S-56 was an American satellite launched by NASA on 4 December 1960 as part of the Explorers program. The satellite was composed of a 3.66 meter (12 foot) diameter inflatable sphere, and was intended to study the density of the upper atmosphere. S-56 was launched on a Scout X-1 rocket, but failed to reach orbit due to the second stage not igniting. It was replaced by the identical Explorer 9 (S 56A) spacecraft.