Last week, after 70 days on the Antarctic ice and more than 900 miles of slogging though the snow, Henry Worsley called for help: He could no longer “slide one ski in front of the other.” The seasoned explorer, who had previously completed several extended Antarctic treks, was dehydrated and malnourished. An airlift took him to Union Glacier base camp, reports the BBC, where he was diagnosed with bacterial peritonitis, an infection of the abdominal lining. He made it to Chile, but it was too late. He died on January 24.

He was just 30 miles short of his goal to make the first solo, unassisted, unsupported crossing of the Antarctic continent.

Worsley might as well have been alone on Mars.

Worsley was attempting to complete Ernest Shackleton’s attempt on the South Pole, one the early 20th-century explorer abandoned about 100 miles short of completion. Plenty of people have made it to the South Pole by now, of course. But the less help you have the more difficult it is to get there.

The extremely badass Felicity Aston skied alone across the continent in 2012, helped by two food supply drops. Others have made the crossing assisted by kite sleds or other power sources. Cecilie Skog made the crossing with a buddy.

But Worsley was doing it the hard way—with no outside inputs at all. And when someone who has the benefit of Iridium satphones and solar-powered battery chargers is doing it the hard way, you know it’s really really hard. Worsley was a perfectly closed system progressing across the underbelly of the planet. He might as well have been alone on Mars.

And there’s a reason that no one has made it this way before: It’s a project that’s almost thermodynamically impossible.

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Body Over Mind

Survival, ultimately, is about having enough heat, enough water, and enough food. And when it comes to surviving in Antarctica, you’d naturally think heat is the biggie. Hypothermia and frostbite are a constant threat in any kind of polar undertaking. But freezing to death isn't actually the biggest risk. “Hypothermia is not normal in people who go to very cold places, provided they are reasonably well-protected," says Mike Tibton, a physiologist in the Extreme Environments Laboratory at the University of Portsmouth. You can get something called non-freezing cold injury, which damages nerves and vasculature, causing chronic pain and cold sensitivity. Cold sure does make life difficult—but it doesn’t make life impossible.

Then there’s water. It’s all around, but it’s bloody hard to drink in Antarctica. Because remember, Antarctica is a desert, and a high-altitude desert at that. Worsley spent part of his journey climbing the Titan Dome, which rises above 10,000 feet. Every minute, you inhale dry-as-bones air, humidify it with moisture in your lungs, and expel that water into the environment. Plus you’re sweating. Tibton estimates that you’d need to drink about six liters a day—which, by the way, you can’t just scoop up from the snow as you walk along. Every drop you drink, you have to gather and melt over your little stove. “Something that for you and I would take 15 seconds, takes 30 to 45 minutes,” says Tibton.

But food, it turns out, is the sticking point. Because it’s straight-up impossible to take enough calories with you to get across the continent of Antarctica. “You get into a vicious spiral of, 'OK, I want to take more food, that means I've got to have more weight, that means I've got to drag more weight, that means my energy expenditure goes up,’” says Tibton. Polar explorers skiing or trudging across the ice, pulling sleds, and keeping their body temperatures up expend around 10,000 calories a day. Ten thousand! That’s 15 In-N-Out Double-Doubles. “I don't think anybody has taken sufficient food with them to remain in energy balance,” says Tibton. “They're drawing on the energy inside them that's stored as fat.”

Robert Falcon Scott and Roald Amundsen and other early explorers got around the problem of food by setting up huge depots or caches of food on the path to their destination in advance. They had another source of calories as well, since they used dogs to pull their sleds. To get to the pole, those guys added up the life forces and energy outputs of dozens of animals and consumed them one by one until they were marching the last miles back to camp and drawing down their own energetic reserves.

Worsley, who said he fully expected to lose weight on his journey, had no such calorie trove beyond what he could carry on the sled he was pulling. In one of his later daily audio diaries, Worsley ruefully alludes to his energy balance, talking about how hungry he is, how much he thinks about food, how skinny his legs now are. "The nutritional aspect of a long journey like this is key," he said, noting that he had based his expected intake on previous missions, "and to be honest I don’t think I paid it as much detail as I should have done."

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Mind Over Body

Worsley was also alone, and that’s its own kind of danger. John Leach, a survival psychologist (and also at the Extreme Environments Laboratory at the University of Portsmouth), tells a story of his training in the armed forces as a survival officer. He was in the desert, he says, and his buddy asked him if he had been drinking water. “I was convinced I had,” says Leach, but his buddy insisted he check his canteen. He hadn’t touched it. "Humans are notoriously bad at monitoring our own condition,” he says. It’s all too easy to get a little bit dehydrated, not realize it, and fall into a slow downward spiral. “If you’re on your own, you’re you’re only reference point. If your reference point starts to drift, you drift with it.”

This is not to say that Worsley himself was drifting, but he did discover that there was another danger to being alone: He notes in his messages that his energy outputs were higher than he had expected—he has to break track through the soft snow and struggle over jagged sastrugi 100 percent of the time instead of switching off the lead position with a partner.

Still, Worsley should have survived. His equation of inputs and outputs was running a severe deficit perhaps—but in the end, germs were Worsley’s undoing. One of the things that happens when you’re starving or dehydrated is that the lining of your gut atrophies. “The bacterial flora in the gut just translocate across,” says Claude Anthony Piantadosi, a professor of medicine at Duke and the author of two books about human exploration and survival. Then you get infection and sepsis. (Physical trauma can cause this too. And heatstroke.)

And when that happens, says Piantadosi, it happens fast. "If you don’t get treated in the first six hours," he says, "Your mortality goes way way up."

Tales of polar exploration are tales of nearly supra-human fortitude. Sometimes there’s no way to call for help. Sometimes you can, but you hesitate. In the 1930s, Richard Byrd overwintered solo on an Antarctic base to gather weather data. He was being poisoned by carbon monoxide but didn't initially call for rescue. Explorer types, by definition, aren't the sorts to hit the eject button at the first (or second, or third) sign of trouble, unwilling to risk the lives of their rescuers in the interest of saving their own.

The risk to the rescuers in Worsley’s case was relatively low (if anything that happens on Antarctica can be said to be low risk). We have helicopters and planes, after all. But the hesitation to call is certainly just as great today as it has ever been in the history of polar exploration. Even so, I wish Worsley had called sooner.