skepticism about this whole thing'". Merbold said that at JSC he was treated as an intruder. Once payload specialists were assigned to a mission, however, full-time astronauts treated them respectfully and often began long-term friendships. Mullane became less critical of them after his first mission; he and Hartsfield approved of Walker, as did Hoffman of Garn after STS-51-D. List of all payload specialists Until Challenger Post-Challenger to Columbia Alternate and back-up (not flown) payload specialists Other statistics Multiple flights Payload specialists who later trained as mission specialists All were international astronauts. Marc Garneau – mission specialist on STS-77, STS-97 Mamoru Mohri – mission specialist on STS-99 Steven MacLean – mission specialist on STS-115 Hans Schlegel – mission specialist on STS-122 Umberto Guidoni – mission specialist on STS-100 Robert Thirsk – completed training, flew on Soyuz TMA-15 Bjarni Tryggvason – completed training, retired in June 2008 without flying again See also List of human spaceflights List of Space Shuttle missions List of Space Shuttle crews Notes References External links Biographies of Payload Specialists on NASA web site. Astronauts