Apollo 15 marked the fourth time we landed on the Moon. The two astronauts walking on the lunar surface were David Scott and James Irwin, and all seemed well, right up until mission control looked at Irwin’s vitals and realized he’d just had a heart attack.
Well, calling it a heart attack, which is a nonscientific term, is a misnomer. What he had was a kind of arrhythmia, where the heart alternates between beating too fast and skipping a beat (“bigeminy”). These are symptoms that would generally make a doctor conclude the something had messed with blood flow in the coronary arteries. At no point did Irwin clutch his chest and gasp in pain, but many people who have heart attacks never do that.
NASA did not inform anyone in the crew about what they’d noticed. Irwin’s heartbeat returned to normal, and the mission continued as planned. Two years later, he had his next heart attack, a major one. He had a few other heart attacks during his remaining years, and a final attack killed him at 61, making him the first and youngest moonwalker to die.