Space travel can be taxing, especially if one’s destination is the moon. For no one was this more true than Jim Irwin, depicted above, the lunar module pilot of Apollo 15. Like the other men to walk on the moon, he had to remain awake for 23 or more hours while carrying out all sorts of activities on the moon’s surface. But unlike the rest of those dozen men, Irwin almost did not make it back.

Irwin and his fellow Apollo 15 moonwalker David Scott were on moon’s surface for about 67 hours, with 19 of those hours spent outside the lunar module. The pair was the first to explore the moon’s surface in a motorized vehicle, later dubbed the “moon buggy,” and are credited with discovering and retrieving the Genesis Rock, a rock sample believed to be roughly 4 billion years old. (For perspective’s sake, the Solar System is believed to be roughly 4.6 billion years old.) For Irwin, the trip sparked another journey — one of religious introspection. As he’d say in his 1973 autobiography, his time on the moon awakened something inside him: “I sensed the beginning of some sort of deep change inside of me.”

But he also felt something else. During his time working on the lunar surface, Irwin’s heart developed an arrhythmia — a series of abnormal heart beats. The heart problems were serious. According to Wikipedia, Dr. Charles Berry, a flight surgeon monitoring the astronauts from back here on Earth told leadership at the Manned Space Center (MSC) that Irwin’s heart problems were “serious” — “If he were on earth, I’d have him in the ICU being treated for a heart attack.” But of course, the MSC could not do anything to assist Irwin.

That was the bad news. The good news is that the doctors in the MSC didn’t have to do much. Berry pondered the issue and came to a realization: “In truth…he’s in an ICU. He’s getting one hundred percent oxygen, he’s being continuously monitored, and best of all, he’s in zero g. Whatever strain his heart is under, well, we can’t do better than zero g.”

Irwin survived the voyage and the trip home; by the time the Apollo 15 crew splashed down back on Earth, his heart rhythm was normal. He suffered a heart attack a few months later but survived for another two decades. His heart condition did, finally, cause him to die on August 8, 1991, almost two decades from the day he left the moon. He was the first of the dozen men to walk on the moon to pass into the other great unknown.

Bonus fact : Not all health problems in space are as serious as Irwin’s was, of course. For example, astronauts can get itchy noses. And that’s a problem, because you can’t simply doff your helmet to scratch it. But don’t worry — NASA thought of that. According to Universe Daily, astronaut helmets come equipped with a tiny piece of Velcro, specifically to be used as a nose scratcher if the need so arises.

From the Archives: E.T.? No Going Home: Above, Dr. Berry extolled the ICU-like virtues of space. But note that he did not claim space to be a sterile environment. This story, perhaps, is why.