While each shuttle astronaut has made unique contributions to America's space program, a few space travelers especially stand out. Here's a look at seven of the brave men and women who've ridden to space on the shuttles:
After flying on the inaugural shuttle flight, the STS-1 mission of Columbia in 1981, Young went on to command another space shuttle mission, the STS-9 flight in 1983, which carried the first Spacelab research module.
When he retired in 2004, he had spent a total of 34 days in space.
After the first shuttle flight, Crippen commanded the shuttle on three subsequent missions in the 1980s. He presided over the first five-person crew, STS-7, flew with the first female American astronaut in space, Sally Ride, on STS-41-C, and commanded the first seven-person crew on STS-41-G.
Overall, Crippen spent 23 days in space over the course of his four shuttle missions.
Ride found her way to the shuttle by being one of 8,000 people to answer a NASA application advertisement in a newspaper. In August 1979, she completed one year of training, and then performed as an on-orbit capsule communicator (CAPCOM), talking to the astronauts from the ground during the STS-2 and STS-3 shuttle missions.
On June 18, 1983, Ride became the first American woman in space as a crewmember on Challenger for the STS-7 mission.
Bluford flew on four space shuttle flights between 1983 and 1992. In addition to his STS-8 flight, he flew on the Spacelab-equipped STS-61-A mission, and two Department of Defense-dedicated missions, STS-39, and STS-53.
Bluford retired in 1993, having logged more than 28 days in space.
Sullivan was a crewmember on three space shuttle missions (STS-41G, STS-31 and STS-45), and logged 22 days in space.
Glenn went on to have a long career in NASA, and also became a U.S. senator in Ohio. In 1998 he flew on the space shuttle Discovery mission STS-95. He was 77 at the time, setting the record of oldest person to go into space. He was also the third seated politician to reach orbit.
Glenn logged a total of nine days in space during his NASA career.
McCandless became the first human to orbit Earth wearing just a spacesuit and a mobile jet pack, called the Manned Maneuvering Unit.
As a mission specialist on two space shuttle missions (STS-41-B and STS-31), McCandless logged more than 312 hours total in space.