was designed to measure the omnidirectional flux of heavy primary cosmic rays in the rigidity range 1 to 15.5 GV. Particles with atomic numbers Z >5, Z >8, and Z >15 were counted separately by an ionization chamber in which each incident particle yielded a pulse. Pulse amplitude was substantially independent of the energy of the incident particle but was proportional to the square of its Z value. Each of the three counting rates was determined every 15 seconds. The experiment performed as planned from launch until 25 October 1959. About 80% of the data acquired for the 25 October 1959, to 31 May 1960, period are useful, with most problems occurring in the lowest Z mode. Very little useful data were acquired after 31 May 1960. Micrometeorite Three photoconducting cadmium sulfide cells were used to measure micrometeorite penetration and molecular sputtering. The three cells were identical in design and effective area (18 sq mm) and were mounted on a magnesium plate on the satellite's equator facing outward perpendicular to the satellite's spin axis. The experiment was exposed to the space environment of micrometeorites, trapped radiation, and sputtering for 38 days of active life. The three cells and a thermistor that was included as part of the experiment performed normally. One cell was penetrated on the 16th day by a particle approximately 10 microns in diameter. Solar X-Ray and Lyman-Alpha Radiation The solar X-ray and Lyman-alpha radiation were measured by means of gas ionization chambers mounted on opposite sides of the upper portion of the double cone configuration of the Explorer 7 satellite. Intensities were monitored in order to obtain a long-term history of solar X-ray and Lyman-alpha fluxes and to correlate these with terrestrial atmospheric responses. The two X-ray detectors ( deep) were filled with argon gas and had beryllium windows (.021 g/sq cm) resulting in a sensitivity to X-rays in the 2 to 8 A range. The Lyman-alpha detectors (on the opposite side), which were circular ionization chambers ( in diameter) filled with nitric oxide gas, had lithium fluoride windows. Their sensitivity was in the 1050 to 1350 A interval. The data, however, were impossible to interpret in terms of incident solar radiation due to both the saturation of detector circuits by Van Allen radiation (150 keV electrons) and electronic difficulties in the feedback amplifier. Thermal Radiation The Explorer 7 thermal radiation experiment was designed to measure incident