On Friday, the Pentagon acknowledged that an October test of the two-stage interceptor rocket encountered more problems than officials initially said. The interceptor hit and destroyed its target, but only after first putting its "cross hairs" on a large decoy balloon released from the Minuteman.

"This is a very demanding program, technologically. We've said that fromthe beginning. We hope that every test we have is a success but I think it would be unrealistic to expect that."

The balloon was there to replicate the kind of countermeasures that a hostile nation might use to confuse an actual U.S. missile defense system. The problems, not mentioned when the Pentagon declared the October 2 test a success, were first reported Friday by The New York Times.

Next Tuesday's test is the second for the Raytheon-built "kill vehicle" and much more difficult than the first. Pentagon officials are struggling to demonstrate the technology necessary for President Clinton to make a decision by this summer on whether to build such a system.

"This test is not as contrived," said a Pentagon official who briefed reporters on the condition he not be identified. "This time, we'll roll in more elements to make sure we have an integrated test with command capability. The real challenge to this is not making it work, it's holding your breath for 30 minutes."

That's about how long it takes for the Minuteman to soar out over the Pacific for an encounter with the intercepting "kill vehicle" launched from a Pacific island more than 4,000 miles (6,437 kilometers) away.

After the Minuteman launches, infrared detectors aboard military satellites will pick up its hot exhaust plume. Those satellites relay the missile's position to a radar site on Kwajalein Atoll in the Marshall Islands. Test officials on the atoll, which abuts the world's largest lagoon, then launch the interceptor. The boosters on that rocket burn for two minutes, then a 120-pound (54-kilogram) "kill vehicle" with its own thrusters flies in space for another six minutes, maneuvering itself to hit the Minuteman at an altitude of 80 to 140 miles (130 to 225 kilometers).

Unlike the first test, the interceptor will be relying on commands coordinated through military test centers in both Kwajalein and Colorado. Program managers at both sites will try to interpret the signals from the early warning satellites and coordinate efforts to direct the interceptor to the target.

"This is a very demanding program, technologically," said Kenneth Bacon, Pentagon spokesman. "We've said that from the beginning. We hope that every test we have is a success but I think it would be unrealistic to expect that."

Should the National Missile Defense program be deployed, it would be "designed to deal with a very specific threat and that threat is from countries like North Korea and Iraq and Iran," Bacon said. "It's a limited missile threat against our country from rogue nations and we believe that it's important to develop a defense against that threat."

If the White House approves deploying such a system this summer, a workable program could be in place by 2005, Pentagon officials said. Two sites are under consideration -- Alaska and North Dakota. Only Alaska, however, is capable of defending all 50 states because of its location. North Dakota could not cover either Alaska or Hawaii.

Tuesday's test already is drawing opposition from a hodge-podge of protesters who plan to picket for 12 hours at the main gate of Vandenberg. Among the protesters are groups representing anti-nuclear activists, environmentalists, Catholic workers and war resisters.

"People all over the world are now vigorously organizing to stop [the missile defense testing]," said organizer Bruce Gagnon, who also has led protests at NASA's Kennedy Space Center against the launches of the nuclear-powered deep space probes Galileo and Cassini. "We cannot allow the Pentagon and aerospace corporations to move the arms race into space. There must be a ban on all weapons in space."