[1] The satellite was composed of a 3.66 m (12.0 ft) diameter inflatable sphere, and was intended to study the density of the upper atmosphere.

The Scout X-1 rocket used to launch Explorer S-56 failed in flight, and the satellite never reached orbit.

[2] The spacecraft consisted of alternating layers of aluminium foil and Mylar polyester film.

Uniformly distributed over the aluminium surface were 5.1 cm (2.0 in)-diameter dots of white paint for thermal control.

The two hemispheres of aluminium foil will be separated with a gap of Mylar at the spacecraft's equator and will serve as the antenna.