In July 1978, he was selected by ESA as a member of the first group of European astronauts. Under agreement between ESA and NASA, he joined the NASA astronaut candidates selected in May 1980 for astronaut training as a mission specialist. His technical assignments in the Astronaut Office have included flight software verification in the Shuttle Avionics Integration Laboratory (SAIL), participation in the development of retrieval techniques for the Tethered Satellite System (TSS), Remote Manipulator System (RMS), and International Space Station (ISS) robotics support. From the Spring of 1996 to the end of 1998, he was Head of the Astronaut Office Robotics Branch. From the year 2000 on, he was assigned to the Astronaut Office EVA (Extravehicular Activity) Branch, while maintaining a position as Lead ESA astronaut in Houston. Nicollier retired from ESA in April 2007. He is a member of the Swiss Astronomical Society, the Astronomical Society of the Pacific, the Swiss Air Force Officers Society (AVIA), and the Swiss Academy of Engineering Sciences and fellow of the British Interplanetary Society. He is also an honorary member of the Swiss Aero Club, the Swiss Society of Engineers and Architects, and the Swiss Astronomy Day Society. Nicollier has spent over 1000 hours in space (42 days, 12 hours and 5 minutes), including one space walk lasting 8 hours and 10 minutes. He served as mission specialist on four missions with four different space shuttles. His first spaceflight was planned to be STS-51-H, which had been scheduled for November 1985, but was cancelled (technical problems). His first spaceflight was planned to be STS-61-K, which had been scheduled for October 1986, but was cancelled following the Space Shuttle Challenger disaster. Nicollier's first spaceflight was as a Mission Specialist on the 8-day Space Shuttle mission aboard "Atlantis", called STS-46, in 1992. The crew deployed the European Retrievable Carrier EURECA, as well as the Tethered Satellite System-1, which was a joint NASA and Italian Space Agency project. His second spaceflight was as a Mission Specialist on the 10-day mission aboard "Endeavour", called STS-61, in 1993. It was the first mission to service the Hubble Space Telescope, which had been launched three years previously. In 1996 he took part in STS-75, aboard Space Shuttle "Columbia", which deployed TSS-1R, which was a follow-up mission to TSS-1 which had been deployed during STS-46.