Astronaut John Young, a NASA legend who went into space six times and was on the first Gemini mission, died at age 87 late Friday after a bout with pneumonia.

Young, who was one of only 12 men to walk on the moon, was the first to fly into space six times, and he also led the first shuttle flight. In a statement, NASA Administrator Robert Lightfoot declared that the U.S. had lost a true "pioneer" of space exploration.

"Today, NASA and the world have lost a pioneer," Lightfoot said Saturday. "Astronaut John Young's storied career spanned three generations of spaceflight; we will stand on his shoulders as we look toward the next human frontier."

Throughout his 42-year career at NASA, Young was the only astronaut to fly aboard the Gemini and Apollo capsules and aboard a space shuttle, doing so two times on each. His sixth and final time in space was in 1983 aboard the Columbia space shuttle – the ninth shuttle mission overall. It carried a European Spacelab module into orbit that served as a proving ground for the type of research carried out today on the International Space Station.

He ultimately became became the chief of the astronaut office from 1974-1987 and was responsible for picking and choosing the astronauts for shuttle missions.

He was preparing for his seventh trip to launch the Hubble Telescope into space when the Challenger shuttle exploded and delayed the telescope's launch. Young was later forced out as chief of the office after criticizing the agency after the Challenger disaster. However, he remained at NASA until he retired in 2004.

Soon after his death, Young was lauded by many as a leader of space exploration and a patriot to emulate.

"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," said former President George H.W. Bush said in a statement. "To us, he represented the best in the American spirit – always looking forward, always reaching higher."

"May his memory serve to inspire future generations of explorers to dare greatly, act boldly, and serve selflessly," Bush added.