This is chapter two of a four-chapter story.

Karen Nyberg was on the space station in 2013 when crewmate Luca Parmitano (#532), out on a spacewalk, called for help. His suit’s cooling system had sprung a leak, and water was filling his helmet. In the weightless freefall of orbit, water doesn’t pool — it forms floating blobs that stick to any surface they touch. Within minutes, water was covering Parmitano’s eyes and nose. He couldn’t see and could barely breathe. He had to get back on board fast before he drowned. But to do that, he needed help from his crewmates.

As Parmitano felt his way back into the airlock, Nyberg floated inside the station, operating the airlock controls. Chris Cassidy (#499), who had been outside working on a different part of the station, joined Parmitano in the chamber. As soon as Cassidy closed the door, Nyberg began repressurizing the airlock.

But the process was agonizingly slow. Parmitano, who had been radioing regular updates on his status to Mission Control in Houston, suddenly went silent. Was he drowning? Nyberg couldn’t tell.

She looked at the air-pressure control and thought about cranking it up full blast to flood the chamber so that they could get Parmitano’s helmet off fast. But she knew that doing so could burst his eardrums — and Cassidy’s, too.

Astronaut Karen Nyberg Dan Winters / for NBC News

As the pressure inched upward, Cassidy squeezed Parmitano’s hand, and the distressed astronaut gave an OK sign. He was able to breathe, and even talk, though no one could hear him because the water had knocked out the communications link in his helmet. And he couldn’t hear because his ears were full of water.

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At last, the pressure equalized. Nyberg flung open the door and quickly removed Parmitano’s helmet so she and Cassidy could wipe the water from his face.

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“I was worried,” Nyberg allows, recalling the moment. But her training kicked in: sometimes the right move in a crisis is to avoid extreme action and just follow standard procedure.

Though a water-filled helmet isn’t a likely scenario, airlock operations was one of myriad procedures Nyberg had practiced during countless hours inside a full-sized mock-up of the International Space Station (ISS) at Johnson Space Center. Much of that training focuses on the routine of a “normal” day — exercising, tending to experiments, speaking with reporters and students on the ground, fixing the amazingly complicated space toilet (which has a reputation for frequent breakdowns), and so on.

But at any moment, and without warning, overseers like Michael Jensen, deputy chief of the training branch at Johnson Space Center, can flip a switch to send smoke spewing into the mock space station. Needless to say, if your response is to break a window, you’re not going to impress anyone. You need to don a gas mask and seek refuge elsewhere on the station. “You don't have time to sit there and think about it," Jensen says.

Astronauts have to be good at solving unexpected and often highly technical problems fast. But they also have to follow procedures reflexively and without deviation, as Nyberg did in the near-drowning mishap. You don’t want to share a spacecraft with people who improvise when they don’t need to.

Raised in a tiny hamlet in central Minnesota, Nyberg could hardly be more different from the swaggering test pilots — all male and mostly former military — who dominated the astronaut corps up through the Apollo era. As a girl, she dreamed of working as an astronaut, though she didn’t become a fighter pilot like Virts (who reckons that being a test pilot and the holder of a degree in applied mathematics made him only “about average” as an applicant). Instead, Nyberg became an expert in mechanical engineering, getting a bachelor’s degree with highest honors at the University of North Dakota before heading to the University of Texas at Austin, where she zipped through a master’s degree and a doctorate in the field in just four years. She landed her first job at NASA while still in school — and secured her first patent for robot-friendly space gear.