I read in a physics textbook that it is not zero gravity in ISS but the fact that the ISS is in "free fall" makes astronauts float around. They do not feel any normal force because the vehicle they are in are also in free fall along with them. Then I watched Neil Degrasse Tyson talk about bone loss in ISS because astronauts are in zero gravity. If it is zero gravity then why is the ISS in orbit? There should be no centripetal force to keep it in orbit. I am confused!