Weightlessness is the condition of zero-gravity (zero-g) or microgravity (near-weightlessness) experienced by all spacecraft and their occupants when in orbit, in any other state of free fall, or traveling through space at constant speed. Weightlessness can be of great benefit in certain areas of research and manufacturing, and large construction work in space. Large masses in orbit do not require support, and their movement is restricted only by inertia. Structures can be designed without provision for support against the forces of gravity – in free space there is no such thing as a static load. On the other hand, long-term exposure to weightlessness has adverse health effects, including muscular deconditioning and bone demineralization.
The first person to write about the possibility of weightlessness may have been Bishop Francis Godwin in his book The Man in the Moone (1638).