Inhabited Earth-orbiting vehicles such as the Space Shuttle return to Earth from speeds of around . Due to the effects of gravity, a vehicle returning from the Moon hits Earth's atmosphere at a much higher speed of around . The g-loading on astronauts during the resulting deceleration can be at the limits of human endurance even during a nominal reentry. Slight variations in the vehicle flight path and reentry angle during a return from the Moon can easily result in fatal levels of deceleration force.