Artificial gravity is the theoretical increase or decrease of apparent gravity (g-force) via artificial means, particularly in space, but also on Earth. It can be practically achieved by the use of different forces, particularly the centripetal force and linear acceleration. The creation of artificial gravity is considered desirable for long-term space travel or habitation, for ease of mobility, for in-space fluid management, and to avoid the adverse long-term health effects of weightlessness. A number of methods for generating artificial gravity have been proposed for many years, as well as an even larger number of science fiction approaches using both real and fictitious forces. Practical outer space applications of artificial gravity for humans have not yet been built and flown, principally due to the large size of the spacecraft that would be required to allow centripetal acceleration rotating spacecraft.