It was the sixth mission in the Small Explorer program (SMEX), selected in October 1997 [1][7] and launched on 5 February 2002, at 20:58:12 UTC.

[5] 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.

Germanium provided not only detections by the photoelectric effect, but inherent spectroscopy through the charge deposition of the incoming ray.

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.

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.

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 proximity of the Sun means not only that these high-energy emissions are orders of magnitude more intense than from any other cosmic source but also that they can be better resolved, both spatially and temporally.

The imaging capability of RHESSI was based on a Fourier transform technique using a set of 9 Rotational Modulation Collimators (RMCs) as opposed to mirrors and lenses.

As the spacecraft rotated, these grids blocked and unblocked any X-rays which may have come from the Sun modulating the photon signal in time.

Multiple RMCs, each with different slit widths, provided coverage over a full range of flare source sizes.

Images were then reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry.

The large area and high sensitivities of the germanium crystal assembly made RHESSI a formidable IPN component.

RHESSI was intended to image at high resolution solar flares in X-rays and gamma rays.

The output from each of the nine detectors, at any given energy, could be Fourier-analyzed to provide a full two-dimensional spatial spectrum of an extended source region on the Sun.

The telemetry data was collected at Berkeley (California), Wallops Flight Facility (WFF), Virginia, Santiago, Chile and Weilheim, Germany.

[16] It was decommissioned on 16 August 2018[4] and remained in a stable low Earth orbit which gradually decayed due to atmospheric drag.