An Italian astronaut who nearly drowned in his helmet during a spacewalk has told of how he felt all alone and frantically tried to come up with a plan to save himself.

Luca Parmitano described how he was unable to see as the water sloshed around in his helmet during a spacewalk outside the International Space Station.

"But worse than that, the water covers my nose - a really awful sensation that I make worse by my vain attempts to move the water by shaking my head," the former test pilot wrote.

"By now, the upper part of the helmet is full of water and I can't even be sure that the next time I breathe, I will fill my lungs with air, and not liquid."