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Walter Schirra Jr., one of the original Mercury Seven astronauts, poses for a studio portrait in this June 1962 file photo. ((Associated Press))

Walter "Wally" Schirra Jr., one of the original Mercury 7 astronauts and the only one to fly on all three of NASA's early space programs, has died at the age of 84, NASA said Thursday.

Schirra's family said he died of natural causes, according to NASA spokesman James Hartsfield.

Schirra flew on three spaceflights: a six-orbit flight around the Earth aboard Mercury 8 in October 1962; a flight with Thomas Stafford in December 1965 aboard Gemini 6; anda flight aboard Apollo 7 in 1968, the first successful Apollo mission.

"With the passing of Wally Schirra, we at NASA note with sorrow the loss of yet another of the pioneers of human spaceflight," NASA administrator Michael Griffin said.

Schirra's Mercury flight lasted nine hours, 15 minutes and reached speeds of 28,255km/h at an altitude of 281 kilometres.

He commanded theGemini 6 flight, which had the distinction of being the first rendezvous between two manned, maneuverable spacecraft. For five hours, Gemini 6 and 7 flew in formation as close as one foot to one another.

The Apollo 7 mission which he commandedwas the first successful human flight using an Apollo spacecraft, and was a major success for NASA after a fire on the launch pad had killed the crew of Apollo 1 almost two years earlier.

He is the only astronaut to have flown missions for the Mercury, Gemini and Apollo programs.

Schirra was one of seven astronauts introduced by NASA on April 9, 1959, just six months after the agency was formed. Known as the Mercury 7 or Original 7, they became the faces of space travel for a generation and included John Glenn, Alan Shepard, Virgil "Gus" Grissom, Donald "Deke" Slayton, Gordon Cooper and Scott Carpenter.

Born in Hackensack, N.J., on March 12, 1923, Schirra received a bachelor of science from the U.S. Naval Academy and graduated from the U.S. Naval Test pilot school before joining NASA.

He retired from the Navy and NASA in 1969 and became a space exploration commentator on CBS.

"Mostly it's lousy out there," Schirra said in 1981 on the occasion of the first space shuttle flight. "It's a hostile environment, and it's trying to kill you. The outside temperature goes from a minus 450 degrees to a plus 300 degrees. You sit in a flying Thermos bottle."

When asked what it was like to view Earth from spaceearlier this year, Schirra replied: "I left Earth three times. I found no place else to go. Please take care of Spaceship Earth."

With files from the Associated Press