Gravity is taken for granted for us dwellers on the surface of our home planet, the earth. When we speak about astronauts in space however, the concept of gravity changes somewhat. Footage of astronauts floating while on board the International Space Station suggests that there is no gravity aboard the station – but at an orbital distance of just 400km, how can this be true?
The answer to this question is that there is indeed gravity at the altitude of the ISS, pulling the station towards the ground all the time. However, the high speed at which the ISS travels cancels out the earth’s gravitational pull, meaning that the combined effect is that astronauts on board the station experience zero gravity.
This does not mean, however, that there is no gravity at the altitude of the ISS – indeed, gravity at 400km altitude is not much less than that experienced on the surface of the earth. However, on-board the ISS, travelling at approximately 7km per second, zero gravity is indeed experienced.