"Lockheed Martin Space Systems heritage in missile defense dates back to the first successful hit-to-kill intercept of a mock ballistic missile warhead outside the earth’s atmosphere on June 10, 1984, by the U.S. Army’s Homing Overlay Experiment (HOE). The Homing Overlay Experiment was a series of four missile tests that were conducted in 1983 and 1984 at Kwajalein Missile Range in the Republic of the Marshall Islands. For each test a Minuteman missile was launched from Vandenberg Air Force Base, Calif., carrying a single mock re-entry vehicle targeted for Kwajalein lagoon more than 4,000 miles away." - Lockheed
"The HOE vehicle was designed to destroy a missile by physically impacting it - a concept known as "hit-to-kill." After separating from the booster, the vehicle would use its onboard sensors to identify and lock-on to the incoming missile and would use other components to direct it to impact. On the last of four HOE tests, a vehicle intercepted an incoming dummy warhead in the first successful demonstration of hit-to-kill technology in June 1984. The United States is now using much smaller hit-to-kill vehicles in deployed ABMs. Lockheed built this HOE test vehicle, and the U.S. Army transferred it to NASM in 1986." - DOD
"In the late 1970s, the development of nuclear-armed ABMs (Anti-Ballistic Missiles), which had culminated in the operationally deployed Safeguard system with LIM-49 Spartan and Sprint missiles, was increasingly regarded as a dead end. The U.S. Army began studies about the feasibility of hit-to-kill vehicles, where an interceptor missile would destroy an incoming ballistic missile just by colliding with it head-on.
The first program, which actually tested a hit-to-kill missile interceptor, was the Army's HOE (Homing Overlay Experiment). The HOE vehicle consisted of the first two stages (Thiokol M55E1 + Aerojet General M56A1) of a LGM-30A/B Minuteman I ICBM, which boosted a large KKV (Kinetic Kill Vehicle) to high altitude. Th e KKV was equipped with an infrared seeker, guidance electronics and a propulsion system. Once in space, the KKV could extend a folded structure similar to an umbrella skeleton of 4 m (13 ft) diameter to enhance its effective cross section. This device, which also had weights attached to the ribs, would destroy the ICBM reentry vehicle on collision.
A total of four intercepts were attempted in the HOE test program. In each test, a Minuteman ICBM with a dummy warhead served as the target. The first attempt on 7 February 1983 missed because a failure in the IR sensor prevented proper target tracking. The next two flights in May and December 1983 failed, too, because of malfunctions in the guidance system. However, the fourth and final test on 10 June 1984 was successful, intercepting the Minuteman RV with a closing speed of about 6.1 km/s (20000 fps) at an altitude of more than 160 km (100 miles).
The successful test came right on time for the SDI (Strategic Defense Initiative) program, which had been officially established in January 1984. One of the primary elements of SDI were ground-based reentry vehicle interceptors without nuclear warheads. The technology tested by HOE formed the base for ERIS (Exoatmospheric Reentry Interceptor Subsystem), the upper-tier component of SDI's ground-based missile defense." - Encyclopedia Astronautica