While in space, astronauts live and function in a weightless environment, both inside and outside of their spacecraft. This situation differs greatly from Earth’s gravity-dominated environment. In order for astronauts to experience the challenges of weightlessness prior to performing actual spacewalks, it is necessary that they undergo simulated weightless conditions here on Earth. (Note: In this article the terms “weightless” and “weightlessness” are used to refer to situations in which the effects of gravity are not observed or felt. Of course, the gravitational force between bodies exists everywhere in the universe, but astronauts in space usually do not experience its effects.)

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