NASA misdiagnosed the earlier leak, believing the water found in the helmet of Italian astronaut Luca Parmitano on July 9 was due to a ruptured drink bag, said space station chief engineer Chris Hansen, who chaired an investigation panel appointed by the U.S. space agency.
"Had the issue been discussed in more detail ... the team likely would have realized that the water experienced in (Parmitano's) helmet was 'out of family' and needed to be investigated further," Hansen wrote in a report released on Wednesday.
Instead, a week later on July 16, Parmitano and NASA astronaut Chris Cassidy put on their spacesuits to continue work outside the space station, a $100 billion research complex that flies about 260 miles (418 km) above Earth.
About 45 minutes into the spacewalk, Parmitano radioed to ground controllers that water was leaking into his helmet.
Investigators said NASA did not immediately recognize the possibility that Parmitano's suit was failing.
Engineers mistakenly believed a water leak would trip a fan to shut down, signaling a suit problem. Instead, the fan remained operational despite water seeping its way around a valve and ultimately inside the spacewalker's helmet.
By the time flight controllers aborted the spacewalk, water obscured Parmitano's vision and impaired his breathing. As the astronaut made his way into the airlock, assisted by Cassidy, he lost radio communications as well.
About 1.5 liters (3.17 pints) of water ended up inside Parmitano's helmet, or about twice as much as what was found after the July 9 spacewalk, the report shows.
Initially, no one suspected that first leak was anything more serious than a burst drink bag.
"This was a really subtle problem. It took us weeks before we started getting to the conclusion that this had happened earlier," Hansen told reporters on a conference call.
Ultimately, engineers figured out that the leak was due to contamination in a pump that is part of the spacesuit's cooling system. The source of the contamination remains under investigation.
The 220-page report includes 49 recommendations to beef up safety protocols, training and communication. NASA says it will implement the findings before scheduling its next series of spacewalks, targeted for this summer.
NASA has twice before been surprised by unknown technical implications of hardware problems, with disastrous results. In 2003, foam insulation falling off the space shuttle's fuel tank during launch caused wing damage that destroyed the Columbia as it glided through the atmosphere for landing, killing seven astronauts.
The 1986 Challenger accident, which also killed seven astronauts, was later blamed on a booster rocket seal that failed during its launch in cold weather.
"The message to all of us is to be really vigilant and to really communicate," said NASA spaceflight chief William Gerstenmaier.
"We're not hiding this stuff. We're actually out trying to describe these things, describe where we can get better. I think that's how we prevent Columbias and Challengers," he said.