Roswell incident 75th anniversary: World's most famous UFO case won't go away

Seventy-five years ago a mysterious discovery on US ranchland triggered the world's most famous UFO, or Unidentified Flying Object, case.

On July 8, 1947, a newspaper in the city of Roswell, New Mexico, ran a story about the United States military confirming debris from a crashed "flying disk" had been found in the area weeks earlier.

Despite a statement from the US Air Force the next day that the wreckage - composed of metallic-looking objects including rubber strips, tinfoil and sticks - was from a weather balloon, the events had captured global attention.

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Former NASA chief historian Roger Launius told 9News the "flying saucer" report was initially put out by the US military for expediency.

"Apparently, it was better from the Air Force's perspective that there was a crashed 'alien' spacecraft out there than to tell the truth," he said.

"But it triggered a media frenzy ... officials in Roswell were inundated by journalists."

The red herring was an attempt to divert the public gaze from the US military's top-secret Project Mogul, involving high-altitude reconnaissance balloons.

Much larger than a conventional weather balloon, a typical Mogul balloon was twice as tall as the Statue of Liberty.

They were developed by the US to spy on Soviet Russia's atomic bomb program.

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"A flying saucer was easier to admit than Project Mogul," Launius said.

In 1947 at the early stages of the Cold War, truth was not a priority for the US government, he said.

But if the military hoped to dispel interest in Roswell with its insistence a balloon had crashed, supported in an official 1948 report, it reignited decades later.

"We get to the late 1970s and early 1980s ... and that period saw a real spike in extraterrestrial interest, from movies to books and other things," Launius said.

Among the most outlandish claims was a film released in 1995 apparently showing autopsies of alien bodies recovered at Roswell and moved to the Pentagon's top secret Area 51 site in Nevada.

It was later proved to be a fake.

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And two years later former US Army public affairs officer Walter Haut said of the Roswell incident in 1947: "We had in our possession a flying saucer."

Launius says claims such as these resulted in the Roswell Incident "getting better with time".

It also spawned books, films and television shows, such as The X-Files.

Determined to quash the frenzy, the US Air Force released an exhaustive report in 1994, titled The Roswell Report, Case Closed.

But despite the document offering numerous explanations to the claims of UFO conspiracy theories, including that reported bodies were crash dummies used in balloon testing, rumours of a cover-up continued.

The city of Roswell, meanwhile, cashed in on its fame.

It now home to a UFO Museum and remains a major destination for alien enthusiasts looking for more evidence of their beliefs.

Others such as Launius insist UFO sightings can only be judged on the evidence at hand.

"They're simply unidentified things you see in the sky. We've all probably seen them," he said.

"And, if you look long enough, you'll probably eventually figure out what it is you're looking at. It's not extraterrestrials."

But he also concedes public interest in Roswell will not go away any time soon.

"We'll still be talking about it in 25 years."