Chris Chandler and Bob Cormack summited Everest in October 1976 as part of the American Bicentennial Everest Expedition that year, the first Americans to make an autumn ascent of Mount Everest according to the "Los Angeles Times". By the 21st century, summer and autumn can be more popular with skiing and snowboard attempts on Mount Everest. During the 1980s, climbing in autumn was actually more popular than in spring. U.S. astronaut Karl Gordon Henize died in October 1993 on an autumn expedition, conducting an experiment on radiation. The amount of background radiation increases with higher altitudes.