Engineering prototype for one of the Tetrahedral Research Satellites also known as TRS-II and ERS-12. One of the sides has been replaced by a Plexiglas panel to display the instruments. The TRS/ERS series was developed for the Department of Defense as a piggyback satellite to be launched with other spacecraft and then detached once in orbit. Silicon solar cells that covered the sides provided power for operation of the on-board instruments and transmitters. The ERS-12 satellite for which this is a prototype was launched aboard an Atlas-Agena vehicle on October 30, 1963 and stayed aloft for 39 hours. The prototype was built by TRW Space Vehicles Division and was donated to NASM by TRW in April 1975.