This is a replica of Explorer 4, an earth satellite whose instrument package was designed and built at the University of Iowa under the supervision of James Van Allen. This specimen contains potted electronics and may be an engineering test model. It was transferred to NASM by NASA's Marshall Space Flight Center in August 1972. Explorer-4 was physically identical to the earlier spacecraft in the Explorer series, consisting of a small bullet shaped payload section attached to a Sergeant solid fuel rocket. The original payload, launched on July 26, 1958 on a Juno was designed to measure the nature of the radiation belts that surround the earth newly discovered by Explorers 1 and 3 and to take measurements in conjunction with the planned Argus high-altitude nuclear explosions that would perturb the belts in explosions on August 27th, August 30th and September 6th. The Explorer 4 flight payload contained four detectors: two shielded scintillation counters and two Anton-type counters for an overall sensitivity range from 20 KeV to over 40 MeV. One of the scintillation counters failed on Sept 3 1958 and the others only on September 19th.