The Proposed Technologies
The technologies under investigation fall into several categories: sensors to detect the initiation of an attack, to track the missiles and warheads through the flight path, and to direct the targeting of defensive interceptors; destructive devices to damage, deflect, or incapacitate missiles or warheads in the various phases of flight; computer systems to process the data acquired by the sensors, to make "kill assessments" from the data in order to determine which elements of the "threat cloud" have been destroyed and which remain to be dealt with, to "hand off" data from one layer of the defensive screen to the next, and to perform other "battle management" functions—including almost certainly the
decision to initiate interception; techniques for discriminating targets from decoys ; and a variety of methods for defeating countermeasures that might be taken by the offense.
The effort to develop sensors draws both on well-established technology and on more exotic ideas that have not yet been developed into usable devices. Conventional infrared sensors, which are now routinely used for early warning of attack and to monitor test-firings, are thought to be particularly well adapted to boost-phase interception because they can readily identify the hot plume of exhaust gases emitted during a launch. Because the sensors must be able to detect both hot and relatively cold objects (the hotter the object, the shorter the wavelength), the sensors must have a wide range of capabilities. The ability to identify and track the plume, however, does not necessarily make it possible to track and target the missile, the position of which must be calculated from the signal produced by the plume. The plumes are, in general, very much larger than the missiles, and the missiles are not usually located in any convenient "central position" or easy-to-find "hotspot" within the plume. Other sensors are being investigated, including those that use an ultraviolet probe, in order to target the missile itself. The sensing problem becomes more acute in the post-boost phase, when the rocket exhausts are much smaller, and even more difficult in the later phases of flight, when the relatively cold warheads must be tracked in space against a background of starts. For this phase, more exotic sensors, such as those that make use of laser and radar technologies (and are therefore called "ladar" sensors), have been suggested.
There are three basic types of destructive devices: (1) conventional ground-based ABMs with nuclear warheads , which are directed by ground-based radars to track warheads in the terminal phase of flight (the system in use when the ABM Treaty was negotiated); (2) kinetic-energy weapons (KEW), which are ground- or space-launched projectiles that use the energy of motion to destroy missiles by colliding with them; and (3) directed-energy weapons (DEW), also known collectively as beam weapons because they make use of powerful beams of electromagnetic radiation produced by lasers (infrared, visible light, or X-rays) or beams of highly accelerated particles—either charged (such as electrons, protons, and ions) or neutral atoms.
The conventional ABM could be outfitted with a kinetic warhead rather than an atomic explosive if tracking and targeting were improved to increase the probability of interception. U.S. and Soviet ABMs developed since the early 1960s have been configured in two modes. One permits high acceleration for rapid interception in the atmosphere (as in the
United States' Sprint), and the other has a relatively slower acceleration for interception above the atmosphere (as in the United States' Spartan). Development is now under way on two kinetic interceptors for endoatmospheric and exoatmospheric interception. Nuclear and nonnuclear systems may both benefit from recent improvements, which have enabled the hardening of radar sites and utilize miniaturized, mobile radar in addition to ladar and infrared systems mounted on aircraft.
Kinetic weapons could also be deployed from a constellation of space-based battle stations. These weapons would be accelerated and directed toward their targets either by chemical rockets or by the as-yet undeveloped electromagnetic launcher, or "railgun." The rocket-propelled projectiles would be fitted with homing devices to correct for any errors in targeting; they might also be equipped with explosives that would detonate near the target, increasing the prospect of a kill. The railgun would use an intense magnetic field to impart speeds greater than 20 km per second—at least several times faster than that achieved with conventional rocket technologies, though still far slower than speeds attained with speed-of-light lasers.
Invented in the 1950s, lasers are essentially devices that produce coherent beams of electromagnetic radiation in the form of light. The radiation may be in the visible, ultraviolet, or X-ray regions of the spectrum. "Lasing" ordinarily occurs when the atoms or molecules in the lasant matter (solid or gaseous) are "excited" by energy—electrical, nuclear, chemical, or optical—pumped in from some external source. The resulting beam can be used to damage missiles and warheads either by thermal means (delivery of intense heat that would burn through the skin of the missile or warhead and alter or destroy sensitive electronic components) or by "impulse kill" (depositing energy in a powerful pulse on the surface of the target, driving a mechanical shock wave through the target). Some lasers are better adapted for one type of kill, some for the other. To hit a target from a great distance requires very accurate targeting, a strong beam, and, for most lasers, large mirrors to focus the beam so as to compensate for its diffusion over long distances.
The lasers usually discussed in connection with strategic defense are the chemical, free-electron, excimer, and X-ray lasers. The process whereby chemicals can be combined to produce the energy needed to pump a laser is well understood, but the required energy levels have yet to be achieved. Furthermore, mirrors larger than any so far produced would have to be developed. Without these mirrors, there is no way to focus the energy to produce the brightness required.
In principle, optical lasers may be based either in space or on the
ground. For space basing, weight and the provision of adequate power are major problems. Distortion and absorption of the beam by the atmosphere are the main problems for ground-based lasers. "Adaptive optics" must be employed to direct the beams through the atmosphere so as to compensate for distortion. The beam would be reflected off space-based relay and "fighting" mirrors to targets in space. The free-electron laser (FEL) is a novel concept. It uses a relativistic electron beam (generated by an accelerator rather than by a conventional lasant) to produce an intense, coherent beam of electrons whose energy is then converted into an extremely high intensity beam of light. Two competing FEL designs are currently under investigation. One, the radio frequency (RF) linac (for "linear accelerator"), is being developed by Los Alamos Laboratory and Boeing. Lawrence Livermore Laboratory and TRW are working on the other, called the induction linac. Once a choice is made between the two designs, a ground-based test version is to be built at the White Sands Missile Range in New Mexico.
The excimer laser normally produces a pulse of sharp laser light, theoretically enabling impulse kill, and would require a smaller mirror than other types of lasers because of its shorter wavelength. Both the FEL and the excimer laser require further development in order to determine whether the requisite power levels can be achieved. The weight required for both, and the power required for the free-electron laser, rule out space basing for the excimer laser and the Induction linac FEL, though not necessarily for the RF linac FEL. Chemical lasers of the hydrogen flouride type, which derive their energy from the chemical reaction of these two substances, may be made light enough for deployment in space basing for the excimer laser and the induction linac FEL, though not necessarily for the RF linac FEL. Chemical lasers of the hydrogen nuclear explosion to pump the lasant, which might be in the form of rods each aimed at a missile. Such a laser would also have to be fired from space, because X-rays are absorbed by the atmosphere, but it could also be "popped up" when needed rather than placed in orbit because it could be more effective at great distances than kinetic weapons and would not need to be housed in a satellite, as chemical lasers are. Research on X-ray lasers still has far to go before application can take place, and its testing in space would violate both the Limited Test Ban Treaty and the Outer Space Treaty. The fission-activated light concept (FALCON), under study at Sandia National Laboratory, also aims to use a nuclear reactor to pump a space-based laser. But it is estimated that the platform for this system would weigh between seven and forty tons, apart from any protective armor.
In order to design computers that are sufficient for the needs of a space shield, two things are required. First, suitable high-speed hardware needs to be developed. Second, and far more problematic, software programming with enough capacity, redundancy, and reliability must also be produced. This software must be able to handle the large and diverse sets of data and data-processing suitable for an unpredictable environment: some elements of the system might be rendered inoperative, and the system would have to operate efficiently without previous testing or "debugging." The degree to which the system would have to be centralized or could be disaggregated is much disputed. The further development of Very High Speed Integrated Circuitry (VHSIC) technology is essential for adequate data processing. The relatively new gallium arsenide computer chip may also improve prospects for achieving the high-speed computations needed for this purpose.
Means for discriminating targets from decoys in space are still quite uncertain, but ladar sensors have been suggested, along with various types of "interactive discrimination." One such technique would be to use beams of neutral particles. Projected at targets, these particles can help to determine mass and thereby help to distinguish between warheads and decoys. In principle, a beam of neutral particles, when directed at a target, can ascertain mass because the object releases secondary radiation (in the form of neutrons and gamma rays) in rough proportion to its mass. Problems remain, though: it has not been demonstrated that an operational system can distinguish neutrons thus generated from neutrons naturally present and that it can be made small enough to be placed in space.
To achieve battle-satellite survivability against efforts to suppress space-based defenses will require hardening the proposed battle stations against attack from lasers and from the effects of nuclear explosions (including electromagnetic pulse, which has been shown in nuclear-test-explosions to disrupt electrical apparatus). Battle satellites may also have to be equipped with counter-countermeasures, such as decoys, protective satellites, rocket motors for maneuvering, and the ability to shoot back. If the satellites carry radar, they would require high-capacity power sources, which in all probability would have to be small nuclear reactors, either carried aboard the satellites or tethered to them.
Adequate launch capacity is yet another essential technological consideration. Space-based elements must be launched into orbit and then serviced and maintained. The need for this launch capacity has led the SDIO to advocate a variety of new high-capacity launch systems. The National Aerospace Plane, if it should become available, is one such system
and could carry much larger payloads than the space shuttle. NASA's difficulties in achieving a reliable shuttle and in maintaining regular launch schedules suggest that the development of this still more complex and demanding launching system will take time, investment, and a further extension of the existing state of the art. The proposal for the Advanced Launch System claims that the system could put payloads into low earth orbit at one-tenth the current cost of $3,00–$5,000 per pound. A Senate staff report has underlined the difficulties involved in achieving this goal: "While the U.S. launched a total of about 350,000 pounds into earth orbit during 1985, SDIO envisions SDI deployment as requiring as much as 5 million pounds in orbit per year. … The capacity of the U.S. to launch payloads into orbit would have to be expanded enormously while the cost would have to come down by at least a factor of ten." Just how realistic this goal is, as the Senate report points out, may be judged by experience with the space shuttle. In 1972 a White House press release predicted that the space shuttle would reduce launch costs from $600–$700 per pound to $100 per pound in 1972 dollars. Currently, space shuttle launches cost $5,000 per pound.