A projectile or kinetic-kill weapon. An example of such a weapon is the electromagnetic rail gun (EMR). This is where a projectile is accelerated to an extremely high velocity by use of electromagnetism. The projectile is placed between two rails (hence rail gun) and an electric current is passed along the two rails (via a connecting armature) forming a magnetic field which accelerated along the gun 'barrel'. Velocities in excess of 10 km/sec have been reached which compares to a rifle bullet which goes perhaps 1 km/sec. The one drawback is that huge amounts of power are required to accelerate projectiles to such speeds.
Lasers. Laser stands for Light Amplification by Stimulated Emission of Radiation and the physicist, Niels Bohr, explained the actual physical principles of such technology as early as 1911. The basic operation of the laser is centred around the fact that electromagnetic radiation is emitted whenever a charged particle losses energy in an electric field. This is due to the actions of an electron as predicted by Niels Bohr, when it moves from a high-energy to a low-energy state within the atom it revolves around. While this happens naturally (as in ordinary light), in a laser, the radiation is stimulated to be emitted by photons. The basic components of a laser are a lasing medium, a pumping system (which acts as an energy source) and a resonant optical cavity. The pumping system acts as an energy source to the lasing medium which stores the energy as electrons trapped at a metastable energy level. Such an action produces a population inversion where there are more electrons in a higher energy state than at ground level, which has to occur before a laser can operate. When this happens, a few electrons decay spontaneously and produce photons, so starting a chain reaction in that they hit other atoms or molecules, which are stimulated to emit photons as well in their transition to a lower energy state. The reaction, if it takes place in an optical cavity, will produce a laser beam as the photons are reflected back into the chamber by the rear mirror to continue the reaction, but the front mirror is only partially reflective, thus allowing some through. The main disadvantage of a laser is that the atmosphere quickly degrades the laser beam (a weapon in space thus wouldn't be affected) unless it uses one of a few wavelengths that are unaffected, such as between 0.3 and 1 micron. Types of lasers include:
Particle beam weapons. While laser beams consist of pure energy, the beams of particle beam weapons consist of sub-atomic particles (such as protons or electrons) which are accelerated to extremely high velocities by electromagnetic fields. The scientific community, in conducting research into high-energy physics has used such beams for a number of years. For them to be used as a weapon, the energy would have to be focused into a narrow, high-intensity beam. Charged particles are necessary so that they can be accelerated by electromagnetic fields, but the particles tend to be mutually repelled within the beam and the beam tends to be disturbed by the magnetic field generated by the Earth. A neutral beam would offer some sort of solution (by stripping the negative charge off the beam by means of and 'Ion stripper') and achieve decent velocities (up to half the speed of light in some cases).
Conventional missiles. This type of ASAT weapon was the earliest to enter service with the United States installing Nike-Zeus missiles on Kwajalein Island in the Pacific Ocean in 1964. They can attack satellites in one of two ways. Either it can approach the target by direct ascent and has to be at high speed to make the interception, having usually only about ten minutes to do the job. The missile either has to have a very accurate homing warhead, or one of very great explosive capacity to effect an intercept. Secondly, there is the co-planar interception technique (the one favoured by the Soviets up until the early 1980s) where the interceptor is launched into the same plane as its target. The missile must be launched when the site is directly beneath the target, which will happen only twice a day. This method has greater flexibility as the missile can be launched slightly before the target by making use of a higher (and therefore slower) orbit to allow the target to catch it up, or a lower (and therefore faster) orbit if the target has already flown passed.
They could either be based on Earth or launched into orbit, but some weapon types are more suitable to Earth-based or orbit-based deployment than others. Alternatively, such weapons could be used for other purposes, such as a ballistic missile defence, as was the hope of Ronald Reagan's 'Strategic Defense Initiative' or 'Star Wars'. With the twentieth century having drawn to a close, an increased and renewed interest (particularly by the US Department of Defence) is apparent in the possibility of using these weapons for ballistic missile defence or for space warfare as the spread of commercially available weapon and computer technology makes a ballistic missile, space-based weapons platform and satellite capability increasingly available to all. Finally, although satellites can be thought of as relatively vulnerable (being fragile and packed with sensitive electronics) they are small, at great distances from the Earth, and travelling at high speed, and there are a great number of them to keep a track of.