Explorer 5 was a United States satellite with a mass of . It was the last of the original series of Explorer satellites built, designed, and operated by the Jet Propulsion Laboratory. Background Explorer 5 was similar in all respects to Explorer 4 and was designed with the same basic science objectives, to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts and to observe the effects of the Project Argus A-bomb detonations. Spacecraft and subsystems Explorer 5 was a long, diameter cylinder and nosecone that comprised the fourth stage of the Jupiter-C launch vehicle. The on-orbit mass (after fuel burnout) was . The spacecraft body was made of stainless AISI-410 steel, thick. The surface was sandblasted, no aluminum oxide striping was used as on earlier Explorer satellites. The base of the cylinder held the Sergeant solid-fuel rocket motor. The Mallory mercury batteries for the low power transmitter were in the upper part of the nose cone. Below these was the low power (10 mW, 108.00 MHz) transmitter for the carrier and sub-carrier signals, which used the stainless steel nose cone as an antenna. Below the nose cone was the detector deck, holding the instrumentation for the radiation experiments, the command receiver, for interrogations, high power playback transmitter (25-30 mW, 108.03 MHz) for interrogation response, cosmic ray experiment electronics, and Mallory mercury batteries for the high power transmitter. The lower spacecraft body was used as the antenna for the high power transmitter. A heat radiation shield was mounted between the payload and the rocket motor. Temperature gauges were mounted at various locations in the spacecraft. The radiation experiment comprised four detectors, two Geiger counters and two scintillation counters. One of the Geiger-Mueller counters was unshielded and one was shielded with 1.6 g/cm2 lead to screen out lower energy particles and radiation. One scintillator was a Cesium (CsI) crystal with a 0.8 mg/cm2 nickel foil window, the other was a plastic scintillator with a 0.14 g/cm2 aluminum window. All radiation experiments were within the spacecraft wall, which provided a protection of 1.2 g/cm2 iron. Juno I launch vehicle The launch vehicle was a Juno I, a variant of the three-stage Jupiter-C with an added fourth propulsive stage, which in this case was the Explorer 4. The first stage was an upgraded Redstone liquid-fueled rocket. The second stage comprised a cluster of eleven Sergeant