Vandenberg Air Force Base is located on the central coast of California, almost midway between San Diego and San Francisco. It's the third largest Air Force Base in the nation, encompassing 98,400 acres. The first military installation on the site, an Army camp, was constructed in late 1941. Years later, the base was handed over to the Air Force, which, in 1957, began construction of the infrastructure and launchpads, beginning with seven Thor ballistic missile launchpads. The first Western Range launch of a Thor missile occurred in December 1958. In February 1959, the world's first polar orbiting satellite, Discoverer I, lifted into space aboard a Thor/Agena from Vandenberg. That same year, Vandenberg housed the first nuclear ICBM to be placed on alert in the United States. By June of 1960, the Western Range had flight-tested 45 ballistic, orbital, and probe launch-vehicle systems. The Air Force annexed more land at the southern tip of the base in 1966 to construct Space Launch Complex 6 for the proposed Manned Orbiting Laboratory, which was never completed. The complex is being refurbished to accommodate the Delta IV evolved expendable launch vehicle.