Scott Carpenter, one of the original members of the Mercury 7 - Nasa's first group of astronauts - has died aged 88, his family has announced.
In 1962 Mr Carpenter became the second American to orbit the earth, piloting the Aurora 7 spacecraft through three revolutions of the earth.
After retiring in 1969 he took up oceanographic activities.
John Glenn, who flew the first orbital mission, is the last surviving member of the Mercury team.
Scott Carpenter's wife, Patty Barrett, said her husband had suffered complications following a stroke in September and died in a Denver hospice.
He lived in Vail, Colorado.
Mr Carpenter, who was born in Boulder, Colorado, was commissioned in the US Navy in 1949 and served as a pilot during the Korean War.
In April 1959 he was selected as one of the original seven Mercury astronauts and underwent training with Nasa, specialising in communication and navigation.
He was the backup pilot for John Glenn during preparation for the first US manned orbital space flight in February 1962, and gave the historic send-off to his teammate: "Godspeed, John Glenn.''
During his own flight, Scott Carpenter's capsule landed 288 miles away from where it was meant to, leaving Nasa and the nation waiting anxiously to see if he had survived.
The Navy recovered him from the Caribbean, floating in his life-raft with his feet propped up.
In a joint lecture with John Glenn 49 years later at the Smithsonian Institution, Mr Carpenter recalled his feelings from that time.
"You're looking out at a totally black sky, seeing an altimeter reading of 90,000ft and realise you are going straight up. And the thought crossed my mind: What am I doing?''
Scott Carpenter did not go back into space but later joined the US Navy's SeaLab II programme and in 1965 spent 30 days under the ocean off the coast of California.
After retirement he founded his company Sea Sciences, working closely with diver and researcher Jacques Cousteau.