The objectives of this investigation are to study the acceleration and storage of energetic electrons accelerated during solar flares by measuring solar X-radiation; to identify gamma-ray burst sources with known celestial objects or phenomena; and to study plasma and energetic charged particle processes in the Jovian magnetosphere. The instrument consists of two hemispherical cesium iodide (sodium) crystals coupled to curved cathode photomultipliers; two small solid-state detectors with an americium 241 radioactive source deposited on the sensors, and a digital electronics unit. The scintillation counters measure X rays in the energy range from 15 to 150 keV, while the solid state detectors measure X rays from 5 to 15 keV. The instrument has a mass of 2.0 kg, uses 2.6 W of power, and has a data rate of 20 bps in storage mode and 40 bps in tracking mode.