Harrison Hagan "Jack" Schmitt (born July 3, 1935) is an American geologist, former NASA astronaut, university professor, and former U.S. senator from New Mexico. He is the most recent living person—and only person without a background in military aviation—to have walked on the Moon.

In December 1972, as a crewmember of Apollo 17, Schmitt became the first member of NASA's first scientist-astronaut group to fly in space. As Apollo 17 was the last of the Apollo missions, he became the twelfth and second-youngest person to walk on the Moon and the second-to-last person to step off the Moon (he boarded the Lunar Module shortly before commander Eugene Cernan). Schmitt remains the only professional scientist to have landed on the Moon. Before training for Apollo 17, he was influential in the geology field for supporting the Apollo program and had helped train Apollo astronauts chosen to visit the lunar surface.

Schmitt resigned from NASA in August 1975 to run for election to the United States Senate as a member from New Mexico. As the Republican candidate in the 1976 election, he defeated Democratic incumbent Joseph Montoya. In the 1982 election, Schmitt was defeated by Democrat Jeff Bingaman.

Born July 3, 1935, in Santa Rita, New Mexico, Schmitt grew up in nearby Silver City, and is a graduate of the Western High School (Class of 1953). He received a Bachelor of Science degree with special field in geology from the California Institute of Technology in 1957 and then spent a year studying geology at the University of Oslo in Norway, as a Fulbright Scholar. He received a Doctor of Philosophy in geology from Harvard University in 1964, based on his geological field studies in Norway.

Before joining NASA as a member of the first group of scientist-astronauts in June 1965, Schmitt worked at the U.S. Geological Survey's Astrogeology Center at Flagstaff, Arizona, developing geological field techniques that would be used by the Apollo crews. Following his selection, Schmitt spent his first year at Air Force UPT learning to become a jet pilot. Upon his return to the astronaut corps in Houston, he played a key role in training Apollo crews to be geologic observers when they were in lunar orbit and competent geologic field workers when they were on the lunar surface. After each of the landing missions, he participated in the examination and evaluation of the returned lunar samples and helped the crews with the scientific aspects of their mission reports.

Schmitt spent considerable time becoming proficient in the CSM and LM systems. In March 1970 he became the first of the scientist-astronauts to be assigned to space flight, joining Richard F. Gordon Jr. (Commander) and Vance Brand (Command Module Pilot) on the Apollo 15 backup crew. The flight rotation put these three in line to fly as prime crew on the third following mission, Apollo 18. When Apollo 18 and Apollo 19 were canceled in September 1970, the community of lunar geologists supporting Apollo felt so strongly about the need to land a professional geologist on the Moon, that they pressured NASA to reassign Schmitt to a remaining flight. As a result, Schmitt was assigned in August 1971 to fly on Apollo 17, replacing Joe Engle as Lunar Module Pilot. Schmitt landed on the Moon with commander Gene Cernan in December 1972.

Schmitt claims to have taken the photograph of the Earth known as The Blue Marble, one of the most widely distributed photographic images in existence. His Apollo 17 crewmates, Gene Cernan (Mission Commander) and Ronald Evans (Command Module Pilot), have made the same claim, and NASA's official position is to credit all three together.

"Perhaps the hardest thing to get used to on the Moon is that the sky is completely black. There's no blue at all."