Explorer 39 was an inflatable sphere, 3.6 m in diameter. It was orbited to make atmospheric density determinations. The spacecraft was successfully launched into a nearly polar, highly elliptical orbit. It was folded and carried into orbit, together with ejection and inflation equipment, as part of the payload of Explorer 40. Two density experiments were performed. One involved the study of systematic density variation, and the other was concerned with nonsystematic density changes. The upper atmospheric densities were derived from sequential observations of the sphere by use of an attached 136.620 MHz radio tracking beacon and by optical tracking. The radio beacon ceased transmitting in June 1971. Since that time it has been necessary to rely solely on the SAO Baker-Nunn camera network for tracking. Explorer 39 has an expected orbital lifetime of 50 years.

Injun 5 was designed to accomplish the following objectives: comprehensive study of the downward flux of charged particles, study of very low frequency (VLF) radio emission in the ionosphere associated with the downward flux, study of geomagnetically trapped protons, alpha particles, and electrons, observation of solar cosmic rays, observation of the continuing decay of the Starfish artificial radiation belt, and study of the temperature and density of electrons and positive ions of thermal and near thermal energy. The spacecraft systems performed normally except for the malfunction of the solar cell power dump device (shortly after launch) which caused the solar cells to deliver a lower power level to the experiments and reduced the time during which the onboard tape recorder could be run. The passive magnetic alignment became effective in mid-December 1968. The spacecraft was turned off from May 31, 1970, to February 18, 1971, after this period it was turned on again. The spacecraft was put in an operational off-mode in early June 1971, and became inoperable shortly thereafter.