A NASA satellite which took images of solar flares in X-rays and gamma rays (energy range 3 keV–17 MeV), launched in 2002 February. It was named in honour of the Romanian-born American astrophysicist Reuven Ramaty (1937–2001), a specialist in solar flare research. Observations ended in 2018 April.

http://hesperia.gsfc.nasa.gov/hessi/ Official mission website.