Anti-satellite weapons (ASATs) are weapons designed to be used against artificial satellites.

The development and design of anti-satellite weapons has followed a number of paths. The initial efforts by the USA and the USSR were using air-launched missiles from the 1950s, from this beginning there were much more exotic proposals.

Air-launched missiles were the first approach because the basic technology was well known. The US began tests of such a system in 1959 but initial results were very discouraging, the first test launch missed by over 6,000 m, and after further failures the project was halted in 1963. Simultaneous US Navy projects were also abandoned although smaller projects did drag on until the early 1970s. The USSR began a similar program in 1967 and actually built and deployed ASAT missiles from around 1976. Stung by the Russian deployment the USAF revived its own ASAT program. From 1977 Vought developed an ASAT to attack satellites in LEO, the three stage missile was fired by an F-15 in a steep climb and carried a miniature homing vehicle (MHV) to track and then destroy the target kinetically. The first test was in 1983 and the first successful interception, of the defunct US satellite P78 SolWind, was on September 13, 1985.

The use of nuclear explosions to destroy satellites was considered after the tests of the first conventional missile systems in the 1960s. Existing guidance technology was insufficient to ensure a strike while a nuclear blast would be sufficient if the weapon was within 1,000 km of the target. However the drawbacks of this excessive destructive radius and the potential of more extensive radiation and EMP damage meant that nuclear ASAT systems did not reach test phase. However, the US adapted the nuclear armed Nike Zeus for ASAT from 1962, codenamed Mudflap the missile was designated DM-15S and a single missile was deployed at Kwajalein, Hawaii until 1966 when the project was ended in favour of the USAF Thor ASAT which ran until 1972. The US also detonated a number of high altitude nuclear weapons in other tests, a 1.4 Mt blast at 400 km over the Pacific in 1958 did some damage to three satellites and also disrupted power transmission and communications across the Pacific. The Outer Space Treaty of 1967 banned the use of nuclear weapons in space.

Other concepts considered included manned and unmanned ASAT from orbit. A manned space vehicle would either rendezvous with a satellite and then either disable or capture it. The military use of automatic self-destruct in satellites would have made this hazardous and the concept was soon altered to a manned vehicle equipped with stand-off weapons. Unmanned orbital ASAT suffered the same problems as air-launched attacks, guidance and interception systems could not be developed sufficiently well to ensure an intercept. Other ideas along the unmanned orbital ASAT included kamikaze satellites, space mine dispensers and single-use space interceptors.

The USSR went for a kamikaze satellite approach because it would be the simplest and cheapest to implement. The designs were named Istrebitel Sputnikov (fighter satellites) and development work began in the early 1960s and the first test flights were made in 1968. The project was halted in 1972 under the terms of SALT I but the system was still deployed and testing of new versions continued up until around 1982 when the entire concept was scrapped, possibly in favour of more advanced orbital ASAT systems, although whether such designs were actually ever deployed is still a matter for heated debate. The Soviet Union also experimented with large ground based ASAT lasers from the 1970's onwards, with a number of US spysats being reportedly 'blinded' during the 70's and 80's.

The US was following a more technical space-based weapon approach. The primary area of research was for directed energy weapons, including the bizarre nuclear explosion powered laser proposal developed at 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 1980's as part of the SDI). The USSR had also researched directed energy weapons, under the Fon project, from 1976 but the technical requirements needed of the high-powered gas dynamic lasers and neutral or charged particle beam systems seemed to be beyond reach. In the early 80's, the Soviet Union also started developing a counterpart to the US air-launched ASAT system, using modified Mig-31's (at least one of which was completed) as the launch platform. After the Soviet Union collapsed, it was proposed to use this aircraft as a launch platform for lofting commercial and science packages into orbit. Recent political developments(see below) may have seen the reactivation of the Russian Air-Launched ASAT program, although there is no confirmation of this as yet.

The Strategic Defense Initiative gave the US and Russian ASAT programs a major boost, ASAT projects were adapted for ABM use and the reverse was also true. The initial US plan was to use the already developed MHV as the basis for a space based constellation of around 40 platforms deploying up to 1,500 of the kinetic interceptors. By 1988 the US project had evolved into an extended four stage development. The initial stage would consist of the Brilliant Pebbles defense system, a satellite constellation of 4,600 kinetic interceptors (KE ASAT), of 45 kg each, in Low Earth orbit, and their associated tracking system. The next stage would deploy the larger platforms and the following phases would include the laser and charged particle beam weapons that would be developed by that time from existing projects such as MIRACL. The first stage was intended to be completed by 2000 at a cost of around $125 billion.

However, research in the US and Russia was proving that the requirements, at least for orbital based energy weapon systems, were, with available technology, close to impossible. Nonetheless, the strategic implications of a possible unforeseen breakthrough in technology forced the USSR to initiate massive spending on research in the 12th Five Year Plan, drawing all the various parts of the project together under the control of GUKOS and matching the US proposed deployment date of 2000.

Both countries began to reduce expenditure from 1989 and the USSR unilaterally discontinued all SDI research in 1992. Research and Development (both of ASAT systems and other space based/deployed weapons) has, however reported to have be been resumed under the Putin government as a counter to renewed US Strategic Defense efforts [post Anti-Ballistic Missile Treaty ]. However the status of these efforts, or indeed how they are being funded, remains unclear. The US greatly reduced expenditure under the Clinton administration but this has been somewhat reversed by George W. Bush.