The accusations of cover-up result, in part, from a few alleged missing images, or images that NASA tampered with, in the series snapped over few days by SOHO (Solar and Heliospheric Observatory).
Plait called the tampering suggestion silly.
Plait said SOHO images have given fringe thinkers and writers plenty to chew on for years now.
"According to them, SOHO images show UFOs, giant planets, the eye of God, and probably even Bigfoot if you look hard enough," he said. "All of these facts are based on these people not understanding how digital cameras work."
Many of the "sightings" claimed in SOHO images are the result of bright spots generated when solar radiation hits the spacecraft's detector, Plait explained.
Other claims result from artifacts, or flaws, known to exist in the camera. SOHO engineers have detailed these shortcoming of digital imaging and have even gone so far as to explain "how to make a UFO" out of a SOHO picture in their effort to combat the conspiracy crowd.
Plait complains that the alleged cover-up is "insulting to NASA and the astronomical community. These conspiracy theorists seem to forget that three people can keep a secret only if two of them are dead. You can hardly ever shut up an astronomer; how would you keep thousands of them quiet?"