By Alan Boyle, Science Editor, NBC News

If you repeat UFO fiction often enough, does it eventually get reported as fact? Yes ... especially if you add in a 2012 doomsday angle and some dodgy astronomical imagery. Fortunately, an Internet truth squad finally knocked down this alien invasion.

Claims that we're about to be visited by alien spaceships are generally a dime a dozen (or a quatloo a dozen?), but for some reason one particular urban legend about "Giant Spaceships Heading Towards Earth" kept itself alive for more than a year, mostly by metastasizing on UFO forums. From the very beginning, the reports pointed to three eerie blue-green shapes on Sky-Map.org's archived imagery from the Digitized Sky Survey. "Trust me you will be very amazed. I WAS FOR SURE!!!!" one commenter wrote in February.

As the story was passed along, another forum commenter (who claimed to be a SETI investigator writing "at great threat to myself") said the spaceships were on a trajectory that would bring them to an area near Washington, D.C., on Dec. 21, 2012 — just in time for the Maya apocalypse. Later versions of the story incorporated the 2012 doomsday angle as well as the attribution to a SETI astrophysicist. Some even gave the researcher a name: Craig Kasnov.

Those are all the elements of a good UFO tale: a supposed insider, sharing seemingly legit evidence about an impending alien invasion with a well-known doomsday deadline. It's clear that thousands of folks wrote about the tale, based on an Internet search of key terms in the text. Sky-Map.org said it recorded nearly 100,000 Web visits on Dec. 2, when the tale was picking up speed on the Internet.

The truth squad finally caught up with the story around Dec. 9, when level-headed forum participants noted that the blue-green shapes were clearly flaws in the photographic plates that were digitized for the sky survey. In each of the three cases, emulsion problems showed up in one of the color-coded plates but not the others — which explained the bluish color. Craig Kasnoff (with a double-f) also weighed in: He wrote that he was indeed involved in the genesis of the SETI @ Home alien-searching project — but he denied that he was an astrophysicist, and denied making any comments about approaching alien spaceships.

"This post may, or may not have, made any contribution to the discussions of 'objects flying towards Earth,'" he wrote. "But I hope it clears up any question regarding my involvement [in] this announcement."

To recap: The weird shapes on the astronomical pictures were nothing more than photographic flaws. The UFO claims had no authority behind them. And the 2012 date merely capitalized on the Maya apocalypse hype.

Bad Astronomy's Phil Plait provides a detailed smackdown of the UFO tale. Discovery News' Ian O'Neill, a charter member of the 2012 truth squad, weighs in as well. O'Neill also provides a handy B.S. detector for the seemingly scientific claims you might come across in the social media mix.

Meanwhile, the UFO beat goes on: The same YouTube user who posted a "Giant Spaceships" video last December has plenty more where that came from.

Bottom line? Watch the skies if you like ... but also watch what you believe.

More UFO tales:

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Alan Boyle, Science Editor, NBC News