Michel was selected as a scientist-astronaut in NASA Astronaut Group 4 in June 1965. He resigned from NASA on 18 August 1969 without having being assigned to any spaceflight missions. Michel believed that it was his decision to resign (after it became clear that he would not be given a flight assignment) that motivated the scientific community to demand that his fellow scientist-astronaut Harrison Schmitt be reassigned to Apollo 17 (replacing Joe Engle) after Schmitt's planned mission on Apollo 18 was cancelled.