THIS summer, a small rocket crucial to the future of the Strategic Defense Initiative (SDI) will roar skyward from a United States government facility at Wallops Island, Va. The rocket will carry a test of ``Brilliant Pebbles.'' Pentagon designs for initial ``star wars'' defenses are becoming increasingly reliant on the concept of Brilliant Pebbles - tiny, self-guided missiles parked in space to guard against nuclear attack.
If this summer's first test of sensor and maneuvering technology goes well, SDI officials will try to fire a ``pebble'' at a moving target next year. If it goes poorly, congressional critics who deride Brilliant Pebbles as a last-gasp strategy - ``Loose Marbles,'' in the words of one - will cite the test as reason to sharply reduce SDI funding.
For all the debate that has swirled around star wars, the US is little closer to a consensus on what to do, or not do, about strategic defenses than it was in 1983, when President Reagan announced the program.
Yet SDI is nearing the point where research begins to give way to full-scale engineering and weapons design.
Congress and President Bush will have to choose: Should SDI be restructured as a research-and-development-only program, or will it proceed with an eye toward deployment?
``Our work is beginning to pay off,'' insisted Air Force Lt. Gen. George Monahan Jr., SDI's director, in a recent meeting with reporters.
Mr. Bush has said he wants to make a decision about SDI deployment in 1993. That is the deadline the SDI organization is working toward. Important decisions loom before then, however. For one thing, congressional action on the SDI budget could make the '93 deadline moot.
The SDI program has cost about $19 billion so far. Between 1984 and 1987 funding grew by almost 50 percent a year. The increase slowed in 1988. In 1989 and fiscal '90, SDI funds shrank after taking inflation into account.
For 1991, the administration is asking for about $4.4 billion in SDI funds, up from this year's $3.6 billion. If Congress holds SDI funding level, or makes reductions, the program will basically tread water. If the budget stays level for the next several years, ``you're back to a technology-only program,'' General Monahan says.
SDI is also on a collision course with the 1972 Anti-Ballistic Missile (ABM) Treaty. Although initial tests of Brilliant Pebbles interceptors won't be a problem, later experiments raise questions of ABM Treaty compliance.
Monahan has won praise from SDI critics for saying that the US needs to decide what it wants to do regarding the ABM pact. Rep. Ronald Dellums (D) of California, an SDI opponent who chairs the House Armed Services subcommittee on R&D, complains that his congressional colleagues continue to support both the ABM Treaty and an SDI program that will violate the treaty.
``It stands logic on its head, but politics isn't always a logical process,'' Congressman Dellums said at a hearing this spring.
As currently envisioned by planners, a Phase-1 SDI system would be limited to enhancing nuclear deterrence by complicating an adversary's attack plans.
Exotic lasers and magnetic rail guns firing shells into space are not part of the Phase-1 design. Instead, plans now call for an initial, two-layer defense: space-based missiles that would fire at intercontinental ballistic missiles rising up after launch; and ground-based rockets that would intercept warheads as they reentered Earth's atmosphere.
SDI sensor satellites now under development include the Boost Surveillance and Tracking System (BSTS), for detection of missile launches, and the Space Surveillance and Tracking System, for tracking warheads through the cold dark of space.
The initial estimate of the cost of a Phase-1 SDI, made in mid-1988, was $115.4 billion. This was lowered to $69.1 billion in October 1988, due to technology advancements and changes in system design, according to the Pentagon. Last November saw another reduction, to $55.3 billion.
Projected costs for Pentagon weapons systems do not typically fall. SDI officials say the advent of Brilliant Pebbles is a big reason that star wars is getting cheaper.
Initially, the space-based rocket layer of SDI was envisioned as consisting of traditional missiles grouped together in ``garages,'' controlled by an elaborate satellite sensor network. Brilliant Pebbles, on the other hand, are small rockets that would float in space individually in ``life jackets'' and would carry their own aiming sensors. Less-central command and control would be necessary. Monahan says that a Brilliant Pebbles SDI system might not need BSTS satellites, further reducing the system estimated cost to $47 billion.
``Since we don't know enough about whether Brilliant Pebbles might work, it's possible to make promises about it, and base your whole plans on where to go on its working,'' says Dietrich Schroeer, a physics professor at the University of North Carolina who has studied SDI technical progress.
SDI may have faded somewhat in the public mind, but it's still spending huge sums of money, notes Mr. Schroeer. ``As long as it's $3.5 billion a year, there will be continued pressure to deploy something,'' he says.
Politics, in the end, will determine whether SDI is deployed. ``I don't know if that decision would be technically driven,'' Monahan says.