This artifact is one of several replicas and flight spare Explorer 1 spacecraft in the collection. It was identified as a fully instrumented flight spare of the Explorer-1 satellite attached to an empty fourth stage Sergeant rocket when it was transferred in 1961 to the collection by the NASA Jet Propulsion Laboratory, the builder of the object. It was initially displayed in the Arts and Industries Building. It was on loan to the Museum of Medical Progress in Madison, WI, (4/70-6/70) and briefly to WETA in Arlington, VA, (6/75-7/75). It was inspected in late 2005 and found to be empty of instrumentation save for the micrometeoroid sensor. But markings in the interior frame indicate it to be "Payload II" which was indeed the flight backup that was sent to James Van Allen's laboratory in Iowa for inspection and testing and then returned to JPL in 1958. That payload was donated to NASM by George Ludwig in 2006 (A20060086). The satellite is displayed in the Milestones of Flight Gallery at NASM.
Explorer-1 was the United States' first successful orbiting satellite. Following the failure of Vanguard in December 1957, the JPL- ABMA group was permitted to adapt the Jupiter-C reentry test vehicle to carry an instrumented satellite into earth orbit. The resulting Explorer-1 satellite was successfully launched and placed into Earth orbit on January 31, 1958. Explorer-1, also known unofficially as Satellite 1958 alpha, transmitted data on micrometeorites and cosmic radiation for 105 days. Data from this and two subsequent Explorer satellites led to the discovery by James Van Allen of a belt of intense radiation surrounding the earth.