Legendary astronaut John Young, who walked on the moon and later commanded the first space shuttle flight, has died aged 87.
Space agency Nasa said he died on Friday at home in Houston following complications from pneumonia.
It said Mr Young was one of its pioneers - the only agency astronaut to go into space as part of the Gemini, Apollo and space shuttle programmes, and the first to fly into space six times. He was the ninth man to walk on the moon.
"Astronaut John Young's storied career spanned three generations of spaceflight," acting Nasa administrator Robert Lightfoot said.
"John was one of that group of early space pioneers whose bravery and commitment sparked our nation's first great achievements in space."
Mr Young became the first person to rocket away from Earth six times. Counting his takeoff from the moon in 1972 as commander of Apollo 16, his blastoff tally stood at seven, for decades a world record.
He flew twice during the two-man Gemini missions of the mid-1960s, twice to the moon during the Apollo programme, and twice more aboard the new space shuttle Columbia in the early 1980s.
His Nasa career lasted 42 years, longer than any other astronaut's, and he was revered among his peers for his dogged dedication to keeping crews safe - and his outspokenness in challenging the space agency's status quo.
Chastened by the 1967 Apollo launch pad fire that killed three astronauts, Mr Young spoke up after the 1986 Challenger launch accident. His hard scrutiny continued well past shuttle Columbia's disintegration during re-entry in 2003.
"Whenever and wherever I found a potential safety issue, I always did my utmost to make some noise about it, by memo or whatever means might best bring attention to it," he wrote in his 2012 memoir, Forever Young.
Mr Young remained an active astronaut into his early 70s and held on to his role as Nasa's conscience until his retirement in 2004.
"You don't want to be politically correct," he said in a 2000 interview. "You want to be right."
Mr Young was in Nasa's second astronaut class, chosen in 1962, along with the likes of Neil Armstrong, Pete Conrad and James Lovell.
He was the first of his group to fly in space: He and Mercury astronaut Gus Grissom made the first manned Gemini mission in 1965.
Unknown to Nasa, Mr Young smuggled a corned beef sandwich on board. The ensuing scandal over two silly minutes of an otherwise triumphant five-hour flight always amazed him.
Sandwiches had already flown in space, he said in his book, but Nasa brass and Congress considered this a multimillion-dollar embarrassment and outlawed corned beef sandwiches in space forever.
Two years later, with Gemini over and Apollo looming, Mr Young asked Mr Grissom why he did not say something about the bad wiring in the new Apollo 1 spacecraft.
He feared it would get him fired, Mr Young said. A few weeks later in 1967 those wires contributed to the fire that killed Mr Grissom, Edward White II and Roger Chaffee in a countdown practice on their Cape Canaveral launch pad.
It was the safety measures put in place after the fire that got 12 men, Mr Young included, safely to the moon and back.
"I can assure you if we had not had that fire and rebuilt the command module ... we could not have done the Apollo programme successfully," he said in 2007.
"So we owe a lot to Gus, and Rog and Ed. They made it possible for the rest of us to do the almost impossible."
Young orbited the moon on Apollo 10 in May 1969 in preparation for the Apollo 11 landing that was to follow. He commanded Apollo 16 three years later, the next-to-last manned lunar voyage, and walked on the moon.
He hung on for the space shuttle, commanding Columbia's successful maiden voyage in 1981 with co-pilot Robert Crippen by his side.
Mr Crippen called flying with Mr Young "a real treat". "Anybody who ever flew in space admired John," he said.
Mr Young made his final trek into orbit aboard Columbia two years later, again as its skipper. His reputation continued to grow and he spoke out on safety measures, even before the Challenger debacle.
"By whatever management methods it takes, we must make Flight Safety first. If we do not consider Flight Safety first all the time at all levels of Nasa, this machinery and this programme will NOT make it," he warned colleagues.
As then chief of the astronaut corps, Mr Young was flying a shuttle training aircraft above Kennedy Space Centre when Challenger ruptured. He took pictures of the nose-diving crew cabin.
Throughout the 1990s and into the 2000s, Mr Young maintained Nasa should be developing massive rockets to lift payloads to the moon to industrialise it, he said, and building space systems for detecting and deflecting comets or asteroids that could threaten Earth.
"The country needs it. The world needs it. Civilisation needs it," he said in 2000, adding "I don't need it. I'm not going to be here that long."
Mr Young spent his last 17 years at Johnson Space Centre in Houston in management, focusing on safety issues. He retired at the end of 2004.
He was born in 1930 and grew up in Orlando, Florida. He became interested early on in aviation and earned an aeronautical engineering degree from Georgia Institute of Technology in 1952.
He went on to join the Navy and serve in Korea. He eventually became a Navy fighter and test pilot.
Mr Young received more than 100 major accolades in his lifetime, including the prestigious Congressional Space Medal of Honour in 1981.
Former president George HW Bush said : "John was more than a good friend; he was a fearless patriot whose courage and commitment to duty helped our nation push back the horizon of discovery at a critical time.
"To us, he represented the best in the American spirit - always looking forward, always reaching higher."
He added: "John leaves a tremendous legacy of accomplishment, in addition to his wonderful family. May his memory serve to inspire future generations of explorers to dare greatly, act boldly and serve selflessly."