After reaching the Amundsen-Scott South Pole Station, a plane evacuating at least one of the station’s 48 workers in need of urgent medical care began the return trip back to South America early Wednesday.

The rescue plane arrived at the research station, operated by the National Science Foundation, on Tuesday following a nine-hour flight from Rothera, a British station on Adelaide Island. Before landing in South America to enable the worker to receive treatment, the evacuation plane will again stop in Rothera, allowing the crew to rest.

National Science Foundation spokesman Peter West said the person being evacuated worked for contractor Lockheed Martin. Another worker may also be on the rescue flight, but the National Science Foundation could not confirm the number of workers on board and declined to provide details about the medical condition of either individual.

Two planes flew into Rothera on Tuesday. While one continued 1,500 more miles to Amundsen-Scott South Pole Station, the other stayed behind in case it needed to assist with search-and-rescue efforts.

Because of the difficulties of flying to the isolated research station, only two other winter rescue missions — in 2001 and 2003 — have occurred since it opened almost 60 years ago.

The sun set in March at the research station, and it will not reappear until September, forcing the pilots to navigate in complete darkness. Planes do not usually fly to the station between February and October because of the challenging flight conditions. In winter, the South Pole’s average temperature is lower than -70 degrees Fahrenheit; on Wednesday, the temperature was -75 degrees Fahrenheit.

Kenn Borek, the Canadian firm contracted by the the National Science Foundation to carry out the rare rescue mission, operates the Twin Otter attempting the evacuation. The plane is uniquely prepared to travel in harsh conditions, with an ability to fly in temperatures as low as -75 degrees Celsius. This type of aircraft has been used in all three evacuation attempts, including the ongoing effort.

The rescue mission also required help from other agencies and organizations, according to the National Science Foundation.

“The evacuation will require contributions from multiple entities involved in the U.S. Antarctic Program including weather forecasts from the U.S. Navy’s Space and Naval Warfare Systems (SPAWAR) Center Atlantic; expertise from the University of Texas Medical Branch; and various contributions from ASC, NSF’s Colorado-based Antarctic logistics contractor as well as assistance from other nations,” the National Science Foundation said in a statement.

But rescues remain dangerous, even with thorough planning, according to Sean Louttit, who piloted the 2001 mission. “You’re the only plane flying on an entire continent,” he told the Washington Post. “You have to be prepared to be totally self-reliant if something goes wrong.”

To avoid medical evacuation, workers on the research station sometimes take extreme measures to treat themselves. In 1999, after Jerri Nielsen diagnosed herself with breast cancer, she administered her own chemotherapy for almost six months while continuing to serve as the station’s doctor.

The National Science Foundation has established safety measures to prevent a need for dangerous rescue efforts. It placed a small clinic at the research station, which frequently communicates with medical experts in the contiguous U.S., and also conducts testing before sending workers to the South Pole.

“We hope that people don’t develop such illnesses, and we put them through a pretty rigorous screening,” Kelly Falkner, the director of polar programs for the National Science Foundation, told NPR station WBUR. “But you can’t predict everything.”