R.P. Lin and the HESSI team
Physics Dept & Space Sciences Lab, University of California, Berkeley, CA, USA

The primary scientific objective of the NASA High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission 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 ~3 keV to ~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 degree inclination orbit. It will provide the first imaging spectroscopy in hard X-rays, with ~2 arcsecond angular resolution, time resolution down to tens of ms, and ~1 keV energy resolution; the first solar gamma-ray line spectroscopy with ~1-5 keV energy resolution; and the first solar gamma-ray line and continuum imaging, with ~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.