The Exoatmospheric Reentry-vehicle Interceptor Subsystem, or ERIS, program, was a component of the United States' Strategic Defense Initiative during the Cold War. The ERIS system was named after Eris, Greek goddess of strife.^[1] ERIS was a kinetic kill system, launched from a ground-based system, and impacting directly to destroy an incoming Intercontinental ballistic missile (ICBM) before the targeted ICBM re-entered the Earth's atmosphere.^[2]

The 1972 Anti-Ballistic Missile Treaty between the U.S. and the Soviet Union permitted each nation to have two ABM sites, each limited to 100 missiles, for a total of 200 ABMs per nation. A 1976 Protocol added to the ABM treaty further limited each nation to a single, 100 missile site.^[3]

In 1985, Lockheed Corporation was awarded a contract to develop an ABM system - the ERIS - that was compliant with the ABM Treaty. ERIS was to be a high altitude component of SDI that would operate in outer space. It would intercept and destroy an incoming ICBM before the ICBM re-entered the atmosphere. ERIS was to be complemented by another ABM system, High Endoatmospheric Defense Interceptor (HEDI), which destroyed enemy missiles in the atmosphere. The ERIS delivery vehicle was composed of the second and third stages of surplus Minuteman I ICBMs. The guidance system was based on technology developed during the earlier Homing Overlay Experiment.^[4] The guidance system used detected the infrared signature of the targeted ICBM.^[1] ERIS used a Kinetic Kill Vehicle (KKV), which destroyed its target by force of impact, not by an explosive charge. The KKV on board ERIS inflated into a large, rigid, octagon-shaped structure moments before impact. This increase in KKV diameter increased the chances of impacting and destroying the target ICBM.^[1] The ERIS received data on its target's location from satellites and radar until the KKV separated from the delivery vehicle. At this time, the infrared guidance system of the KKV took over, controlling the KKV until impact.^[1] It was also believed that ERIS would be a viable weapon in anti-satellite operations.^[2]

The development of ERIS came late in the Cold War. After the collapse of the Soviet Union, and effective end of the Cold War, the Strategic Defense Initiative was reorganized. ERIS was cancelled as part of the reorganization, and the system was never directly implemented. However, technology developed from ERIS is currently used in other U.S. missile defense systems, including Terminal High Altitude Area Defense (THAAD) and Ground-Based Midcourse Defense (GMD).^[4]