Washington — Considerable uncertainties remain about the cost, effectiveness, and survivability of ``star wars'' defenses against ballistic missiles, according to a major congressional study released Tuesday. The report by Congress's Office of Technology Assessment (OTA) concludes that computer software could be the weak point in the Strategic Defense Initiative (SDI). There is a ``significant probability'' that software would fail catastrophically in the system's first battle, says OTA project director Thomas Karas.
``It looks very difficult'' to reach intial ``star wars'' technical goals by 1994 or 1995, according to Mr. Karas.
SDI officials have long said their job is to provide data allowing a decision on ``star wars'' deployment in the mid-1990s.
The OTA study is only the latest in a string of recent studies calling SDI goals and structure into question. Last month, the Pentagon's own Defense Science Board recommended that the program be replanned, aiming toward a number of small initial deployment steps rather than a major Phase One system.
These small steps would begin with a limited number of interceptor rockets based on the ground at a single site. Surveillance and weapon capability would be gradually updated as technology matured. In this way ``star wars'' could proceed despite uncertainties about technology, funding, and political acceptability, according to the Defense Science Board.
Such a gradually emplaced system would not make it ``virtually impossible'' for nuclear warheads to hit the United States, a goal President Reagan reiterated at the Moscow summit. Its value, according to SDI officials, would be to disrupt Soviet attack plans, making an attack less likely to be launched in the first place.
According to the long-awaited OTA report, even after a number of these small steps had been taken, only a ``modest fraction'' of attacking warheads would be shot down. A ``star wars'' system with interceptor rockets based in space would shoot down about 2,500 of 10,000 incoming reentry vehicles, according to a rough OTA estimate.
OTA also points out that the Soviets could frustate space-based interceptor rockets by the relatively simple expedient of gradually increasing the lift-off speed of their ballistic missiles.
Thus, space interceptors ``would face fairly predictable obsolescence,'' says OTA. Lasers and other directed energy weapons would be necessary for an SDI system to retain its effectiveness.
Among the technologies OTA says still need much work are sensors for discriminating between warheads and decoys, communications and computers, and weapons to defend the system itself against Soviet attack.
Project director Karas said he was ``surprised'' at the lack of attention the SDI organization has given to defending space-based SDI platforms. Three chapters in the OTA report on possible Soviet countermeasures to SDI were withheld, as the Pentagon has refused to declassify them.