SALT LAKE CITY — As Americans gear up to mark a half-century since Apollo 11 landed on the moon, a new survey by Satellite Internet finds 10 percent of Americans believe NASA faked the whole adventure — up from six percent in 1999.

The number is even higher among the younger set than among those who lived during that era: Nearly 1 in 5 millennials doubt we landed on the moon, the survey says.

Here's an especially odd fun fact, though: Three-fourths of those who say they think the moon landing was faked also believe that aliens have landed on Earth. That apparently seems more plausible than a reverse visitation of earthlings to the moon.

Ask moon-landing deniers why they don't believe that Neil Armstrong took his "one small step for man, one giant leap for mankind" and most note that Old Glory was rippling in an apparent breeze in the photo taken on the moon. "No wind on the moon," they say, though these debunkers have been debunked repeatedly by the explanation that Armstrong and fellow astronaut Buzz Aldrin wiggled the flag pole back and forth to plant it in the lunar terrain, then photographed it immediately, while it still twanged and vibrated, causing the ripples.

Had those eager astronauts waited a couple of minutes to take the photo, the ripples probably wouldn't have become an issue. But those espousing the "faked it" theory also decry images of the astronauts reflected in each other's helmets that don't show either man peering into a camera to take the photograph. National Geographic says that one's easy: Their cameras were mounted to the chest part of their spacesuits — not unlike modern police chest cameras. And photos taken of the astronauts clearly show that.

Experts probably thought they'd need their hands for other things — like planting flags and gathering moon rocks. And maybe zero-gravity didn't seem like the best place to risk losing a camera that was used to take once-in-a-lifetime photos, either.

A scene from the documentary film "Apollo 11."
A scene from the documentary film "Apollo 11." | Neon CNN Films

Discussion of whether the moon landing really happened — and the vast majority believe it did — points out how tricky history can be. If you only believe what you personally witness, then who's to say that any moment in history will stand the test of time?

The Holocaust — with its slaughter of millions of Jews and artists and others who drew the wrath of Adolf Hitler and his Nazi forces in World War II — has been denied repeatedly. As recently as this week, it was labeled "fake news" in flyers found outside a temple in Marblehead, Massachusetts.

The Al Jazeera news organization in May suspended two journalists for a video that alters and downplays the facts of the Holocaust, which are well-known and thoroughly documented. But while moon-landing denial is deemed relatively harmless and appeals mainly to conspiracy theorists — especially those who don't trust the government — Holocaust denial is seen more ominously as anti-Semitic and racist, a tool used to foment hate.

Will future generations deny that terrorists hijacked planes and flew them into the World Trade Center and the Pentagon — something today's young generation is more apt to accept as fact because it lived through the events?

It's already happening as numerous conspiracy theories surround the 9/11 attack. Even Popular Mechanics has busied itself refuting sporadic but persistent claims, explaining the science behind things like the fact some windows at the Pentagon remained intact. They were "blast resistant" and did what they were supposed to based on where they were in relation to the impact of the hijacked plane.

A scene from the film "Apollo 11."
A scene from the film "Apollo 11." | Neon CNN Films

Research suggests that conspiracy theories are not benign.

Research in the European Journal of Social Psychology found that, "specifically, conspiracy theories are consequential as they have a real impact on people's health, relationships and safety; they are universal in that belief in them is widespread across times, cultures and social settings; they are emotional given that negative emotions and not rational deliberations cause conspiracy beliefs; and they are social as conspiracy beliefs are closely associated with psychological motivations underlying intergroup conflict."

For Vox, Jane Coaston wrote that "conspiracy theories can diminish faith in institutions and government, lead to distrust of science and medicine, and — even worse — inspire acts of violence. And conspiracy theories can be and are being used by those with political influence to shore up their power and mitigate opposition."