Overview
Project Orion was a study into a "Nuclear Pulse" spaceship that would be propelled by exploding a series of atomic bombs behind a massive steel pusher plate. Led by physicists Ted Taylor and Freeman Dyson, the craft was designed to achieve both high thrust and high efficiency, theoretically capable of round-trips to Mars in just 125 days.
The "Military Space Battleship"
While NASA focused on exploration, the U.S. Air Force saw potential for a "military Orion"—a heavily armored space cruiser capable of carrying hundreds of nuclear warheads. This led to theories that the project wasn't truly canceled in 1964 but was instead moved into "deep black" classification to build a secret space fleet.
Termination
The official project was ended following the 1963 Partial Test Ban Treaty, which banned nuclear explosions in the atmosphere and space. However, the robustness of the design—which could tolerate accelerations of up to 100g for uncrewed craft—ensures that Orion remains a favorite topic for secret space program theorists.