Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, originally High Energy Solar Spectroscopic Imager or HESSI or Explorer 81) was a NASA solar flare observatory. It was the sixth mission in the Small Explorer program (SMEX), selected in October 1997 and launched on 5 February 2002, at 20:58:12 UTC. Its primary mission was to explore the physics of particle acceleration and energy release in solar flares. The spacecraft re-entered Earth's atmosphere at 00:21 UTC on 20 April 2023, 21 years after its launch.
Spacecraft HESSI was renamed to RHESSI on 29 March 2002 in honor of Dr. Reuven Ramaty, a pioneer in the area of high energy solar physics. RHESSI was the first space mission named after a NASA scientist. RHESSI was built by Spectrum Astro for Goddard Space Flight Center and was operated by the Space Sciences Laboratory in Berkeley, California. The principal investigator from 2002 to 2012 was Robert Lin, who was succeeded by Säm Krucker. The entire spacecraft rotated to provide the necessary signal modulation. The four, fixed solar panels were designed to provide enough gyroscopic moment to stabilize rotation about the solar vector. This largely eliminated the need for attitude control. The instrument detectors were nine high-purity germanium crystals. Each was cooled to cryogenic temperatures by a mechanical cryocooler. Germanium provided not only detections by the photoelectric effect, but inherent spectroscopy through the charge deposition of the incoming ray. The crystals were housed in a cryostat, and mounted with low-conductivity straps. A tubular telescope structure formed the bulk of the spacecraft. Its purpose was to hold the collimators above the Ge crystals at known, fixed positions. The satellite bus consisted of the structure and mechanisms, the power system (including the battery, solar panels, and control electronics), the attitude control system, thermal control system, command and data handling system (C&DH), and telecommunications system. The spacecraft structure provided support for the telescope and other components. It was manufactured out of aluminum parts to be light weight but strong. The equipment platform had a honeycomb structure to further reduce the weight. The spacecraft was manufactured in Gilbert, Arizona by Spectrum Astro, Inc. The Imaging Telescope Assembly consisted of the telescope tube, grid trays, Solar aspect system (SAS), and Roll angle system (RAS). It was constructed, assembled, aligned, and tested at the Paul Scherrer Institute in Switzerland. The front and rear grid trays were attached to the telescope tube. It maintained the separation and alignment of the trays. Nine grids were mounted on a grid tray at each end of the telescope tube. The grid pairs modulated the transmission of solar flare X-ray and gamma ray emissions through to the detectors as the spacecraft spins around the axis of the telescope tube. The modulated count rates in the nine detectors were used in computers on the ground to construct images of solar flares in different energy bands. The five coarse grids (square) were constructed by Van Beek Consultancy in Netherlands. The four fine grids (round) were constructed by Thermo Electron Tecomet in Massachusetts. All grids were characterized both optically and with X-rays at Goddard Space Flight Center before being shipped to the Paul Scherrer Institute for integration into the imaging telescope assembly. The spectrometer contained nine germanium detectors that were positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by the ORTEC division of Perkin Elmer Instruments. When they were cooled to cryogenic temperatures and a high voltage was put across them (up to 4000 volts), they converted incoming X-rays and gamma-rays to pulses of electric current. The amount of current was proportional to the energy of the photon, and was measured by sensitive electronics designed at the Lawrence Berkeley National Laboratory and the Space Sciences Laboratory, at Berkeley, California. The detectors were cooled with an electromechanical Stirling-cycle cryocooler built by SunPower Inc., and flight qualified at Goddard Space Flight Center. It maintained them at the required operating temperature of −198 °C (−324.4 °F), or 75° above absolute zero).
Mission concept RHESSI was designed to image solar flares in energetic photons from soft X-rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide high resolution spectroscopy up to gamma-ray energies of ~20 MeV. Furthermore, it had the capability to perform spatially resolved spectroscopy with high spectral resolution.
Scientific objectives Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission was to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum could be obtained at each point of the image. This new approach enabled researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of the fundamental high-energy processes at the core of the solar flare phenomena. The primary scientific objective of RHESSI was to understand the following processes that take place in the magnetized plasmas of the solar atmosphere during a flare:
Impulsive energy release Particle acceleration Particle and energy transport These high-energy processes play a major role at sites throughout the Universe ranging from magnetospheres to active galaxies. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of space physics and astrophysics. The high energy processes of interest include the following:
The rapid release of energy stored in unstable magnetic configurations The equally rapid conversion of this energy into the kinetic energy of hot plasma and accelerated particles (primarily electrons, protons and ions) The transport of these particles through the solar atmosphere and into interplanetary space The subsequent heating of the ambient solar atmosphere These processes involve:
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