The High Energy Solar Spectroscopic Imager (HESSI) mission

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. The HESSI instrument utilizes Fourier-transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors to make observations of X-rays and γ-rays from ∼3 keV to ∼17 MeV. It will provide the first imaging spectroscopy in hard X-rays, with ∼2 arcsec angular resolution, time resolution of 2s for full image (tens of ms for crude image), and ∼1 keV energy resolution; the first solar γ-ray line spectroscopy with ∼1-5 keV energy resolution; and the first solar γ-ray line and continuum imaging, with ∼36-arcsec angular resolution. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft, and is planned to be launched in July 2000 into a 600 km-altitude, 38 deg. inclination orbit. HESSI will provide detailed information on the energetic particle populations at the Sun, for comparison to measurements of solar energetic particles by ACE, WIND, and other interplanetary spacecraft