WASHINGTON — More than 33 years after the United States landed men on the moon, NASA is spending more than $15,000 to convince people that it really did happen and that the space agency didn't make it all up.

Stubborn conspiracy theorists claim that NASA's six Apollo-program moon landings were faked. After decades of belittling and ignoring them, NASA has decided to fight back. It hired James Oberg, a Houston-based former aerospace engineer and award-winning author of 10 books on space, to confront skeptics point by point. Many scientists already have done that on the Internet, but skeptics remain unconvinced.

"Ignoring it only fans the flames of people who are naturally suspicious," Oberg said this week in an interview.

Last year, Fox television twice broadcast a show entitled "Conspiracy Theory: Did We Really Land on the Moon?," and NBC's "Today" show staged a debate on the topic. Last month Buzz Aldrin, the second man on the moon, punched a conspiracy theorist who had been pestering him to swear on a stack of Bibles that the landing was real.

After the Fox show first aired, NASA put out a one-paragraph press release titled "Apollo: Yes, We Did."

Yet a 1999 poll found that 11 percent of the American public doubted the moon landing happened, and Fox officials said such skepticism increased to about 20 percent after their show, which was seen by about 15 million viewers.

Stephen Garber, NASA's acting chief historian, said Oberg's 10-chapter, 30,000-word monograph "is not going to convince the people who believe in these myths. Hopefully, it'll speak to other people who are broad-minded."

The book will expose "space myths writ large (and will) look at some of these broader issues of how these myths get initiated and promulgated," Garber said.

Oberg "has got one hell of a job ahead of him," said skeptic Ralph Rene, a New Jersey carpenter who said he's self-taught in physics and has self-published two books. One book claims the moon landing didn't happen; the other criticizes Isaac Newton's grasp of physics. "I could care less what they do."

"It's a real shame that it has to be done," said Sonoma State University astronomy professor Phil Plait. He runs the badastronomy.com Web site, which debunks space myths, including the moon hoax.

"It's beneath NASA's dignity to give these twinkies the time of day," Plait said. "The problem is, if you ignore a problem, it doesn't go away."

But confronting conspiracy theorists usually doesn't convince them either, said historian Gregory Camp, author of "Selling Fear: Conspiracy Theories and End-Times Paranoia."

"The true believers in that kind of thing already have the answers — at least in their eyes," Camp said. "It's incredible that a book like that has to be written."