Reuven Ramaty High Energy Solar Spectroscopic Imager

Lore & Background

The Imaging Telescope Assembly consisted of the telescope tube, grid trays, Solar aspect system, and Roll angle system, constructed and tested at the Paul Scherrer Institute in Switzerland. Nine grids were mounted on a grid tray at each end of the telescope tube; grid pairs modulated transmission of solar flare X-ray and gamma-ray emissions as the spacecraft spun. The modulated count rates in the nine detectors were used on the ground to construct images. Five coarse grids were built by Van Beek Consultancy in the Netherlands, and four fine grids by Thermo Electron Tecomet in Massachusetts. The spectrometer's nine germanium detectors, pure to over one part in a trillion, were manufactured by ORTEC division of Perkin Elmer Instruments. When cooled and with high voltage up to 4000 volts, they converted incoming X-rays and gamma-rays to electric current pulses proportional to photon energy, measured by electronics designed at Lawrence Berkeley National Laboratory and Space Sciences Laboratory. The detectors were cooled by a Stirling-cycle cryocooler built by SunPower Inc., maintaining −198 °C.

Reader's Guide

RHESSI's significance lies in its novel combination of high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, enabling detailed energy spectra at each image point. This allowed researchers to determine where particles are accelerated and to what energies during solar flares, advancing understanding of fundamental high-energy processes in magnetized plasmas. The mission's primary scientific objectives included understanding impulsive energy release, particle acceleration, and particle and energy transport in the solar atmosphere. These processes are relevant throughout the universe, from magnetospheres to active galaxies. RHESSI provided spatial resolution of 2 arcseconds at X-ray energies from ~4 keV to ~100 keV, 7 arcseconds to ~400 keV, and 36 arcseconds for gamma-ray lines above 1 MeV. It also observed gamma-ray bursts from off-solar directions, though without positional imaging. The spacecraft re-entered Earth's atmosphere on 20 April 2023, after 21 years of operation.

Did You Know?

• RHESSI was the first space mission named after a NASA scientist, Dr. Reuven Ramaty.
• The spacecraft rotated to provide signal modulation, and its four fixed solar panels stabilized rotation about the solar vector.
• The germanium detectors were cooled to −198 °C by a Stirling-cycle cryocooler.

Mission Concept & Scientific Goals

RHESSI was conceived to tackle a fundamental question in solar physics: where and how particles get accelerated during solar flares. Researchers understood that flares release enormous energy, converting it into the kinetic energy of electrons, protons, and ions, yet the precise mechanisms remained unclear. The mission's breakthrough was its dual capability—imaging hard X-rays and gamma rays while simultaneously performing high-resolution spectroscopy at every point in the image. This spatially resolved spectroscopy was unprecedented, allowing scientists to trace energy spectra across the flare region rather than averaging over the entire event. The primary targets were impulsive energy release from unstable magnetic configurations, the rapid conversion of that energy into particle kinetic energy, and the subsequent transport of those particles through the solar atmosphere and into interplanetary space. These processes, occurring at particle energies reaching many GeV and temperatures of tens to hundreds of millions of degrees, are not confined to the Sun. They recur across the universe in magnetospheres, active galaxies, and other high-energy environments, making RHESSI's findings a cornerstone for space physics and astrophysics broadly.

Engineering & Spacecraft Design

Rather than relying on conventional reaction wheels or thrusters, RHESSI employed an elegant spinning design. The entire spacecraft rotated around its telescope axis, and four fixed solar panels provided sufficient gyroscopic moment to stabilize that rotation about the solar vector, largely eliminating the need for active attitude control. The bulk of the spacecraft was a tubular telescope structure that held the collimator grids at precise, fixed positions above the detector crystals. The satellite bus was built from lightweight aluminum parts with a honeycomb equipment platform to minimize mass. Spectrum Astro in Gilbert, Arizona, manufactured the spacecraft for Goddard Space Flight Center. The Imaging Telescope Assembly—comprising the telescope tube, front and rear grid trays, the Solar Aspect System, and the Roll Angle System—was constructed, aligned, and tested at the Paul Scherrer Institute in Switzerland. Nine grids were mounted on each grid tray, and the pairs modulated X-ray and gamma-ray transmission as the spacecraft spun, enabling ground-based reconstruction of flare images across different energy bands.

Detection Technology & Instrumentation

At the heart of RHESSI were nine high-purity germanium crystals, each manufactured by the ORTEC division of Perkin Elmer Instruments to a purity exceeding one part in a trillion. These artificially grown crystals served a dual purpose: detecting incoming photons via the photoelectric effect and providing inherent spectroscopy through the charge deposition of each ray. To operate, the crystals had to be cooled to minus 198 degrees Celsius, just 75 degrees above absolute zero, by an electromechanical Stirling-cycle cryocooler built by SunPower Inc. and flight-qualified at Goddard. At these cryogenic temperatures, with up to 4,000 volts applied across them, the crystals converted incoming X-rays and gamma rays into measurable pulses of electric current, with the current magnitude proportional to photon energy. Sensitive electronics designed at Lawrence Berkeley National Laboratory and the Space Sciences Laboratory in California measured these pulses. The crystals were housed in a cryostat and mounted with low-conductivity straps for thermal isolation. The five coarse square grids were built by Van Beek Consultancy in the Netherlands, while the four fine round grids came from Thermo Electron Tecomet in Massachusetts, all characterized optically and with X-rays at Goddard before integration.

Mission Timeline, Naming & Legacy

RHESSI's journey began in October 1997 when it was selected as the sixth mission in NASA's Small Explorer program. It launched on 5 February 2002 at 20:58:12 UTC, and on 29 March 2002, just weeks into its mission, it was renamed in honor of Dr. Reuven Ramaty, a pioneer in high-energy solar physics. This made RHESSI the first space mission to carry the name of a NASA scientist. Robert Lin served as principal investigator from 2002 to 2012, after which Säm Krucker assumed the role. The spacecraft operated for a remarkable 21 years, far exceeding typical mission lifespans for a small explorer. On 20 April 2023 at 00:21 UTC, RHESSI re-entered Earth's atmosphere, concluding its long service. Operated by the Space Sciences Laboratory in Berkeley, California, and built for Goddard Space Flight Center, the mission left a lasting imprint on our understanding of solar flare physics and the high-energy processes that govern magnetized plasmas throughout the cosmos.