A small NASA explorer satellite to investigate the basic physics of particle acceleration and the explosive energy released in solar flares. It was launched on 5 February 2002 with Reuven Ramaty's name added to the mission; he was a founding member of the HESSI team and died in 2001. Its objectives include discovering the location, frequency, and evolution of impulsive energy in the Sun's corona; studying the acceleration of electrons, protons, and heavier ions in flares; and investigating the heating of plasma to tens of millions of degrees. Hard X-ray (high-energy X-rays) and gamma-ray imaging will be used during the mission. Besides the Sun, RHESSI will also study the Crab nebula and other areas of the sky.

The sixth Small Explorer (SMEX) mission, it was expected to have a life of up to three years, but in February 2017 the satellite celebrated 15 years in space; until it was decommissioned in August 2018 it was the only active observatory providing imaging spectroscopy of the energetic electrons that carry such a predominant part of the energy released in a flare.

https://hesperia.gsfc.nasa.gov/rhessi3/ Full details of the workings, scientific goals, and status of the Rheuven Ramaty High-Energy Solar Spectroscopic Imager. The site's articles are clearly laid out and easy to read, with animations and an image gallery.