Reuven Ramaty (1937–2001) was a pioneer in the fields of solar physics, gamma-ray astronomy, nuclear astrophysics, and cosmic rays. He was a HESSI Co-Investigator and one of the founding members of the HESSI team. His active involvement and enthusiastic support were critical for HESSI’s selection by NASA as the sixth Small Explorer (SMEX) mission. Following the launch on 5 February 2002, HESSI was renamed in his honor. It became the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the first space mission named after a NASA scientist.