One of the biggest misconceptions about gravity is that it doesn't exist in space. But the truth is, gravity is what keeps the planets in line. It surrounds earth, weaves its way through our solar system, and glues the galaxy together. So why do astronauts float? Is there less gravity? Well, not really. When cosmonauts bob around the International Space Station (ISS), you’re actually watching them fall. Yeah, they’re free-falling at an enormous speed toward the earth’s horizon. Hard to picture? Well, imagine jumping down an elevator shaft in a very tall building (bear with us)...
You’d pick up a lot of speed and eventually reach terminal velocity (this is the maximum speed an object can fall in a particular environment). Now, imagine the walls of the elevator disappear… and the city that surrounds the building vanishes… and the bottom of the elevator stretches out so far beneath you it’s just a dot. You’d still be falling, but you’d have no reference point from which to ‘view’ your fall. The only thing that might imply you're falling is the air rushing past your face and through your hair, which doesn't happen to astronauts. So, let’s get rid of the air. Let’s say it’s the same amount of atmosphere that exists outside the ISS.
The station orbits within the thermosphere, which is basically a vacuum. And the 'air' in the station is a precise concoction of oxygen and other essential gasses. The nifty thing about vacuums is that all objects inside the vacuum have the same terminal velocity, which means they fall at the same rate.
Back to the elevator shaft. Let’s put you inside a shipping container, and throw that container down the shaft. You and the container would be falling together. And, because everything falls at the same rate in a vacuum, everything would be going at the same speed - your cheeks, legs, blood and hair held in a constant state of falling inside the metal container. As we remove the elevator walls, and the city disappears, and the bottom of the shaft becomes a dot, you'd look weightless, floating peacefully inside the container. Sounds a lot like an astronaut floating around the ISS.
But wait, if they're hurtling toward the earth, why don’t we see astronauts dropping out of the sky? Well, the station is orbiting so fast that it’s falling over the curvature of the planet. Imagine you threw a tennis ball - it might travel a few feet and then drop to the ground. But what if you threw it so hard and fast that when it started to drop, the horizon fell away underneath it, never allowing it to hit the ground? If the ISS were to slow down, it would drop to a lower orbit and its trajectory would fall short of the horizon, sending it straight to earth. Amazingly, it’s going at just the right speed to fall continuously... Ahh, physics.
Image by Bruce Christianson