 A plasma probe was mounted on the sphere to measure energy and momentum distribution of protons above a few kilovolts to study the radiation effect of solar flares.The micrometeorite detector and sun scanner were mounted on the sphere as well.The only difference between Pioneer P-31 and the earlier Pioneer P-30 was the addition of a solid state detector sensitive to low energy protons on the satellite and an STL-designed rubidium frequency standard experiment placed on a pod attached to the booster.