancy pools use water to give astronauts an Earth-based experience similar to the low gravity of space. In neutral buoyancy, precise weighting means that gravity is balanced by buoyancy, so you remain suspended at the same level underwater, rather than floating up or sinking down.

Being in a neutral buoyancy pool isn’t exactly the same as being in space. For example, water drags on anything moving through it, making movements slower, whereas there’s almost no resistance in space. It’s also possible to swim in water, of course, which isn’t an option open to astronauts in space. However, neutral buoyancy gives a sense of what it’s like to move around when you can’t make use of your own weight to, for example, stabilise yourself on a surface.

Because of this, astronauts often practise moving or performing tasks in neutral buoyancy, wearing spacesuits. Some neutral buoyancy training facilities include full-scale mock-ups of the equipment and environments the astronauts are expected to encounter in space, such as International Space Station components. This way, they can familiarise themselves with the space station and how to move around it before travelling to the real thing.

Pools can also be used to simulate other gravitational conditions, such as the low gravity of the moon. The European Space Agency’s short video ‘Moondive’ gives a quick introduction to some of the space-based tasks that can be simulated at their Neutral Buoyancy Facility.

At the moment, there’s no way to simulate microgravity perfectly for long periods on Earth. However, with parabolic flights and neutral buoyancy, astronauts can get a taste of weightlessness in advance, giving them an idea of what to expect when they venture into space.

If you’re interested in how microgravity affects other living things, we’ve written before about the challenges of growing plants in space.

Cover image: NASA

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