The Pentagon announced yesterday that captured alien beings, either dead or alive, are not being held in secret U.S. military installations. Alien spaceships have never invaded Earth, particularly not in the vicinity of Roswell, N.M., 50 years ago, top officials said.

That, essentially, was how the Air Force tried yesterday to debunk one of the central tenets of UFO mythology. At an otherworldly news briefing, staid military officers handed out copies of "The Roswell Report: Case Closed," a 231-page investigation of an incident that has created a pop-culture cottage industry.

The Air Force launched this preemptive strike as the town of Roswell, population 50,000, prepares to celebrate the golden anniversary of an event that has made it a holy city for UFO cultists -- tens of thousands of pilgrims are expected there next week.

Pentagon briefers showed a packed room of reporters photos and video of NASA test craft that looked eerily like flying saucers, saying these helped explain why so many people claim to have seen such airships. Referring to decades-old reports of alien corpses, Col. John Haynes read a statement saying, in part: "Bodies observed in the New Mexico desert were actually anthropomorphic test dummies that were carried aloft by U.S. Air Force high-altitude balloons for scientific research."

Previous government denials about alien landings have not been universally accepted, and yesterday's was no exception.

"You're confident you're not part of any coverup, wittingly or unwittingly?" a reporter pressed. "I'm totally confident," Haynes replied.

In early July 1947, officials at the Roswell Army Air Field collected the remains of what they initially reported to be a wrecked "flying disc." That press release was quickly revised to say it was only a weather balloon. But the lore grew to include tales of almond-eyed little gray men being spirited away for autopsies and even interrogations.

Several magazines, including Time and Popular Mechanics, have devoted recent cover stories to the Roswell incident. Last summer's top-grossing movie, "Independence Day," used the premise that the captured Roswell aliens were being stored in the Nevada desert at a classified Air Force base popularly known as Area 51. Haynes, whose usual job is declassifying documents, deflected several questions yesterday about Area 51, saying, "I don't have the access,' end of quote."

Such secrecy has helped to feed speculation about Roswell. The Air Force last attempted to demystify the incident in September 1994, issuing a report acknowledging that the shiny debris found on a sheep ranch outside Roswell was no ordinary weather balloon. It was the remains of a secret listening device designed to detect Russian nuclear blasts. Part of Project MOGUL, the device included an array of foil kites and was held aloft by a 600-foot-long train of balloons. (That disclosure came only after Rep. Steve Schiff {R-N.M.} pressed for an investigation by the General Accounting Office.)

"There were no alien' passengers therein," said the 1994 report. "It is recommended that this document serve as the final Air Force report related to the Roswell matter."

But skeptics weren't satisfied. What about the persistent reports of humanoid bodies, they asked? Capt. James McAndrew, the author of the initial report, was given permission to keep researching Roswell. He turned up archival film footage and photos of dummy drops that were widely used in the early '50s in New Mexico to test parachutes and ejection equipment. In the new report, he concludes that people who told of seeing aliens -- most accounts are secondhand -- were confused or the victim of hazy memories.

"Dummies . . . were most likely the aliens' associated with the Roswell Incident,' " says the report, noting that witnesses said the figures were bald and had no eyebrows. Other rumors of aliens turning up at military hospitals were connected to a 1956 air tanker crash that left 11 men dead, and a 1959 "manned balloon mishap" that injured two pilots, according to the new Air Force report.

"If you find that people talk about things over a period of time, they begin to lose exactly when the date was," Haynes said.

"It doesn't wash. I think the Air Force is truly getting desperate," said Stanton Friedman, a nuclear physicist who calls himself "the original investigator of Roswell." "None of that work {with test dummies} was done before the 1950s. How many lies can we stand from the Air Force? It's time for the Air Force to come clean."

But other longtime UFO researchers have come to agree with the Air Force's conclusions about Roswell. "No UFO crashed at Roswell -- with or without aliens. It did not happen. Period," declares Kent Jeffrey, who collected more than 20,000 signatures from people opposed to secrecy about the Roswell matter.

Kal Korff, another former true believer, writes in this month's Skeptical Inquirer magazine: "The Roswell mystery has been solved, and there is no credible evidence that the remains of an extraterrestrial spacecraft was involved."

Officials had no immediate estimate of how much was spent on the new Roswell report (available from the Government Printing Office for $18) but said they have no intention of ever probing UFOs again. "It's a waste of taxpayers' money to keep investigating reports that never turned out to be true," said Defense Department spokesman Kenneth Bacon.

"Do you really think this will put this whole thing to rest?" a reporter asked.

"Of course not," Bacon said. "Probably in another decade another colonel will be here giving another briefing on the Roswell incident." CAPTION: Guide Dennis Balthaser of the UFO Museum and Research Center in Roswell, N.M., explains an exhibit. CAPTION: Lts. Eugene Schwartz, left, and Raymond Madson flank "Sierra Sam," a dummy used in Air Force parachute drop tests. CAPTION: Air Force Col. John Haynes at the Pentagon yesterday, attempting to lay the "Roswell incident" to rest, "totally confident" there was no coverup. CAPTION: The "Case Closed" report includes this photo of part of NASA's Voyager space probe, which the Air Force says is typical of the objects people may have mistaken for alien spacecraft.