What! The International Space Station has been invaded by an alien? People back on Earth got concerned for the team aboard the station, but eventually learned they were goofed because it was an invasion concocted no less than by renowned Chris Hadfield, Canadian astronaut and the commander of Expedition 35. Still all part of April Fool's day global phenomenon commemoration.

Using his Twitter account, Mr Hadfield relayed finding a "debris" in the shape of a saucer floating in space.

His next photo earned gasps because it showed a clearer, now more defined outline of a UFO flying high above the Earth and apparently going towards the space station.

"Orbital debris seems to be on a course moving a bit faster than ISS," Mr Hadfield wrote. "I'll try to take more pictures if it swings by."

Over the course of less than four hours, Mr Hadfield build the momentum of credibility of his April Fool's joke, continuously posting photos and updates.

"Wow, what a huge piece of debris! Maybe I can grab it with the Canadarm2...," he said.

And then, he reported the inevitable: "The object appears to be coming closer to the Station. I think it might be trying to board us!"

The invader? A small green alien held by a terrified looking commander on both hands.

"I don't know what it is or what it wants, but it keeps repeating "Sloof Lirpa" over and over. Alert the press."

Sounds alien? "Sloof Lirpa" is actually April Fools spelled backwards.

But some knew way before even Mr Hadfield got to close his joke's curtain call that the ISS commander was playing a joke.

"Astronaut humour," Ivor Tossell (@ivortossell) commented.

Another follower, Steve McPherson (@steventurous), wrote, "I didn't know they had Photoshop in space."

Other followers played along.

"Quick someone give Canadarm2 a baseball bat #homerun," Pat Dunn (AstonPat) wrote.

And to cap off his joke, Mr Hadfield posted the photo below.