The Air Force presently deploys 450 Minuteman III ICBMs in fixed, landbased launch facilities (or "silos"), located on three bases that encompass vast regions of the United States. Three missile wings-the 90th Missile Wing, 91st Missile Wing, and 341st Missile Wing, which fall under the command of Air Force Global Strike Command-operate the ICBM force. Each ICBM carries up to three W78 warheads, or a single W87 warhead. In fiscal year 2007, the Air Force began deploying W87 warheads, while reducing the number of deployed W78 warheads. To support U.S. Strategic Command's operational requirements, the Air Force maintains nearly all of the ICBMs on alert at any given time.