An astrophysicist by training, a pilot, and the first Swiss citizen to fly in space, Claude Nicollier has spent his career where understanding has to become an act carried out correctly the first time. Selected in 1978 for the first class of European Space Agency astronauts, he flew four Space Shuttle missions between 1992 and 1999, two of them to repair the Hubble Space Telescope, and became the first European to work outside the shuttle in open space. Trained as a Swiss Air Force pilot, an airline pilot and a test pilot, he went on to lead flight testing for Solar Impulse and has taught space mission design at EPFL since 2004. His experience is one of long preparation: years of work condensed into an intervention that allows no second attempt, in an environment that forgives nothing. Within the Club des Sentinelles, he brings that culture of anticipation: seeing what can fail before it does, and preserving the capacity to act.