You may have seen it happen in the movies, or on TV: an astronaut swims their way through space to get from point A to point B. It may look a little fun, but unfortunately, humans can’t swim through space because there’s not enough stuff to swim through.

What Is the Physics of Swimming?

We can easily swim through water because water is dense enough to allow us to push back on it with our hands and get something for our effort. By conservation of momentum, when we push on the water, the water pushes back on us, propelling us forward.

Why Can’t You Swim in Space?

However, the same swimming motions don’t work out so well for us in the air because our atmosphere is 830 times less dense than water. When we wave our hands and push on the air, the air is technically pushing back on us, but not enough to count.

Airplanes and flying animals can still manage it though, by finding the right combination of weight, thrust, and lift. In the end it’s all the same, however, just conservation of momentum: all flying objects push on the air, and the air pushes back on them.

Read More: How Mathematician Emmy Noether’s Theorem Changed Physics

Is There an End to Space?

The higher up in elevation you go, the less dense our atmosphere becomes, which is why high-altitude mountain climbers need to carry along supplemental oxygen – there just isn’t enough of it around up there. Technically, there is no outer edge to our atmosphere. The air just keeps getting progressively thinner the farther away you get from the Earth.

What Is the Kármán Line?

But in 1957 the Hungarian-American engineer and physicist Theodore von Kármán attempted to calculate an “edge” to space. He discovered that at an altitude around 100 kilometers, the listing force supplied by an aircraft moving through the atmosphere drops to essentially zero.

Since then, space agencies have taken this “von Kármán” line as a working, practical dentition of the edge of space. Aircraft simply don’t have enough air above this altitude to rely on normal lifting forces, and so they have to use something else to propel themselves forward: rockets.

Read More: The Kármán Line: Where Does Space Begin?

How Does a Rocket Move In Space?

Contrary to some perceptions, rockets do not work by pushing against anything in space. They can work in absolute vacuum. The key is again, conservation of momentum.

Instead of pushing against anything, a rocket forces its exhaust out one end, and in response, the rocket moves forward. You can replicate this feat at home. Get in a rolling office chair on a smooth floor and start throwing objects in one direction; with momentum conserved, you will start rolling in the opposite direction.

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What Is In Space?

Outer space is far emptier than anything we can construct in our vacuum chambers on Earth. Don’t get me wrong, there’s still a bunch of stuff: stray atoms, molecules, and dust grains floating around, highly charged subatomic particles zipping by, and the ever present radiation soaking the universe. But there just isn’t a lot of it. Across the entire cosmos, the average density of all the matter is roughly one hydrogen atom per cubic meter.

You can see just how thin the material of space is in the Sun’s atmosphere, the corona. Typically only visible during total solar eclipses, the corona has an exceedingly hot temperature of over a million degrees. But if you were to float through it, you wouldn’t feel a thing. You can only register that temperature when the particles that make up the corona slam into you and deposit their energy, and there aren’t enough particles around to make you notice.

Read More: What’s Hotter Than the Surface of the Sun? The Solar Corona

How Do You Move In Space?

Even though you can’t swim in outer space, there is a way for you to sail through it. The radiation that the Sun emits carries momentum with it. Even though photons, the fundamental particles of light, don’t have mass, they still have energy, and that energy packs a (very, very tiny) punch. It’s not enough to affect anything on Earth, let alone notice it. But in space, you can use this nonstop radiation pressure to push yourself around, as long as you’re relatively close to the solar system.

Testing Solar Sails in Space

Several private and governmental space agencies have already tested solar sails. There was even once an unintended use of the effect. When NASA’s planet-hunting Kepler space telescope lost control of some of its reaction wheels, which the observatory used to reorient itself, the engineers behind the mission concocted a clever scheme. They angled the Kepler spacecraft just right and used radiation pressure from the Sun to keep it pointing steady. The idea worked, and Kepler could continue its mission.

So you won’t need to grab your space-swimsuit and take a dip in that inky blackness anytime soon. But scientists and engineers continue to test and develop solar sails. They hope that someday they can be used as a form of slow, but incredibly cheap, transportation of materials among the planets.

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