Cooperating in a way that would have been unimaginable during the Cold War, U.S. and Russian military officers sat down side by side at the headquarters of the U.S. Space Command today and began jointly monitoring missile early warning equipment to assure their nations that no Y2K glitches will propel the world toward Armageddon when the new millennium dawns.

And other than a brief malfunction of one hot line telephone connection between the two nations, everything went as planned. Which is to say, nothing much happened.

"Operations have gone like clockwork," said Navy Capt. Michael W. Luginbuhl, vice director of operations for the space command headquarters at Peterson Air Force Base.

The hot line malfunction "was the only glitch," agreed Col. Sergey Kaplin, department chief of the general staff of the Russian Federation, speaking through an interpreter. "As you know we have multiple lines of communication."

After training together for more than a week, the first joint missile-watching teams this morning began what will be round-the-clock monitoring of the sky through mid-January from the $4.5 million Center for Y2K Strategic Stability, set up under an agreement signed 15 months ago by Defense Secretary William S. Cohen and his Russian counterpart, Marshal Igor D. Sergeyev.

Ultimately, the experiment begun here will evolve into a permanent Joint Warning Center to be built near Moscow and staffed by officers from both nations to prevent an unintended exchange of nuclear-tipped missiles.

Officers from both countries today agreed that monitoring data from the complex U.S. system of ground radar systems and space satellites -- funneled here from the nearby operations center 1,800 feet beneath the top of Cheyenne Mountain -- is largely an exercise in public relations as far as any Y2K dangers are concerned. The United States and Russia, they said, had long since fixed and tested all their critical nuclear warmaking computer systems so that nothing will fail when the year 2000 begins and computers must recognize that the digits 00 in the software mean 2000 and not 1900.

"My greatest concern," said Luginbuhl as he addressed a small army of journalists gathered here, "is that I'll trip when I walk out of here over one of the wires. . . . We've spent a long time preparing for this and we are Y2K-ready."

As for the Russians, Kaplin said, "all our systems have been checked and we are sure they will work well. My only concern is I don't speak English and I have to work through an interpreter."

The 19 Russian officers here for the joint monitoring exercise are not working inside Cheyenne Mountain, the hardened, ultra-secure command center for the North American Aerospace Defense Command (NORAD) that was completed in 1962 and is protected by 25-ton blast doors. Instead, key early warning data is being transferred from the command center to the Y2K center at Peterson, including the time, position, and trajectory of any missiles launched anywhere in the world.

The idea is that if a computer glitch occurs and falsely indicates a missile launch, the joint command will be able to immediately verify that it is not a true launch and prevent a retaliatory strike. Or, in the ultimate nightmare scenario, if a computer malfunction results in an accidental launch, both sides would be able to monitor it together and communicate with their national commands to prevent an escalating nuclear disaster.

And even though the visiting Russian officers will not have access to the full range of sensitive data that pours into NORAD from around the world, Kaplin said "the information we get lets us solve all the problems and complete our mission."

U.S. officers went out of their way today to stress that computers never have total command of the 500 Minuteman and 50 Peacekeeper intercontinental ballistic missiles, and that any launch ultimately relies on "a human in the loop."

"We are very confident we will not have any accidental launches of ICBMs," said Lt. Col. Gary Warren, Y2K manager for the Space Command. He detailed how the Air Force spent five years testing the 400 separate computer systems, 97 of them "mission critical" ones, involved in nuclear defense. Of those systems, he said, only 21 needed Y2K repairs and none of those were highly critical to the nuclear defense mission.

This afternoon, commanders of the first U.S.-Russian shift to monitor the early warning systems emerged from the Y2K center and said everything had proceeded as planned and that they expect nothing different when they proceed through a series of Y2K "rollovers" as the new year begins in key nuclear facilities around the world.

The most important of those demarcations will come at 12:01 a.m. Greenwich Mean Time, the time that U.S. forces operate on, and 12:01 a.m. Moscow time, which governs Russian forces.

"It went very smoothly," reported Air Force Lt. Col. David Hale. Though "we don't expect any problems on the U.S. or Russian side," Hale said it is possible that other nations with less-sophisticated missile and computer systems may experience Y2K anomalies.

But after so many years of being adversaries staring across a potential nuclear abyss, both sides were asked, how is it possible to have complete trust in one another at such a sensitive time?

"I believe this faith is very strong because it let us construct this center," said Kaplin, the Russian commander.

"At some point you have to trust, to have the faith," said Luginbuhl. "I don't think anyone has anything to gain by presenting false information or data."