This is the engineering model for the Interplanetary Monitoring Platform A (IMP-A or Explorer 18). It is the first of a series of seven satellites designed to be launched before and during Apollo flights. IMP-A carried instruments designed to measure cosmic rays, the solar wind, and interplanetary magnetic fields beyond the earth's magnetic field. The sphere on top of the boom contained a sensitive magnetometer. Four windmill-like solar panels provided operating power. The satellite was launched on November 26, 1963 aboard a Delta vehicle resulting in a highly elliptical orbit that ranged from 110 to 122,800 miles. Data from the mission led to the discovery of a new layer of radiation beyond the Van Allen belt and confirmed the existence of a standing shock wave in the solar wind around the earth's magnetic field. This object was transferred to NASM by NASA in May 1976.