Why swimming in space doesn't move astronauts

HostWe have all seen those movies where people float around inside a starship like they're fish in a tank. It looks so easy and fun, like you could just kick your legs and glide from one side of the room to the other. But in real life, if you find yourself stuck right in the middle of a big room on the space station, and you can't reach a wall or a handle, you might be in for a long wait. Why is it that waving your arms and legs around doesn't actually get you anywhere in space?

GuestIt's a funny thing to think about, but you can get stranded in the air just as easily as you could get stranded on a desert island. On Earth, we think of swimming as something that just works because we have water to push against. Water is heavy and thick. When you pull your hand through a pool, you're grabbing a big chunk of stuff and shoving it behind you. Because you push the water back, the water pushes you forward. But in a space station, you're not in water. You're in air. Even though there's no gravity pulling you down, the rules for how you move are still the same. You need something to push off of to get going.

HostBut we have air here on Earth too, and I can feel it when I run fast or put my hand out a car window. It feels like a real thing I can touch. So shouldn't a fast enough doggy-paddle still give you at least a little bit of a nudge if you try hard enough?

GuestYou would think so, but the numbers are just totally against you. Think about how much thinner air is than water. It's about eight hundred times less heavy. So, to get the same push you get from one easy stroke in a swimming pool, you would've to wave your arms hundreds of times faster than any human can move. You're trying to swim in a ghost. There just isn't enough stuff in the air for your hands to grab onto and move your whole body. Your arms move through it so easily that the air doesn't push back hard enough to move your weight. It would be like trying to swim in a pool filled with nothing but feathers or smoke.

HostOkay, I get that the air is too light to help much. But even if I'm not pushing against the air, if I kick my legs really hard in one direction, doesn't that force have to send me the other way? That's how a rocket works, right? It just shoots stuff out the back to go forward.

GuestWell, a rocket works because it's actually throwing something away from itself. It shoots hot gas out the back. When you kick your legs in space, you're not throwing anything away. Your legs are still attached to your body. So, when you kick your legs out, your hips move back a little bit to keep everything even. Then, when you pull your legs back in to kick again, you just pull your hips right back to where they started. You end up wiggling in place like a bug on its back. To move forward, you have to let go of some part of yourself or push off something that's not you. You can't just move your own parts around and expect your whole self to travel across the room.

HostThat sounds like a total nightmare. You're just hovering there, wiggling, and getting nowhere. But what if I just waited? Wouldn't the air flow in the station eventually drift me toward a wall or something I could grab?

GuestIt might, but you would need to be very patient. The space station has fans to keep the air moving so the astronauts don't end up in a bubble of the bad air they just breathed out. If you wait long enough, that tiny breeze might eventually blow you toward a handrail. But that could take a long time, and you might just drift in a circle. There's a story from the station about an astronaut who got stuck right in the middle of a wide-open module. He was just out of reach of every single wall. He was stuck there for a while just flailing around, and he couldn't get a grip on anything.

HostWait, if he was stuck, how did he get out? Did he have to wait for someone to come swim over and tow him to a wall?

GuestHe actually used a trick that sounds like a joke, but it's pure science. He took off his shirt and threw it as hard as he could in the opposite direction. Since the shirt has a little bit of weight, throwing it away gave his body a tiny shove in the other direction. It was just enough of a push to drift him toward a wall. It's the same way a garden hose kicks back when you turn it on. He had to give something up to get moving. If he hadn't been wearing a shirt, he could've tried blowing air out of his mouth really hard. It's a very weak engine, but in a world where nothing is rubbing against you to slow you down, even a tiny puff of breath will eventually move you. It just takes a lot of huffing and puffing to move a full-grown person even an inch.

HostIt's so strange because we think of space as this place where you're totally free to move, but you're actually more trapped than you're on the ground.

GuestYou really are. On Earth, we have the floor to push against. Even if we're in a chair with wheels, we can use our feet. But in that open air of a space station, you're a prisoner of your own weight unless you have something you can toss away.

GuestThe most useful thing you can have in that spot is a heavy tool or a bunch of clothes to throw, because without a wall to push, you're just a person wiggling in the dark.

HostThose fans on the station are doing a lot more than just keeping the air fresh if they're the only thing saving you from a long afternoon of floating in the middle of a room.