BY JONATHAN S. LANDAY
Mercury News Washington Bureau

WASHINGTON -- Final preparations were under way Thursday for a $100 million test of the proposed U.S. missile defense system amid a fresh barrage of charges by U.S. experts that it will not work and will undermine U.S. national security.

``The system would offer little protection and would do grave harm to this nation's core security interests,'' said a letter to President Clinton from 50 Nobel Prize-winning scientists. They urged him not to authorize the deployment of the system even if the test tonight succeeds.

Theodore Postol, a Massachusetts Institute of Technology physicist and outspoken critic of the plan, sent a separate letter to Clinton charging that defense officials who demonstrate ``technical illiteracy'' were misleading the president about the system's capabilities.

The Pentagon said final preparations were going forward.

Weather and technical conditions permitting, a modified Minuteman intercontinental ballistic missile fired from Vandenberg Air Force Base will loft a target warhead and a Mylar balloon decoy into space above the Pacific Ocean.

Twenty minutes later and 4,300 miles away, an interceptor -- called a kill vehicle -- will be launched atop a booster rocket from the Pacific atoll of Kwajalein.

Must detect warhead

If all goes according to plan, the interceptor will distinguish between the decoy and the mock warhead and head for the warhead. It will then vaporize the warhead in a collision 144 miles above the Earth. Their convergence speed will approach 15,000 mph.

The test is scheduled to take place during a four-hour period beginning at 10 p.m. (7 p.m. PDT). It will take scientists up to two weeks to analyze the results and report them to Defense Secretary William Cohen, who will recommend later this summer whether to proceed with building the first phase. Tonight's test would be the third full-scale trial of the system's tracking satellites, prototype ground-based radars and prototype interceptor. The first test in October succeeded. January's test failure was blamed on a coolant leak in the interceptor.

Even the successful test prompted questions. Some experts say tests involving only one decoy do not prove the system reliable. They contend that U.S. foes would employ many decoys. Pentagon officials insist that technologies are being developed to allow the interceptor to identify and reject multiple decoys.

The Pentagon originally said that two out of the three tests would have to succeed for Cohen to recommend to Clinton that he proceed with the system's deployment. But in recent weeks, defense officials have said that Cohen could still give the go-ahead even if today's test fails.

Numerous scientists have criticized the program, saying it has flaws that will prevent it from achieving its goal of protecting all 50 states. The system is intended to defend against limited nuclear, chemical or biological attacks by countries developing long-range missiles, such as North Korea.

Might spark arms race

The 50 Nobel laureates warned that building the system would ignite a new arms race with Russia and China. Both nations fear the system is aimed at them and have threatened to deploy more nuclear-tipped intercontinental ballistic missiles so they could overwhelm it.

The scientists also said no technology exists that would enable the kill vehicle to keep ahead of improvements to offensive missiles.

``Even if the next planned test of the proposed anti-ballistic missile system works as planned, any movement toward deployment would be premature, wasteful and dangerous,'' said the letter. It was sponsored by the Federation of American Scientists, a Washington, D.C.-based arms control organization, and drafted by Hans Bethe, who worked on the first U.S. atomic bombs.

`Technical illiteracy'

Postol said that his letter to Clinton ``points out an unbroken series of statements that indicate both bias and technical illiteracy of the staff who would be supporting the president's decision.''

In a telephone interview, Postol said he recommended that Clinton ask for an independent review of the system's technical capabilities.

The cost of building and operating the full system has been estimated at $60 billion, but defense officials concede it could cost much more.