 Pioneer P-31 was virtually identical to the earlier Pioneer P-30 satellite which failed, a 1-meter diameter sphere with a propulsion system mounted on the bottom giving a total length of 1.4 meters.The mass of the structure and aluminum alloy shell was about 30 kg and the propulsion units roughly 90 kg.Four solar panels, each 60 x 60 cm and containing 2200 solar cells in 22 100-cell nodules, extended from the sides of the spherical shell in a "paddle-wheel" configuration with a total span of about 2.7 meters.