This is a fully instrumented back-up for the Explorer 12 spacecraft. Explorer 12 was the first in a series of four satellites designed specifically to study the behavior of energetic particles near the Earth. They carried instruments to provide data on the solar wind, the interplanetary field, trapped radiation, and cosmic rays. The octagonal spacecraft carried six experiments contributed by the Goddard Space Flight Center, the NASA Ames Research Center, the University of New Hampshire, and Iowa State University. Power was supplied by four windmill-like solar panels. The satellite was placed into a highly elliptical orbit that ranged from 170 to 50,000 miles on December 6, 1961. It transmitted scientific data during its lifetime of 112 days. The satellite was manufactured at NASA's Goddard Space Flight Center where the instruments were integrated and tested.