Mercury Seven In January 1959, Slayton was selected as one of the candidates for
NASA's
Project Mercury, the first U.S. crewed space flight program. Despite his initial lack of interest in spaceflight, he agreed to pursue astronaut selection. After an initial interview at the temporary NASA headquarters in the
Dolley Madison House in
Washington, D.C., Slayton was psychologically and physically tested at the
Lovelace Clinic in
Albuquerque, New Mexico, along with fellow future astronauts
Scott Carpenter and
Jim Lovell. On April 2, 1959, Slayton was notified of his selection as an astronaut. He moved his family from Edwards Air Force Base to a housing development near
Fort Eustis, where he was neighbors with fellow
Mercury Seven astronauts
Gus Grissom and
Wally Schirra. In 1959, during a
centrifuge training course, he underwent an
electrocardiogram that found that he had erratic heart activity. He received further medical evaluation at
Brooks Air Force Base and was diagnosed with idiopathic paroxysmal
atrial fibrillation, Initially, Slayton's ineligibility was only for his assigned mission, and he attempted to improve his health by exercising more regularly and abstaining from alcohol. NASA leadership determined that Slayton was still at risk for atrial fibrillation and removed his eligibility to fly any of the remaining Mercury missions. Slayton continued to be responsible for making crew assignments, and determined the astronauts that would fly on the
Gemini and
Apollo missions. in 1971 Slayton was examined at the
Mayo Clinic after a long period without heart fibrillation, and was determined to not have a coronary condition. On March 13, 1972, NASA announced that Slayton had returned to flight status. The Apollo and
Soyuz spacecraft both launched on July 15, 1975. On July 17, the two craft rendezvoused in orbit and the American astronauts conducted crew transfers with cosmonauts
Alexei Leonov and
Valeri Kubasov. At the end of the flight, an erroneous switch setting led to noxious
nitrogen tetroxide fumes from the command module's RCS thrusters being sucked into the cabin during landing and the crew was hospitalized as a precaution in
Honolulu,
Hawaii, for two weeks. During hospitalization, a
lesion was discovered on Slayton's lung and removed. It was determined to be benign, but he would have likely been grounded from ASTP if it had been discovered before the flight. He was 51 years old, making him the oldest astronaut to fly in space at the time.
Space Shuttle program Prior to the ASTP flight, Slayton was assigned by
Chris Kraft to manage the
Approach and Landing Tests (ALT) of the Space Shuttle program. The ALT program developed the tests for the
Space Shuttle orbiter Enterprise, and modified several
F-104 Starfighters and
T-38 Talons to train the astronauts. While working on the Space Shuttle, he also assisted in developing the
Shuttle Carrier Aircraft. The ALT program ended in late 1977 and Slayton agreed to manage the Space Shuttle's Orbital Flight Tests (OFT). During the selection of the
Group 8 astronauts, Slayton advocated for fewer selections, with the expectation that two-man crews would fly the regular satellite deployment missions. He officially retired in 1980, but continued to serve in an advisory role for
STS-1, and flew a T-38 chase plane during the landing of
STS-2. He formally left NASA on February 27, 1982, and had flown 7,164 hours. ==Post-NASA career==