Inertia is the property of an object that causes it to resist changes to its state of motion. Being an inherent property of mass, it is present even in the absence of gravity. For example, although a spacecraft may be located well away from any gravitating mass, its inertia must still be overcome in order for it to speed up, slow down or change direction. Inertia is a familiar concept in physics but its origin is poorly understood. It is believed to be a property of bodies primarily due to their interaction with masses at the edges of the universe. Momentum (mass times velocity) is a measure of inertia; changes in momentum, resulting from the action of an unbalanced force, permit a measure of inertial mass.