Rotating spacecraft. Illustration showing how a spinning spacecraft can simulate gravity. The astronaut standing in the rim of the rotating craft would fly off at a tangent if not for the enclosing wall of the spacecraft. Centripetal force (short red arrow) is the force that makes a body follow a curved path, preventing it from continuing in a straight line, and is directed towards the centre of the circle. The astronaut experiences an outward 'centrifugal force', which is really the his own inertia acting against the circular rotating movement of the wall. This apparent force can be used to simulate the force of gravity.