Forty years after Neil Armstrong became the first man to walk on the moon, there are still those who insist that his giant leap for mankind happened on a film set in Arizona, not on the lunar surface.

The deniers insist that NASA went to extraordinary lengths and great expense to stage a moon landing in a film studio because it wanted to distract a public weary of the Vietnam war, or felt it had to beat the Soviet Union in the space race but feared it didn't have the technology.

Or maybe the US space agency headed to the studio because it was cheaper and less risky than flying to the moon, the deniers have also said.

They put forward thoeries -- like the astronauts would have been fried by radiation when they passed through the Van Allen belts on their way to the moon -- to back up their claims that a moon shot in 1969 would have been impossible.

Most of the deniers were tipped into the realm of lunar landing disbelief after seeing photos of Armstrong and Aldrin on the moon -- or on a set somewhere, as they would insist --, astronomer Phil Plait said on the SETI Institute's "Are We Alone" radio program this week.

The first thing they tend to notice is the starless sky, Plait said on the radio show which is hosted by Seth Shostak, senior astronomer at SETI, a nonprofit based in California that aims to explain the origin and nature of life in the universe.

"There's no atmosphere on the moon, so you would expect the stars to be brighter," Plait said.

But even photos of the night sky on earth would show no stars unless they were shot with an exposure of several seconds -- which they couldn't be for the moon landing because the pictures of Armstrong and Aldrin were taken when the sun was up, he said.

At the shutterspeeds the astronauts would have used, "you just don't see stars, whether you're on earth or on the moon," Plait said.

The deniers also point out that the American flag shown in video footage of the lunar landing flutters -- even though there's no air on the moon -- and have grips with shadows on the lunar surface.

The negationists fell dormant for several years but resurfaced in 2001 when Fox TV aired a program called "Conspiracy Theory: Did We Land on the Moon?"

The show portrayed NASA as little more than "a blundering movie producer," wrote Dr Tony Phillips on the Science@NASA website.

Lunar landing negationists will probably fade into the background again when the United States returns to the moon, but will they die out entirely? Probably not, said Shostak.

"We'll go back to the moon and find all this hardware and take pictures of it and say, 'Look! Their bootprints!'

"And people who like to think that the US government has nothing better to do than fake a moon landing will say, 'Well, you faked that, too.'"