Jul.20.2017 / 12:50 AM ET

This is chapter three of a four-chapter story.

The glamorous parts of spaceflight ascending skyward on a pillar of fire, floating gracefully against a backdrop of stars are in some ways the easiest on the astronauts minds and bodies, as long as nothing goes wrong. As NASA eyes the long-term future of human space exploration and missions to Mars, medical and psychological challenges are among those that loom largest.

When Scott Kelly (#391) returned in 2016 from a nearly one-year-long stay on the space station, he had to be carried from his landing capsule (as do most ISS astronauts) and spend more than a month in rehabilitation to regain his strength. Freed from their constant defiance of gravity, muscles atrophy and bones weaken during spaceflight. We dont yet fully understand how to keep astronauts fit, both mentally and physically, on journeys that could last two years or more and require them to do heavy lifting on another planet. To help answer those questions, scientists have been studying Kelly since he got back and comparing his health to that of his twin brother, Mark (#409), who stayed on Earth.

NASA researchers are also going to rely heavily on astronauts who have medical training people like Kjell Lindgren, who joined the corps in 2009 after earning his M.D. at the University of Colorado and then completing a residency in emergency medicine, a postdoc fellowship, another residency in space medicine, and a masters degree in public health. Lindgren then worked for NASA as a ground-based crew surgeon for both Space Shuttle and ISS missions. At that point, he seemed pretty qualified.

The 50-person panel of mostly current astronauts who review applicants no doubt noted that Lindgren, who was born in Taiwan but raised mostly in England, was a champion parachutist at the U.S. Air Force Academy. It takes a certain daredevil spirit to be an astronaut because its one of the few jobs whose holders know, with 100 percent certainty, that every mission will do them harm. Beyond the ever-present risk of some acute disaster, the toll of microgravity and radiation on astronauts in space is very real. Whether the damage is reversible upon return to Earth or can be prevented by new technologies is an important question that scientists, including spacefaring ones like Lindgren, have to answer before NASA sends anyone to Mars.

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Like all newly enrolled astronaut candidates, Lindgren completed two years of basic training before he earned the job title of astronaut. As part of that regimen, every candidate must become a pilot in a supersonic T-38 jet trainer or at least qualify as a navigator in the T-38s second seat. Lindgren already had his wings, but many candidates are relative newcomers to aviation.

Does it seem reckless for NASA to put astronaut candidates into a cramped cockpit, surrounded by gauges and switches, and ask them to do risky high-speed maneuvers? Computer simulators are now quite realistic, after all Johnson Space Center has one of the best, which astronauts use to practice docking and robotic maneuvers. And T-38 training has actually claimed the lives of several astronauts, though none in recent years. Nevertheless, NASA considers the flights to be indispensable practice at making life-or-death decisions without hesitation.

Its one thing to screw up in a ground-based simulator, Virts says, quite another to do so in a T-38 flying at 40,000 feet. Make a mistake while flying at 500 miles an hour, and your ability to remain alive will be sorely compromised.

After completing basic training, Lindgren spent two years working at ground assignments while waiting to be given a mission in space. Part of that time was spent as capsule communicator, or CAPCOM, sitting in the control room at Johnson Space Center and talking to whomever was up in orbit. As a doctor, Lindgren is able to pick up on signs of the psychological issues that astronauts can develop: depression, frustration at the lack of downtime, and third-quarter phenomenon a drop in motivation when the excitement of a mission has worn off but its end is not yet in sight.

Finally, in 2013, Lindgren got called off the bench and into the game. He would be going to the ISS for a 141-day mission. All he had to do was complete two more years of mission-specific training to get ready for the launch in 2015. In his case, that meant studying up on more than 100 different science experiments that he and his crewmates would be running, plus rehearsing two spacewalks that he would take with Kelly.

For someone who sailed through many years of medical studies, that probably didnt seem too onerous. Astronauts know that for every day they spend in space they will likely spend many more on the ground polishing their skills and doing public outreach, helping plan or run other astronauts missions, and whatever else NASA asks them to do.

PREVIOUS CHAPTER: Saving a Spaceman from Drowning

NEXT CHAPTER: For Astronauts, Crazy Risks Come with the Job

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Inner Strength for Outer Space - NBCNews.com