Most often, we imagine how astronauts float in “zero gravity”, as if the attractive force of the Earth disappears as soon as you go a few hundred kilometers above the surface. In reality, this is not true. The International Space Station is still deep in the zone of influence of Earth’s gravitational field, at an altitude where gravity is still almost 90% of the strength we feel on the ground.
What we see as weightlessness is actually constant free fall. The ISS and all the people and objects in it fall around the Earth at a speed of about 28,000 km/h. The station is not floating because it is out of the reach of gravity, but because it is moving sideways so fast that it is constantly “waving” the planet.
The ISS flies at an average altitude of slightly more than 400 km. That sounds like a lot, but it’s small compared to the Earth itself, whose radius is about 6,400 km. Gravity weakens with distance, but does not drop off dramatically at that height. NASA explains it simply: a person who weighs 45 kg on Earth, at the top of an imaginary ladder that reaches the ISS, would have a sense of weight as if he were about 41 kg. In other words, if the station somehow stood on a huge tower, the astronauts would normally stand on the floor in it.
The ISS and astronauts are still deeply affected by Earth and its gravity, but it moves fast enough to miss the planet regularly
The difference is that the ISS does not stand still. She keeps falling. Isaac Newton explained this idea with a thought experiment with a cannon on a high mountain. If you fire the cannonball weaker, it falls close. If you shoot it harder, it falls further. If you fire it fast enough, the Earth’s surface curves under it at the same speed as it falls, so it doesn’t hit the ground, but continues to circle.
That’s the orbit: falling steadily toward the planet, with a high enough lateral velocity that the body keeps missing the surface. ISS does just that. The earth is constantly attracting it, but the station is moving so quickly to the side that it does not have time to fall to the surface.
That’s why astronauts float. The station, crew and all objects in it fall together at the same speed, so nothing presses against the floor. The astronaut does not feel weight because there is no support “pushing” him back. A similar feeling occurs for a moment during the sudden fall of a roller coaster or an elevator, only that the fall does not stop in orbit.
The key is speed. The ISS moves at about 7.7 km per second and orbits the Earth in about 90 minutes. The crew therefore sees the sunrise or sunset approximately every 45 minutes, that is, about 16 sunrises and 16 sunsets during one day.
This is why scientists don’t like the term “zero gravity”, but instead use the term microgravity. Gravity still works, but the apparent weight inside the craft is very small. However, there are minor residual effects: the resistance of the very thin upper atmosphere, the gravitational differences along the station, and the movement of the crew itself.
Even at an altitude of 400 km, the atmosphere is not completely empty. This weak drag slowly slows the ISS down and lowers its orbit. That is why the station is periodically raised additionally, so that it does not lose height over time and re-enter the atmosphere. That’s why it’s important to understand the difference. Astronauts do not float because they have escaped gravity. They are in constant decline around Earth, at almost 8 km per second, but they are moving fast enough to never touch the ground, Space Daily explains.