Artificial gravity is an acceleration resulting from the application of a force. In the context of manned spaceflight, artificial gravity may alleviate the adverse health effects of prolonged weightlessness.Astronauts routinely experience intermittent accelerations caused by the force of a rocket engine. However, the term artificial gravity usually refers to a sustained Normal force provided by a rigid structure in uniform circular motion. Practical outer space applications of artificial gravity for humans have not yet been built and flown, principally because of the large size of the spacecraft required to produce a useful centripetal acceleration, comparable to little g.