PUNTA ARENAS, Chile — A weather window opened on Halloween morning, the typical stiff winds and polar fog relenting, and the flight to Antarctica was cleared for takeoff.

For nearly a week, Colin O’Brady, a 33-year-old American adventure athlete, and British Army Captain Louis Rudd, 49, had been waiting in Punta Arenas, Chile, on the Strait of Magellan, near the shattered end of the South American continent.

In separate buildings blocks away from one another, they had been immersed in similar tasks: weighing and re-bagging their freeze-dried provisions and sorting through polar-grade gear.

Their stashes included sleeping bags good for conditions up to minus 40 Fahrenheit, portable solar panels, cross-country skis, hand-held satellite phones and modems, and a GPS tracker programmed with way points to lead them step by frozen step across the highest, driest and by far the coldest continent on earth.