or molybdenum wire grids of width mounted parallel to the rotation axis of the tube pointing at the Sun. The tube rotates about its axis as the spacecraft spins at a rate of 15 rpm. During a rotation, a photon from any point on the Sun can either pass through a grid-pair or be blocked by one or other of the grids. This causes a modulation of the intensity of photons emanating from that point. The depth of modulation is zero for the photons arriving exactly along the spin axis and gradually increases to the off-axis photons. Behind each grid-pair is a cryogenic (75 K) germanium detector of 7.1 cm diameter and thickness. The output from each of the nine detectors, at any given energy, can be Fourier-analyzed to provide a full two-dimensional spatial spectrum of an extended source region on the Sun. The full spatial spectrum is possible because each wire grid pair has a different slit width, spacing and wire thickness. Data accumulation is about 16 Gb during a 10-minutes rotation. The telemetry data was collected at Berkeley (California), Wallops Flight Facility (WFF), Virginia, Santiago, Chile and Weilheim, Germany. Science analysis of the data was involved close collaboration with many dedictated ground based and satellite based solar observatories. A secondary goal of RHESSI is to observe astronomical sources such as Crab Nebula. Results RHESSI observations have changed our perspective on solar flares, particularly on high-energy processes in flares. RHESSI observations has led to numerous publications in scientific journals and presentations at conferences. Through 2017, RHESSI is referenced in 2,474 publications, books, and presentations. RHESSI was the first satellite to image gamma rays from a solar flare. RHESSI was the first satellite to accurately measure terrestrial gamma-ray flashes that come from thunderstorms, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher frequency on average than the average for cosmic sources. Following communication difficulties, RHESSI ceased science operations on 11 April 2018 at 01:50 UTC. RHESSI was decommissioned on 16 August 2018, and remains in a stable low Earth orbit. However, since it has no means of propulsion, atmospheric drag will eventually pull the spacecraft into atmosphere of Earth, which may occur as early as 2022. See also Explorer program References External links RHESSI website by NASA's Goddard Space Flight Center HESSI archive website by the Space Sciences Laboratory RHESSI data