How to simulate gravity in space?

Anti-gravity doesn't exist, but its effects can be imitated. In a rotating habitat, any object tends to follow a straight-line path while the wall curves beneath it: the occupant ends up pressed against the outer wall, which becomes a floor. The strength of this artificial gravity depends on the radius and the rotation speed. A large ring spins slowly and stays comfortable; a narrow module must spin fast, and head movements then trigger nausea and visual illusions. Hence the dilemma: either a huge structure to launch and assemble, or a discomfort that's hard to sustain for months.

No crewed station has ever been fitted with artificial gravity. Rotating-ring projects remain studies, and the only centrifuges flown on board are small laboratory devices that subject cells, seeds, or samples to a chosen acceleration, in order to separate the effect of microgravity from that of radiation. On the ground, by contrast, space research uses short-arm centrifuges, where a lying volunteer endures an acceleration directed from feet to head, to test countermeasures.

With no floor available, the crew replaces mechanical load with exercise: a treadmill with a pulling harness, a bike, a resistance-adjustable strength machine, every day. Add to that compression garments, close medical monitoring, and rehabilitation upon return. None of this restores the effects of real weight, particularly on the distribution of body fluids — one of the arguments in favor of rotation for very long flights.