John Young, NASA’s longest-serving astronaut, who flew in space six times, walked on the Moon, commanded the first space shuttle, and became the conscience of the astronaut corps, advocating for safety reforms in the wake of the 1986 Challenger disaster, died following complications from pneumonia on January 5, 2018, at his home in Houston. He was 87.

John Watts Young was born on September 24, 1930, in San Francisco, California. When he was 18 months old, Young’s parents moved, first to Georgia and then Orlando, Florida, where he attended elementary and high school. Young earned his bachelor of science degree in aeronautical engineering from the Georgia Institute of Technology in 1952. After graduation, he entered the U.S. Navy, serving on the destroyer USS Laws in the Korean War and then entering flight training before being assigned to a fighter squadron for four years.

Young graduated from the U.S. Navy Test Pilot School in 1959 and served at the Naval Air Test Center at Naval Air Station Patuxent River in Maryland, where he evaluated Crusader and Phantom fighter weapons systems. In 1962, he set world time-to-climb records to 3000- and 25,000-meter (82,021- and 9,843-feet) altitudes in the F-4 Phantom. Young retired from the U.S. Navy with the rank of captain in 1976. Over the course of his flying career, he logged more than 15,275 hours in props, jets, helicopters, and rocket jets, including more than 9200 hours in NASA’s T-38 astronaut training jets.

Selected alongside Neil Armstrong and Jim Lovell with NASA’s second group of astronauts in 1962, Young flew two Gemini missions, two Apollo missions, and two space shuttle missions. He was one of only three astronauts to launch to the Moon twice and was the ninth person to set foot on the lunar surface. In total, Young logged 34 days, 19 hours, and 39 minutes flying in space, including 20 hours and 14 minutes walking on the Moon.

Young made the first of his six missions as the pilot on the maiden flight of Gemini, NASA’s two-seater spacecraft. Flying with original Mercury astronaut Virgil “Gus” Grissom, Young launched on the nearly five-hour Gemini 3 mission on March 23, 1965, putting the new vehicle through its paces while also taking a bite or two from a later infamous corned beef sandwich that he smuggled onboard the flight. Young commanded his second spaceflight, Gemini 10, in July 1966. The three-day mission climbed to more than 760 kilometers (400 miles) above Earth to measure the risk posed by radiation, conducted the program’s first double rendezvous with two Agena target vehicles, and included two spacewalks by pilot Michael Collins.

On the Apollo 10 mission in May 1969, Young became the first person to orbit the Moon alone. During the flight, which was a full-up dress rehearsal for the first lunar landing two months later, Young remained onboard the command module “Charlie Brown” while his crewmates, Thomas Stafford and Eugene Cernan, flew “Snoopy,” the Apollo 10 lunar module, to within 14 kilometers (47,000 feet) of the Moon’s surface. On their return to Earth, Young, Stafford, and Cernan set a record for the highest speed achieved by astronauts onboard a spacecraft: 39,897 kilometers per hour (24,791 miles per hour).

Young got his chance to walk on the Moon in April 1972, as commander of Apollo 16, the fifth and penultimate Apollo lunar landing. Young and Charles Duke landed the “Orion” lunar module in the Descartes highlands for a nearly three-day stay. “There you are, mysterious and unknown Descartes highland plains,” described Young, as he took his first steps on the Moon. Exhibiting his dry wit, Young then compared his situation to a Joel Chandler Harris story, adapted for the Disney movie “Song of the South,” to express how fortunate he felt to be on the moon. “I’m sure glad they got ol’ Br’er Rabbit here,” he remarked, “back in the briar patch where he belongs.”

Over the course of three excursions across the boulder-strewn surface, Young and Duke explored more than 26 kilometers (16 miles), becoming the second crew to drive a lunar rover. As they went, they collected 96 kilograms (211 pounds) of Moon rocks and lunar soil, which they brought back to Earth with Apollo 16 command module pilot Thomas “Ken” Mattingly. During their first moonwalk, Young and Duke received word from Mission Control that the U.S. Congress had approved the funding to develop the space shuttle. “The country needs that shuttle mighty bad,” Young said in response. “You’ll see.”

Although he had no way of knowing it at the time, Young would next make history commanding the first flight of the space shuttle nine years later, almost to the day. Young and Robert Crippen launched on space shuttle Columbia on April 12, 1981. Because of the way the orbiter had been designed, it could not be tested in space without a crew. For two days and six hours, Young and Crippen tested Columbia’s systems before returning to Earth like no other orbital spacecraft had done before — with wings, gliding to a touchdown on the dry lake bed at Edwards Air Force Base in southern California.

Young’s then-record sixth space mission returned him to the commander’s seat on board Columbia for the orbiter’s sixth mission in November 1983. This time, Young led a crew of five, including the first international astronaut to fly on the shuttle, Ulf Merbold of the European Space Agency (ESA). STS-9 also marked the the first flight of the European-built Spacelab laboratory, a pressurized module that was mounted inside the orbiter’s payload bay. The 10-day mission carried out 72 experiments in astronomy, astrobiology, material sciences, and Earth observation. On December 8, 1983, Columbia made a pre-dawn landing at Edwards, returning Young to Earth for the last time.

In addition to his own six spaceflights, Young also served on five backup crews, including backup pilot for Gemini 6; backup command module pilot for the second Apollo mission (as slated before the Apollo 1 fire) and Apollo 7, the first crewed Apollo launch; and backup commander for Apollo 13 and Apollo 17. In 1974, Young was named the fifth chief of the Astronaut Office, after serving for a year as the office’s space shuttle branch chief. For 13 years, Young led NASA’s astronaut corps, overseeing the crews assigned to the Apollo-Soyuz Test Project, the approach and landing tests with the prototype orbiter Enterprise, and the first 25 space shuttle missions.

After the loss of space shuttle Challenger and its seven-person crew in January 1986, Young penned internal memos critical of NASA’s attention to safety, a topic he had championed since his days flying Gemini. Young expressed concern over schedule pressure and wrote that other astronauts who had launched on missions preceding the ill-fated STS-51L mission were “very lucky” to be alive. Young was subsequently reassigned to be special assistant to the director of the Johnson Space Center for engineering, operations, and safety until 1996, when he was named the associate director for technical affairs, a position he held until his retirement from NASA on December 31, 2004.

Young was the recipient of many honors for his contributions to space exploration, including the Congressional Space Medal of Honor, NASA Distinguished Service Medal, Rotary National Space Achievement Award, and six honorary doctorates. Young was inducted into the National Aviation Hall of Fame in 1988 and Astronaut Hall of Fame in 1993. He was awarded the NASA Ambassador of Exploration in 2005, including a Moon rock he assigned for display at the Houston Museum of Natural Science, and was bestowed the General James E. Hill Lifetime Space Achievement Award from the Space Foundation in 2010. A stretch of Florida State Road 423 that runs through Orlando is named John Young Parkway in his honor.

“NASA and the world have lost a pioneer,” said NASA acting administrator Robert Lightfoot in a statement. “John Young’s storied career spanned three generations of spaceflight; we will stand on his shoulders as we look toward the next human frontier.” Continued Lightfoot, “John was one of that group of early space pioneers whose bravery and commitment sparked our nation’s first great achievements in space. But, not content with that, his hands-on contributions continued long after the last of his six spaceflights — a world record at the time of his retirement from the cockpit. Between his service in the U.S. Navy, where he retired at the rank of captain, and his later work as a civilian at NASA, John spent his entire life in service to our country. On STS-9, his final spaceflight, and in an iconic display of test pilot ‘cool,’ he landed the space shuttle with a fire in the back end. John Young was at the forefront of human space exploration with his poise, talent, and tenacity. He was in every way the ‘astronaut’s astronaut.’ We will miss him.”