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."