We describe the satellite Injun 3 and its scientific payload, which has been used in geophysical studies reported in the accompanying papers. The satellite was launched on December 13, 1962, into an orbit with apogee altitude 2785 km, perigee altitude 237 km, orbital inclination 70.4°, and period 116 minutes. One axis is continually aligned to within a few degrees of the local geomagnetic field vector by a permanent magnet. Eight directional detectors point at right angles to this axis and therefore measure trapped, or Van Allen, radiation. There are three omnidirectional particle detectors. Six particle detectors point upward in the northern hemisphere and therefore detect particles that are being precipitated to cause auroras. Two photometers, with 5577 A and 3914 A filters, point down the axis to view these auroras in the northern hemisphere, while another 5577 A photometer serves the same purpose in the southern hemisphere. With a loop antenna, very‐low‐frequency electromagnetic radiation in the frequency range about 500 cps to about 10 kc/s is detected, and a frequency and amplitude analysis is performed. The characteristics of the proton detectors which cover the energy range ∼50 kev to more than 100 Mev and the electron detectors which cover the range ∼5 kev to ∼5 Mev are