International Space Station astronauts Chris Cassidy and Luca Parmitano had been conducting their second spacewalk of the month. The walk had been intended to last over six hours. It was being carried out in order to perform routine maintenace and to prepare the ISS for the arrival of a Russian laboratory module later this year.

However, just over an hour into the spacewalk, Parmitano complained that his helmet was filling with water, and the liquid was getting into his nose and eyes. The incident, which was captured on NASA's livestream of the procedure, led to mission control cancelling the spacewalk.

Upon both astronauts returning safely to the ISS pressure chamber 1 hour and 32 minutes after the spacewalk began, Cassidy reported Parmitano looked OK but "miserable".

It is not yet clear what caused the water leak, with early suggestions that it could have been a leaking drinking water supply. It is not believed to have been sweat.

The spacewalk has been postponed until a future date. Former ISS Commander Chris Hadfield said ending the spacewalk was the right call:

Lots of water is inexplicably collecting in Luca's helmet. Where is it coming from? To be safe, NASA is stopping the spacewalk. Good idea.

— Chris Hadfield (@Cmdr_Hadfield) July 16, 2013