Astronaut Bruce McCandless II during a spacewalk in 1984. (AP)

Bruce McCandless II, a NASA astronaut who was the first person to fly freely and untethered in space during a 1984 space-shuttle mission, died Dec. 21 in California. He was 80.

His death was announced by NASA, but details about the place and cause of death were not disclosed.

Mr. McCandless was photographed floating in space with a hefty spacewalker's jet pack, alone in the cosmic blackness above a blue Earth. He traveled more than 300 feet away from the space shuttle Challenger during the spacewalk.

"The iconic photo of Bruce soaring effortlessly in space has inspired generations of Americans to believe that there is no limit to the human potential," Sen. John McCain said in a statement. The Arizona Republican and Mr. McCandless were classmates at the U.S. Naval Academy.

McCandless said he wasn't nervous about the historic spacewalk.

"I was grossly overtrained," he told the Daily Camera of Boulder, Colo., in 2006. "I was just anxious to get out there and fly. I felt very comfortable . . . It got so cold my teeth were chattering and I was shivering, but that was a very minor thing."

Astronaut Bruce McCandless II in 1982. (AP)

During that flight, Mr. McCandless and fellow astronaut Robert L. Stewart pioneered the use of NASA's backpack device that allowed astronauts walking in space to propel themselves from the shuttle. Stewart became the second person to fly untethered two hours after Mr. McCandless.

"I'd been told of the quiet vacuum you experience in space, but with three radio links saying, 'How's your oxygen holding out?' 'Stay away from the engines!' 'When's my turn?' it wasn't that peaceful," Mr. McCandless wrote in the Guardian in 2015.

"It was a wonderful feeling," he added, "a mix of personal elation and professional pride: It had taken many years to get to that point."

Mr. McCandless was later part of the 1990 shuttle crew that delivered the Hubble Space Telescope to orbit. Previously, he served as the NASA mission control capsule communicator in Houston as Neil Armstrong and Buzz Aldrin walked on the moon in 1969.

During his spacewalk, Mr. McCandless wrote, "My wife was at mission control, and there was quite a bit of apprehension. I wanted to say something similar to Neil when he landed on the moon, so I said, 'It may have been a small step for Neil, but it's a heck of a big leap for me.' That loosened the tension a bit."

Bruce McCandless II was born June 8, 1937, in Boston and graduated from high school in Long Beach, Calif. His father was a Navy rear admiral who received the Medal of Honor during World War II.

Mr. McCandless graduated from the Naval Academy in 1958. He received a master's degree in electrical engineering from Stanford University in 1965 and a second master's degree, in business administration, from the University of Houston at Clear Lake in 1987.

He was a naval aviator who participated in the Cuban blockade in the 1962 missile crisis. Mr. McCandless was selected for astronaut training in 1966 during the Gemini program and was a backup pilot for the first manned Skylab mission in 1973. After leaving NASA, he worked for Lockheed Martin Space Systems in Colorado.

His first wife, the former Bernice Doyle, died in 2014 after 53 years of marriage.

Survivors include his wife, ­Ellen Shields McCandless of Conifer, Colo.; two children from his first marriage; a brother; two sisters; and two granddaughters.