High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.