Swirling with lunacy and paranoia, the theories warn of mayhem and cataclysm. They fill books and websites, inspiring hand-wringing among gullible people. The claim: The world is ending on Friday, the final chapter in an ancient Mayan prophecy carved into stone calendars thousands of years ago.

The stories are a jumble, based on everything from New Age mysticism to biblical “end times.” In some accounts, a giant secret planet is about to slam into Earth, or a solar storm will wipe out the human race. None has any basis in fact, scientists say, but a poll this summer found 12 percent of Americans are worried. Some teenagers have even talked of suicide.

As Dec. 21, 2012, draws near, however, the U.S. government has a secret weapon to hold back the tidal wave of misinformation and pseudoscientific quackery: a bespectacled 72-year-old scientist, often clad in a rumpled cardigan, sitting in a two-story office building off Highway 101 in Mountain View.

David Morrison is Kryptonite for the world’s conspiracy craziness. A Harvard-trained astrophysicist who studied under Carl Sagan, Morrison is the senior scientist at the Astrobiology Institute at NASA Ames Research Center. He has worked on many of America’s top space missions, from Mariner to Voyager to Galileo, and published more than 155 technical papers and a dozen books on astronomy.

These days he has emerged as NASA’s most prominent Debunker of Doomsday, answering questions from people all over the world on his website, giving speeches and talking to the media. While some of his colleagues wonder if he’s wasting his time, Morrison holds out hope that reason and facts can win out, even in an age of Internet hoaxes and hype.

“I got my first doomsday question four years ago and wondered what the heck it was,” he said. “Perhaps I made the mistake of answering them, but since then I’ve gotten a little over 2,000 emails. I got 200 last weekend.”

Five days a week, Morrison calmly and logically explains to the masses through his “Ask an Astrobiologist” website why our days are not numbered.

One of the most common rumors is of a mysterious planet named Nibiru hiding behind the sun, ready to slam into Earth.

“Impossible,” Morrison said. “Earth goes around the sun. We see all sides of the sun. We’d see it.”

And he adds, if a planet were about to hit Earth in a few days, we’d really see it. “It would look like the moon in the sky,” he said. “You’d see it in the daytime. You wouldn’t have to ask the government.”

Kamikaze comets or asteroids? The more than 100,000 professional and amateur astronomers around the world would see those too, years before they got close to us, he said.

Solar flares? Sure, the sun has pulses and storms, which sometimes can disrupt electronics on Earth.

“That’s one of the few things here that is real,” Morrison said. “We know the sun has an activity cycle every 11 years. The peak is late next spring. There will be flares. But they don’t hurt us. This cycle the flares are weaker than last time.”

In recent months, Morrison has appeared in Web videos on NASA’s site, which he says is getting more clicks on doomsday topics than any issue except the Mars rover mission.

Although many claims are spawned by everything from religious zealotry to hucksters selling books, he said, there is a serious side.

“I get questions from people saying, ‘I’m 11 years old, and I can’t sleep, I can’t eat.’ I have had kids saying they are considering suicide, mothers emailing me saying they are considering killing their children before the end of times.”

Andrew Fraknoi, chairman of the astronomy department at Foothill College in Los Altos Hills, said Morrison’s work is heroic.

“He has taken on a thankless task,” said Fraknoi, who also is speaking out to debunk doomsday fears. “He feels that we as scientists have an obligation to respond, to reassure the public and to give the public the fact-based view of the universe. That is so absent from so many realms of our social discourse today.”

The latest angst, say archaeologists and experts on Mayan culture, is based on a big misunderstanding. The Maya, whose civilization flourished in Mexico and Guatemala from 2000 B.C. to 1000 A.D., built pyramids and observatories. Their calendar was based on 394-year cycles called baktuns. The 13th of those cycles since the date of the Mayan creation story 5,126 years ago ends Friday.

But that doesn’t mean they thought the world was going to end, said Rosemary Joyce, a professor of anthropology at UC Berkeley.

“It’s not the end of the calendar,” Joyce said. “It’s the end of a cycle. It rolls over, like an odometer.”

Joyce said the fears began generations ago, when scholars who hadn’t yet learned how to read Mayan hieroglyphics mistakenly concluded that they were describing mystical prophecies. New Age activists embraced the ideas in the 1960s and 1970s, and today the misread history has blended with “end times” fantasies and spread on the Internet. The 2009 disaster movie “2012,” featuring floods, massive earthquakes and other computer-generated mayhem, further put a spotlight on the issue.

Several million people of Maya heritage are still around today, including more than 40,000 in the Bay Area. They don’t believe the world is ending, said Alberto Perez, program director at the Maya Association of the Bay Area.

“I have the sense that it bothers people in our community that we are perceived in this almost-negative way, like we predicted the end of the world,” he said. “We didn’t. We’re worried about day-to-day things: jobs, education, immigration, health care.”

Many people who know NASA’s Morrison are wondering what he’ll do when we wake up Dec. 22 and Earth is still here. Will he pen a grand “I told you so?”

“No,” he said with a chuckle. “I’m going to stop answering questions about this. I’m worn out.”