Project Orion was a study of a starship intended to be directly propelled by a series of explosions of atomic bombs behind the craft (nuclear pulse propulsion). Early versions of this vehicle were proposed to take off from the ground; later versions were presented for use only in space. Six non-nuclear tests were conducted using models. The project was eventually abandoned for multiple reasons, such as the Partial Test Ban Treaty, which banned nuclear explosions in space, as well as concerns over nuclear fallout.