firing Nike nuclear missiles against oncoming ICBMs, thus exploding nuclear warheads over the North Pole. This idea was soon scrapped and work began on Project Defender in 1958. Project Defender attempted to destroy Soviet ICBMs at launch with satellite weapon systems, which orbited over Russia. This programme proved infeasible with the technology from that era. Work then began on the Sentinel Program which used anti-ballistic missiles (ABM) to shoot down incoming ICBMs. In the late 1950s United States Air Force considered detonating an atomic bomb on the Moon to display U.S. superiority to the Soviet Union and the rest of the world (Project A119). In 1959, a feasibility study of a possible military base on the Moon (Project Horizon) was conducted. In 1958, a plan for a 21-airman underground Air Force base on the Moon by 1968 was developed (Lunex Project). The Safeguard Program was deployed in the mid-1970s and was based on the Sentinel Program. Since the ABM treaty only allowed for construction of a single ABM facility to protect either the nation's capital city or an ICBM field, the Stanley R. Mickelsen Safeguard Complex was constructed near Nekoma, North Dakota to protect the Grand Forks ICBM facility. Though it was only operational as an ABM facility for less than a year, the Perimeter Acquisition Radar (PAR), one of Safeguard's components, was still operational as of 2005. One major problem with the Safeguard Program, and past ABM systems, was that the interceptor missiles, though state-of-the-art, required nuclear warheads to destroy incoming ICBMs. Future ABMs will likely be more accurate and use hit-to-kill or conventional warheads to knock down incoming warheads. The technology involved in such systems was shaky at best, and deployment was limited by the ABM treaty of 1972. In 1983, American president Ronald Reagan proposed the Strategic Defense Initiative (SDI), a space-based system to protect the United States from attack by strategic nuclear missiles. The plan was ridiculed by some as unrealistic and expensive, and Dr. Carol Rosin nicknamed the policy "Star Wars", after the popular science-fiction movie franchise. Astronomer Carl Sagan pointed out that in order to defeat SDI, the Soviet Union had only to build more missiles, allowing them to overcome the defence by sheer force of numbers. Proponents of SDI said the strategy of technology would hasten the Soviet Union's downfall. According to this doctrine, Communist leaders were forced to either shift large portions