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The sign accompanying the satellite. It reads

Explorer XXXIII (Engineering Test Unit)

Launched by the Goddard Space Flight Center on July 1, 1966, atop a Delta Rocket, the 206-pound (93.4-kilogram) Explorer XXXIII overshot its location for placement in a lunar orbit. Placed in a highly elongated orbit about the Earth, with the most distant part of the orbit beyond the moon, Explorer XXXIII was still able to perform its intended mission of interplanetary magnetic field studies of solar plasma and energetic particles measurement. It still orbits the Earth, but current location information is not maintained.