Explorer 9 was the first in a series of 3.66 m inflatable spheres to be successfully placed into orbit solely for the determination of atmospheric densities. It was identical in its objectives and configuration to the earlier unsuccessful launch of Explorer S-56.
The spacecraft consisted of alternating layers of aluminum foil and Mylar polyester film. Uniformly distributed over the aluminum surface were 5.1 cm-diameter dots of white paint for thermal control. The sphere was packed in a tube 21.6 cm in diameter and 48.3 cm long and mounted in the nose of the fourth stage of the launch vehicle. Upon separation of the third and fourth stages, the ejection bellows, a nitrogen gas bottle, inflated the sphere and a separation spring ejected it out into its own orbit.
The two hemispheres of aluminum foil were separated with a gap of Mylar at the spacecraft's equator and served as the antenna. A 136 MHz, 15 mW beacon was carried for tracking purposes, but the beacon failed on the first orbit and the SAO Baker-Nunn camera network had to be relied upon for tracking. Power was supplied by solar cells and rechargable batteries.
Explorer 9 was the first spacecraft placed in orbit by an all-solid rocket and the first spacecraft successfully launched into orbit from Wallops Island.
The spacecraft reentered the earth's atmosphere on April 9, 1964.