Story highlights Carpenter suffered complications after a stroke

John Glenn is the lone survivor of the Mercury Seven

"We have lost a true pioneer," says NASA administrator

Carpenter was the second American to orbit Earth

Astronaut Scott Carpenter, the second American to orbit Earth, died Thursday, NASA said. He was 88.

"We have lost a true pioneer. I shall long remember him not only for his smarts and courage but his incredible humor. He kept us all grounded," said NASA Administrator Charlie Bolden. "We will miss him greatly."

Carpenter was one of the original Mercury 7 astronauts chosen by NASA, which said he died from complications after a stroke. He was a backup pilot for John Glenn ahead of America's first manned orbital space flight in February 1962.

Carpenter flew the second American manned orbital flight in May of that year. Flight time was four hours and 54 minutes, according to a NASA biography.

Carpenter's spacecraft overshot its landing target by some 250 nautical miles, giving rise to fears about his fate.

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With Carpenter's death, Glenn is the lone survivor of the Mercury 7, which included Carpenter, Glenn, L. Gordon Cooper, Virgil "Gus" Grissom, Walter Schirra, Alan Shepard and Donald "Deke" Slayton.

Soviet cosmonaut Yuri Gagarin was the first person to orbit Earth in April 1961.

'Pioneer' of inner and outer space

Carpenter was born in Boulder, Colorado, and attended the University of Colorado, where he studied aeronautical engineering. He retired from the Navy in 1969, after some two decades of service.

Post-NASA, Carpenter explored underwater environments as an aquanaut in the Navy's Man in the Sea Project -- at one point living and working on the ocean floor for 30 days straight. He later served as director of the Navy's aquanaut operations.

"I still can't make up my mind whether I like outer or inner space better," Carpenter said last year -- adding with a smile, "But there's a difference in glory."

Carpenter's memoir, "For Spacious Skies: The Uncommon Journey of a Mercury Astronaut," was published in 2003. He also wrote two novels.

"He was one of the good guys and a good friend, a pioneer who made significant contributions to our country," said Dick Gordon, command module pilot for Apollo 12.