A Twin Otter plane flies from the South Pole after a 2003 rescue mission. (Photo: Jason Medley, NSF)

The South Pole, this time of year, is in the height of winter. But that doesn’t mean operations at the Amundsen-Scott research station stop. Every year, dozens of scientists hunker down for months to keep the station running and do research.

But getting sick there can be deadly. That’s because, in the middle of winter, rescue operations can be very dangerous in the total darkness and extreme cold. 

Just two such operations have been launched before, one in 2001, and again two years later. But, earlier this week, the National Science Foundation, which provides funding for the station, said they were launching a third: up to two workers had fallen ill, and needed to be evacuated. 

The NSF did not give details on the workers’ conditions, but in the past, the agency has set a high bar for such flights, refusing, four years ago, for example, to retrieve one worker who had a stroke

That’s mostly because the flights are so dangerous. This year’s mission involves two planes, which took off Tuesday and hope to land at Amundsen-Scott on Sunday, weather permitting. One plane plans to pick up the workers, while the other will stay back in case it’s needed for a search-and-rescue mission. 

The planes, known as Twin Otters, are designed especially for cold-temperature flying, when the air is at its thinnest, the NSF said. 

“It’s a very serious decision that we take to move in this direction,” Kelly Falkner, the director of polar programs for the NSF, told the Washington Post. ”We try to balance our decisions with all of the risks involved.”