Neil Armstrong, 81, became the first man to walk on the moon on July 20, 1969. After setting foot on the moon's surface, he famously stated, “That's one small step for [a] man, one giant leap for mankind.” Armstrong was the commander of the moon landing mission. Known as Apollo 11, the mission completed a national goal set eight years earlier by President John F. Kennedy. In addition to the moon milestone, Armstrong performed the first successful docking of two vehicles in space. Before becoming an astronaut, Armstrong served as an engineer and aircraft research pilot.

Buzz Aldrin, 81, was the second man to walk on the moon, joining Neil Armstrong about 20 minutes after Armstrong took his first steps. Aldrin was the lunar module pilot for the Apollo 11 mission. Before joining the U.S. space agency, he flew fighter jets as a member of the Air Force during the Korean War. NASA says Aldrin has remained at the forefront of efforts to ensure the U.S. continues to hold a leading role in manned space exploration.

Michael Collins, 81, was the command module pilot for Apollo 11. He remained aboard the command module as Neil Armstrong and Buzz Aldrin descended to the moon's surface in their lunar module. Earlier in his career, Collins served as a flight test officer for the Air Force, primarily working with jet fighters. After leaving NASA, he served for a while as the director of the National Air and Space Museum in Washington.

John Glenn, 90, became the first American to orbit the Earth in 1962, completing a successful three-orbit mission. He also became the oldest person to fly in space when at the age of 77-years-old he flew aboard the space shuttle Discovery in 1998 to study the effects of space flight on the elderly. He served in both World War II and the Korean war as a pilot and later held the position of U.S. senator for nearly 25 years. He is also credited with setting a speed record for flying across the U.S., an accomplishment he achieved in 1957 as a test pilot.