December 31, 2007:  After 14 years of effort, the U.S. Air Force has completed replacing the Minuteman ICBMs 1960s era NS-20 guidance system with the NS-50 guidance system. The design of the NS-50 was frozen in the 1990s, and installation of the NS-50s began in 2000. By early December, 2007, all 500 of the U.S. Minuteman missiles had the new guidance system. The NS-20 was upgraded over the decades, but the basic design remained much the same. The new NS-50 is more reliable and easier to maintain. The NS-50 will remain in service until the Minuteman missiles reach the end of their useful lives at about 2020.  The guidance system replacement is a major part of a refurbishment program that is costing about $5 million per missile.