The leak was so bad that Luca Parmitano, Italy's first spacewalker, couldn't hear or speak as the spacewalk came to an abrupt end. He asked his spacewalking partner, Christopher Cassidy, for help getting back in.
"He looks miserable. But OK," Cassidy assured Mission Control in Houston.
The source of the leak wasn't immediately known but a possible culprit was the helmet drink bag that astronauts sip from during spacewalks, although Parmitano later reported the water had an odd taste. Water also is piped through the long underwear worn under a spacesuit, for cooling.
Parmitano reported, "It's a lot of water," before he could no longer talk. His crewmates quickly yanked off his helmet, once he was back inside.
NASA seldom cuts a spacewalk short. Tuesday's problem left them with no choice. Parmitano could have choked on the floating water droplets in his helmet.
The trouble cropped up barely an hour into what was to be a six-hour spacewalk to perform cable work and other routine maintenance that had stacked up over the past couple years.
It was the astronauts' second spacewalk in eight days.
Parmitano startled everyone when he announced that he felt a lot of water on the back of his head.
At first, he thought it was sweat because of all his exertion on the job. But he was repeatedly assured it was not sweat. Cassidy said it might be water from his drink bag; it looked like a half-liter of water had leaked out.
The water eventually got into Parmitano's eyes. That's when NASA ordered the two men back inside. Then the water drenched his nose and mouth, and he had trouble hearing on the radio lines.
Cassidy quickly cleaned up the work site once Parmitano was back in the air lock, then joined him back in the space station.
The four astronauts who anxiously monitored the drama from inside hustled to remove Parmitano's helmet. They clustered around him, eight hands pulling off his helmet and using towels to mop his bald head. Balls of water floated away.
Parmitano looked relatively fine on NASA TV as he gestured with his hands to show his crewmates where the water had crept over his head.
Cassidy told Mission Control: "To him, the water clearly did not taste like our normal drinking water." A smiling Parmitano then chimed in: "Just so you know, I'm alive and I can answer those questions, too."
Mission Control praised the crew for its fast effort and promptly scheduled a radio hookup with flight surgeons on the ground. Engineers, meanwhile, scrambled to determine the source of the leak.
It was the fastest end to a spacewalk since 2004 when Russian and American spacewalkers were ordered back in by Mission Control outside Moscow because of spacesuit trouble. That spacewalk lasted a mere 14 minutes. Tuesday's spacewalk lasted one hour and 32 minutes.
This was the second spacewalk for Parmitano, 36, a former test pilot and Italian Air Force officer. He became the first Italian to conduct a spacewalk last Tuesday, more than a month after moving into the space station.
Cassidy, 43, a former Navy SEAL, is a veteran spacewalker midway through a six-month station stint.