Unidentified flying objects have been spotted a few hundred miles above the Earth's surface by Nasa cameras.
The objects were captured near Nasa's International Space Station (ISS) on the U Stream camera. The film was then uploaded to YouTube by user Streetcap1.
Two of the space enthusiast's films show UFO activity near the ISS - on 14 January a cigar-shaped object is seen moving very slowly across the sky until it eventually disappears and on 15 January, a series of lights is filmed in a triangular formation. A few of the lights disappear and then return.
They also appear to move independently from one another until they eventually fade and vanish.
Streetcap1 had posted a number of previous videos claiming to show UFOs in space.
At the end of November, he uploaded a film said to show two UFOs flying along the Earth's contour.
Scott Waring, author of the website UFO Sightings Daily, said he had spotted a UFO in images taken during the Apollo 12 mission in 1969.
He claimed the images were blurred and had their colour changed by Nasa to hide the truth.
"Often Nasa gets caught altering photos - but for what reason? To hide the facts and make it harder for you to discover the truth that aliens exist."
One sceptic insisted that the films from the International Space Station do not show UFOs, although he has not been entirely dismissive about the issue.
James Oberg, a former space engineer who specialised in space shuttle operations for Nasa, said: "Spacecraft-generated 'dandruff' has been seen since the very first human spaceflights, when the non-intuitive relative motions and impossible-to-judge distances in the earthly environment of outer space tricked observers into misinterpreting visual stimuli.
"It shouldn't be surprising that only half a century later, most folks watching YouTube videos are still totally flummoxed by what they understandably can't comprehend."
Olberg, who is now the space news consultant for NBC, said it was important, however, to monitor footage for abnormalities in case something out of the ordinary did appear: "It's good to keep scanning space video for possible anomalies and reporting them quickly.
"There is always a real chance that it could be a genuine anomaly, either a spacecraft malfunction or other threat, expected or unexpected. In the past, missions have failed because a clue that should have been seen out the window was overlooked."