In another swing of the strategic arms pendulum, the antiballistic missile system is making a comeback.
Part of its steadily growing support comes from technological gains the past few years and hopes more will come -- Buck Rogers-like devices such as the long wave length infrared detectors, computers, and nonnuclear interceptors that promise to overcome flaws in earlier ABM concepts.
Additional support comes from politicians and defense officials who see ABMs as necessary to protect the new MX intercontinental ballistic missile whether or not it goes into a mobile basing system.
This new strategy of using ABMs to protect hardened missile silos rather than open cities also makes the system more practical.
Another group of boosters are policymakers who would rather postpone spending billions on the unpopular MX deployment plan and put several hundred million more in research on an ABM program that might pan out in the future.
Finally, there are the hardline anti-Soviet officials and members of Congress who want an ABMsystem revived in order to destroy SALT I and thus, they hope, undermine any future arms control negotiations with the Russians.
At the Republican platform committee drafting sessions, it took a last-minute effort by Reagan supporters to delete a demand for deployment of an ABM system. In its place they wrote in a demand for "vigorous research and development" of such a system.
In 1972, the two superpowers agreed to limit ABM sites to two apiece, a number cut to one several years later.
The publicly announced reason for the ABM treaty was that if one side or the other built a system that effectively protected its cities from the missiles of an opponent, it would feel free to launch a first strike ICBM attack of its own without fear of a devastating response.
In fact, it is now generally recognized by scientists and military men that neither side in 1972 was close to developing a workable ABM system to protect cities.
The U.S. system, for example, had long-range large-yield, nuclear-tipped Spartan missiles that were to intercept incoming Soviet warheads in space. Those Russian warheads that got through the Spartan blast were to be targeted by ground-based radar that would direct Spring interceptor missiles.
It was subsequently discovered that the Spartan explosions would create large areas of the atmosphere where the radars used to direct the Sprint could not operate.The radars were vulnerable to an attack that could blind the entire system; and finally the computers attached to the radars were incapable of keeping up with the incoming warheads.
Although the SALT I treaty halted deployment of new ABM sites, both countries were permitted to continue research on missile defense systems -- and both did.
The roughly $250 million that the Pentagon has put into exploring ABM technology the past eight years has begun to pay off. Even ABM critics admit that.
But everyone also agrees that serious hurdles remain before a workable system is in hand.
As now conceived, the future ABM will, like its predecessor, have both long-range and short-range interceptors. That, however, is where the comparisons end.
The heart of the new long-range system is an airborne rather than ground-based detection and guidance system.
At the initial sighting of a Soviet launch, from satellites in orbit above Russia, the United States would fire several rocket-borne probes in trajectories that would keep them above the atmosphere. Each probe would contain an infrared telescope that would scan the pathway of the launched Soviet rockets.
The highly sensitive infrared devices would be able to detect the big pieces of the Soviet missiles, such as the initial stages and fuel tanks, at ranges of 5,000 kilometers. They also could see, after some minutes, the smaller objects, including the buses carrying many warheads and single warheads that were launched.
All this data would be pumped into the probe's onboard computers, which would distinquish real warheads from decoys and then compute their potential impact points on U.S. soil.
That information would go to a central battle computer that would decide which targets would be defended and which interceptor rockets should be launched.
The long-range interceptors, also guided by infrared sensors and their on-board computers, would carry and launch their own dozen or more non-nuclear kill vehicles.
The long-range system thus avoids two of the major flaws of the old ABM system. Its guidance systems are invulnerable since they are airborne; its kill systems will not make it impossible for ground-based radars to continue functioning.
The homing sensor in the nonnuclear kill device has not been flight tested but has been studied extensively in computer simultations. Scientists are designing both a conventional explosive and fragment array nose for the device.
The idea is to strike the Soviet warhead in space hard enough so that its explosive detonate or at least part of its heat shield is cut away so that during the reentry into the atmosphere it would burn up.
The new second line of this modernized ABM system is termed LoADS (for low altitude defense system) and parallels the concept planned in 1972. However it would be used only for missile fields and its intercept of incoming warheads would take place only a few kilometers above the ground.
Since there are no cities within miles of missile fields, there is no concern about fallout from nuclear explosions that close to the ground.
The single-stage intercept missile along with its radars would be relatively small and could be put either in hardenen silos or made mobile and hidden in shelters much as the MX missile will be.
The Senate Armed Services Committee was impressed enough with the LoADS concept to recommend an acceleration and the full Senate went along by authorizing an additional $25 million for the program in fiscal 1981.
ABM supporters are aware that the SALT I treaty would have to be abrogated before the U.S. tests some components now planned for the new missile defense system -- particularly the nonnuclear interceptors -- since the treaty bars either side from testing interceptors with more than one warhead.
A handful of Senate Republicans -- many opposed to SALT I -- joined in an amendment to the fiscal 1981 defense authorization bill that directs the defense secretary to report to Congress by Feb. 1 on the future for ABMs.
As Sen. Pete V. Comenici (R-N,M.) put it during the Senate debate, "It is important to remember that the next United States/Soviet review of the ABM treaty will occur in 1982,"
Information from the defense secretary's ABM reviews, Domenici said, "would serve as the basis for congressional reexamination of the merits of the ABM treaty in a changing strategic environment."