ALIEN conspiracy theorists are confident they've caught NASA in the act of covering up the fact we are not alone in the universe.

A video posted on YouTube yesterday showed how an image of Saturn's moons Dione and Titan, taken by NASA's Cassini orbiter, had been Photoshopped before being added to a Picture of the Day website.

In the video, "DominatorPS3" turned up the brightness levels on the photo to show that a "huge" object can be seen behind the smaller moon, Dione.

Clearly visible are brush strokes that show how the rainbow aura of the object has been blacked out.

"More solid proof of NASA/ government coverups," DominatorPS3 said.

"... and this is recent. You can do this yourself!!"

Of course, being the internet, it didn't take long for the truth to get out there, so to speak.

The person responsible for the manipulation, Emily Lakdawalla, told a forum of excitable theorists that she made the changes because of the way Cassini takes photos.

"Cassini takes colour pictures by snapping three sequential photos through red, green, and blue filters," she said.

"In the time that separated the three frames, Dione moved, so if I did a simple color composite I would be able to make Titan look right, but not Dione; or Dione look right, but not Titan.

"So I aligned Dione, cut it out, and then aligned Titan, and then had to account for the missing bits of shadow where the bits of Dione had been in two of the three channels."

She explained the process further at Planetary.org but it still wasn't enough for the alien hunters, particularly DominatorPS3.

"Thanks for the feedback and explanation," he said in a comment on the YouTube post.

"However I still remain sceptical because someone still could have ordered her to 'cut it or something out'.

"But thanks for putting that info up so others can read it; I am not trying to convince people what I believe."