Science Puzzle
Which Way Is Down in Space?
Astronauts on the space station float, tumble and drift. Nothing falls to any floor. It is called zero gravity, and it looks as if gravity has switched off.
But the station orbits only 400 kilometres up, where Earth's gravity is still about 90 percent as strong as it is at your feet. So why does everything float?
The Answer
Gravity has not switched off at all. The astronauts, the station and everything inside are all falling toward the Earth, continuously, and have been since the moment they reached orbit. They float because they are all falling together, at exactly the same rate, so nothing presses against anything else. There is no floor pushing up on your feet, so you feel weightless.
Why do they never land? Because they are also travelling sideways at around 28000 kilometres per hour. Fire a cannonball hard enough and it curves out over the horizon before it hits the ground. Fire it hard enough and the Earth's surface curves away beneath it exactly as fast as it falls. It will fall forever without ever reaching the ground. That is an orbit: not an escape from gravity, but an endless fall around the planet.
So there is no down in orbit, and the proper name for that floating sensation is not zero gravity but freefall.
The principle: Orbits and freefall. Weightlessness is caused by everything falling together, and an orbit is a fall with enough sideways speed that the ground curves away as fast as you descend.