Even before the Safeguard program was deployed and then dismantled, the U.S. Army’s Advanced Ballistic Missile Defense Agency (ABMDA) began to exploit the emerging long-wave infrared sensor technology that allowed detecting and tracking objects against the cold space background. Studies conducted in the mid- to late 1960s defined midcourse defense options based on interceptors with long-wave infrared sensors capable of detecting potential intercontinental ballistic missile (ICBM) threats thousands of kilometers away on their ballistic trajectories, observing them for more than 100 sec while closing on the threat, thereby maximizing the opportunity for discriminating warheads from countermeasures and other objects in the threat complex and finally homing on the object that posed the most credible threat to intercept it. Simulations in 1969 lent confidence to the notion that this optical homing could be accomplished with sufficient accuracy to achieve a direct hit, thereby destroying the target by the force of the collision, at closing velocities approaching or exceeding 10 km/sec. At the same time, technology was dramatically improving the ability to track rocket boosters from space and predict their trajectories with handover volumes compatible with the acquisition-and-divert capabilities of the interceptors. With a moderate-sized onboard long-wave infrared (LWIR) track (while scan or staring mosaic sensor uncapped once above 80 km altitude), the interceptor could view the threat against the deep space background as it closed in and could thus search and acquire individual objects hundreds of miles away, typically about one-third the range of the threat missile. Moreover, the same sensor could be used to home in on the target once it was designated to achieve miss distances consistent with nonnuclear kill. This work led to flight experiments for verification, but because of the ABM treaty, more than 10 years elapsed before a technology flight experiment—called the Homing Overlay Experiment—was initiated that led to the successful intercept of an ICBM reentry vehicle (RV) launched from Vandenberg Air Force Base in California in 1984. This experiment was followed by the Exoatmospheric Reentry Interceptor System (ERIS), which reduced the size of the kill vehicle (KV) to a more operational configuration that successfully intercepted in 1991. While all of these experimental interceptors had, in varying degrees, the onboard processing to track and discriminate among tens of objects, including celestial objects, in the field of view, there was still concern about the ease of creating relatively lightweight countermeasures that would be effective above the atmosphere.
In 2001, the National Missile Defense (NMD) program transitioned to the Ground-Based Midcourse Defense (GMD) system and was directed to be deployed by 2004. It is currently emplaced at Fort Greely, Alaska. The attraction of midcourse (exoatmospheric) defense is that interceptors at a few sites can protect targets anywhere in the entire country, committing the earliest intercepts only after assessing an attack with multiple phenomenology. Put another way, in principle, it can adapt in real time to defend against whatever is threatened and still have sufficient shot opportunities to deal with imperfections in target desig-