Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic or tactical purposes. Although no ASAT system has been utilised in warfare, a few countries (China, India, Russia, and the United States) have successfully shot down their own satellites to demonstrate their ASAT capabilities in a show of force. ASATs have also been used to remove decommissioned satellites.
ASAT roles include: defensive measures against an adversary's space-based and nuclear weapons, a force multiplier for a nuclear first strike, a countermeasure against an adversary's anti-ballistic missile defence (ABM), an asymmetric counter to a technologically superior adversary, and a counter-value weapon.
Use of ASATs generates space debris, which can collide with other satellites and generate more space debris. A cascading multiplication of space debris could cause Earth to suffer from Kessler syndrome.
History
The development and design of anti-satellite weapons has followed a number of paths. The initial efforts by the United States and the Soviet Union used ground-launched missiles from the 1950s; many more exotic proposals came afterwards.
United States
In the late 1950s, the US Air Force started a series of advanced strategic missile projects under the designation Weapon System WS-199A. One of the projects studied under the 199A umbrella was Martin's Bold Orion air-launched ballistic missile (ALBM) for the B-47 Stratojet, based on the rocket motor from the Sergeant missile. Twelve test launches were carried out between 26 May 1958 and 13 October 1959, but these were generally unsuccessful and further work as an ALBM ended. The system was then modified with the addition of an Altair upper stage to create an anti-satellite weapon with a range. Only one test flight of the anti-satellite mission was carried out, making a mock attack on the Explorer 6 at an altitude of . To record its flight path, the Bold Orion transmitted telemetry to the ground, ejected flares to aid visual tracking, and was continuously tracked by radar. The missile successfully passed within of the satellite, which would be suitable for use with a nuclear weapon, but useless for conventional warheads.
A similar project carried out under 199A, Lockheed's High Virgo, was initially another ALBM for the B-58 Hustler, likewise based on the Sergeant. It too was adapted for the anti-satellite role, and made an attempted intercept on Explorer 5 on 22 September 1959. However, shortly after launch communications with the missile were lost and the camera packs could not be recovered to see if the test was successful. In any event, work on the WS-199 projects ended with the start of the GAM-87 Skybolt project. Simultaneous US Navy projects were also abandoned although smaller projects did continue until the early 1970s.
The use of high-altitude nuclear explosions to destroy satellites was considered after the tests of the first conventional missile systems in the 1960s. During the Hardtack Teak test in 1958 observers noted the damaging effects of the electromagnetic pulse (EMP) caused by the explosions on electronic equipment, and during the Starfish Prime test in 1962 the EMP from a warhead detonated over the Pacific damaged three satellites and also disrupted power transmission and communications across the Pacific. Further testing of weapons effects was carried out under the DOMINIC I series. An adapted version of the nuclear armed Nike Zeus was used for an ASAT from 1962. Codenamed Mudflap, the missile was designated DM-15S and a single missile was deployed at the Kwajalein atoll until 1966 when the project was ended in favour of the USAF Thor-based Program 437 ASAT which was operational until 6 March 1975.
Another area of research was directed-energy weapons, including a nuclear-explosion powered X-ray laser proposal developed at Lawrence Livermore National Laboratory (LLNL) in 1968. Other research was based on more conventional lasers or masers and developed to include the idea of a satellite with a fixed laser and a deployable mirror for targeting. LLNL continued to consider more edgy technology but their X-ray laser system development was cancelled in 1977 (although research into X-ray lasers was resurrected during the 1980s as part of the SDI).