This is a full-size display mock-up of the Explorer 3 spacecraft (1958 Gamma 1) instrument section with the outer casing removed to display the interior. Components are identified with small labels, including the Geiger counter and batteries. It was prepared for display at the Jet Propulsion Laboratory before its transfer in 1975. All examples of Explorer originally had shipping containers built for repeated use.

Explorer 3, externally nearly identical to Explorer 1 but with different electronics, was successfully placed into a highly elliptical Earth orbit on March 26, 1958 and was the first to carry a tape recorder to provide continuous data acquisition in a day when tracking and data receiving stations were not world-wide. The satellite transmitted data on micrometeorites and cosmic radiation for some 93 days. Data from this and the Explorer 1 satellite led to the discovery by James Van Allen that a radiation belt surrounds the earth.

When it was originally transferred to NASM it was identified as an Explorer 1 package on the invoice. This was corrected in 1984, reversed in the 1990s, and finally re-corrected in 2005.