Leroy Gordon Cooper Jr. (March 6, 1927 – October 4, 2004) was an American aerospace engineer, test pilot, United States Air Force pilot, and the youngest of the seven original astronauts in Project Mercury, the first human space program of the United States. Cooper learned to fly as a child, and after service in the United States Marine Corps during World War II, he was commissioned into the United States Air Force in 1949. After service as a fighter pilot, he qualified as a test pilot in 1956, and was selected as an astronaut in 1959.

In 1963 Cooper piloted the longest and last Mercury spaceflight, Mercury-Atlas 9. During that 34-hour mission he became the first American to spend an entire day in space, the first to sleep in space, and the last American launched on an entirely solo orbital mission. Despite a series of severe equipment failures, he successfully completed the mission under manual control, guiding his spacecraft, which he named Faith 7, to a splashdown just 4 miles (6.4 km) ahead of the recovery ship. Cooper became the first astronaut to make a second orbital flight when he flew as command pilot of Gemini 5 in 1965. Along with pilot Pete Conrad, he set a new space endurance record by traveling 3,312,993 miles (5,331,745 km) in 190 hours and 56 minutes—just short of eight days—showing that astronauts could survive in space for the length of time necessary to go from the Earth to the Moon and back.

Cooper liked to race cars and boats, and entered the $28,000 Salton City 500 miles (800 km) boat race, and the Southwest Championship Drag Boat races in 1965, and the 1967 Orange Bowl Regatta with fire fighter Red Adair. In 1968, he entered the 24 Hours of Daytona, but NASA management ordered him to withdraw due to the dangers involved. After serving as backup commander of the Apollo 10 mission, he was superseded by Alan Shepard. He retired from NASA and the Air Force with the rank of colonel in 1970.

Leroy Gordon Cooper Jr. was born on March 6, 1927, in Shawnee, Oklahoma, the only child of Leroy Gordon Cooper Sr. and his wife, Hattie Lee née Herd. His mother was a school teacher. His father enlisted in the United States Navy during World War I, and served on the presidential yacht USS Mayflower. After the war, Cooper Sr. completed his high school education; Hattie Lee was one of his teachers, although she was only two years older than he. He joined the Oklahoma National Guard, flying a Curtiss JN-4 biplane, despite never having formal military pilot training. He graduated from college and law school, and became a state district judge. He was called to active duty during World War II, and served in the Pacific Theater in the Judge Advocate General's Corps. He transferred to United States Air Force (USAF) after it was formed in 1947, and was stationed at Hickam Air Force Base, Hawaii Territory. Cooper Sr. retired from the USAF with the rank of colonel in 1957.

Cooper attended Jefferson Elementary School and Shawnee High School, where he was on the football and track teams. During his senior year in high school, he played halfback on the football team that played in the state football championship. He was active in the Boy Scouts of America, where he achieved its second highest rank, Life Scout. His parents owned a Command-Aire 3C3 biplane, and he learned to fly at a young age. His father sat him on cushions so he could see and rigged the rudder pedals with blocks so he could reach them. He unofficially soloed when he was 12 years old, and earned his pilot certification in a Piper J-3 Cub when he was 16. His family moved to Murray, Kentucky, when his father was called back into service, and he graduated from Murray High School in June 1945.

After Cooper learned that the United States Army and Navy flying schools were not taking any more candidates, he enlisted in the United States Marine Corps. He left for Parris Island as soon as he graduated from high school, but World War II ended before he saw overseas service. He was assigned to the Naval Academy Preparatory School as an alternate for an appointment to the United States Naval Academy at Annapolis, Maryland, but the primary appointee was accepted, and Cooper was assigned to guard duty in Washington, D.C. He was serving with the Presidential Honor Guard when he was discharged from the Marine Corps in 1946.

Cooper went to Hawaii to live with his parents. He started attending the University of Hawaiʻi at Mānoa, and bought his own J-3 Cub. There he met his first wife, Trudy B. Olson (1927–1994) of Seattle, through the local flying club. She was active in flying, and would later become the only wife of a Mercury Seven astronaut to have a private pilot certification. They were married on August 29, 1947, in Honolulu, when both were 20 years old. They had two daughters.

At college, Cooper was active in the Reserve Officers' Training Corps (ROTC), which led to his being commissioned as a second lieutenant in the U.S. Army in June 1949. He was able to transfer his commission to the United States Air Force in September 1949. He received flight training at Perrin Air Force Base, Texas and Williams Air Force Base, Arizona, in the T-6 Texan.