Nuclear missile silos are clustered together, why?
Five hundred Minuteman III missiles are deployed at four bases in the north- central United States: Minot AFB and Grand Forks AFB, North Dakota, Malmstrom AFB, Montana, and F. E. Warren AFB, Wyoming.
The LGM-30 Minuteman missiles are dispersed in hardened silos to protect against attack and connected to an underground launch control center through a system of hardened cables. Launch crews, consisting of two officers, perform around-the-clock alert in the launch control center.
A variety of communication systems provide the National Command Authorities with highly reliable, virtually instantaneous direct contact with each launch crew. Should command capability be lost between the launch control center and remote missile launch facilities, specially-configured EC-135 airborne launch control center aircraft automatically assume command and control of the isolated missile or missiles.
The silos have the most direct route to targets in the former USSR, moslty in routes over the northern ice packs. The silos are located away from large population centers in the east and west coasts.
Each missile squadron has five flights interconnected by a network of hardened, buried electronic cables. A missile flight consists of a Missile Alert Facility (MAF) electronically tied to 10 Launch Facilities (LFs). The LFs are located 3 to 16 miles from the MAF, with each LF situated 3 to 9 miles apart.
These facilities are interconnected by a redundant network of hardened, buried cables, or a single path of hardened, buried cables plus a medium frequency radio (Grand Forks AFB and Squadron IV at Malmstrom AFB). Any MAF within a squadron can monitor status, command tests and launch any of the missiles within its own squadron.
Also, the Airborne Launch Control Center (ALCC), under restricted conditions, can launch any missile within the Minuteman Force. Each MAF is staffed 24 hours a day by a missile combat crew of two officers.
"Launch on warning" policies or defensive stance would theoretically not allow for pre-emptive strikes to take out the 'clusters' as you put it. By the time launches and trajectories have been confirmed, the birds will be away within minutes, assuming of course that the national command authority has given the order.
