Federal officials said late Wednesday a small plane with two sick workers is headed from Antarctica to Chile, the latest leg in a daring rescue mission from a remote U.S. research station at the South Pole.

After making a stop for a few hours at a British station on the edge of Antarctica, the two ailing U.S. workers left for Chile on another plane. A spokeswoman for the British station said the two are expected to arrive in Chile for medical treatment Wednesday evening.

The rescue team left the U.S. Amundsen-Scott station at the South Pole on Wednesday morning for the 1,500-mile trip to the British post Rothera.

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Cold and dark usually prevent planes from flying to the polar outpost from February to October.

National Science Foundation spokesman Peter West would not reveal the patients' names or conditions.

Cold and dark usually prevent planes from flying to the polar outpost from February to October.

There have only been three medical evacuations from the station since 1999, reports CBS News' Dana Jacobson. Temperatures are often too cold for routine flights to and from the South Pole, which is why this rescue mission is so rare and dangerous.

Winter officially arrived on Antarctica this week, plunging the South Pole into perpetual darkness. The temperature reached minus 76 degrees Fahrenheit Tuesday.

This year 48 people planned to spend the winter at the Amundsen-Scott South Pole Station, including a doctor and physician's assistant, but with at least one person requiring outside treatment, rescue mission was deemed necessary.

A similar mission was needed in 2003 when scientist Barry McCue was airlifted so he could get emergency gallbladder surgery.

"They came up to me and said, 'You could die any day now,'" McCue recalled.

Another evacuation plane in 2001 flew through billowing winds and temperatures so cold they can freeze jet fuel and hydraulics -- conditions most aircraft just can't handle.

Sean Louttit was a pilot on both rescues.

"You don't really know what to expect until you open the door and you hit it face on," he said.

The current mission actually started last Tuesday, when two twin otter planes took off from Canada.

Both aircraft finally reached Antarctica Monday, but only one was sent Tuesday morning to make perhaps the most dangerous leg of the trip: the near 10-hour flight to the South Pole.

The rescue crew -- a pilot, co-pilot, engineer and medic -- arrived at the station around 5:20 p.m. ET. After a roughly 10-hour layover to give them rest, the evacuation began.