For a ballistic missile to become a precision-guided weapon, it must be equipped with a navigation system — simply put, a "brain" that continuously monitors its position, velocity and orientation.
The United States and the Soviet Union pioneered the development of such systems, and one of them is thought to be the pinnacle of Cold War-era military technology.
This "doomsday navigator" is called the Advanced Inertial Reference Sphere (AIRS), and it was used to navigate the highly-accurate LGM-118A Peacekeeper missile, also designated as the MX (Missile Experimental).
A photo of the inertial guidance module was featured in a book by photographer and author Martin Miller, who had catalogued weapons of mass destruction (click here to access) stockpiled by the United States in the late 20th century.
Miller has captured a close-up image of the Advanced Inertial Reference Sphere (AIRS) guidance system. "Rather than being gimbal-mounted, the sphere floats in a fluorocarbon fluid within an outer shell," he writes in the book.
There are three accelerometers and three gyroscopes in the module, which consists of a beryllium sphere floating in a fluorocarbon fluid within an outer shell — making it possible for the sphere to rotate in any direction.
"The gyroscopes and accelerometers are positioned within the sphere as are the three hydraulic thrust valves and turbo-pumps used to maintain the stable orientation of the sphere," Miller said.
Consisting of some 19,000 parts, AIRS modules bring the strike precision of the MX Peacekeeper missile to within 40 metres. According to the scarce information available, it was the most precise inertial navigation system ever created.
However, the module appeared to be extremely expensive: according to the Nuclear Weapons Archive blog, in 1989, a single accelerometer used in the AIRS cost $300,000 and took six months to manufacture.
Development of the LGM-118As first began in 1971, and there were originally plans for 100 missiles. Congress, however, capped the number at 50 in 1984. The last such missile was decommissioned in September 2005.