|Mission type||Earth science|
|Manufacturer||Langley Research Center|
|Launch mass||9.4 kg (21 lb)|
|Start of mission|
|Launch date||8 August 1968, 20:12UTC|
|Rocket||Scout B S165C|
|Launch site||Vandenberg SLC-5|
|End of mission|
|Decay date||22 June 1981|
|Perigee altitude||670 km (420 mi)|
|Apogee altitude||2,538 km (1,577 mi)|
|Epoch||8 August 1968|
Explorer 39, also known as AD-C (Air Density C), was an American scientific satellite belonging to series Air Density. It was launched on August 8, 1968, join with Explorer 40, from Launch Complex 5 of the Vandenberg Air Force Base, through a Scout rocket. Explorer 39 orbited the Earth once every 118.2 minutes, at an inclination of 80.6°. Its perigee was 680 kilometres (420 mi) and apogee was 2,522 kilometres (1,567 mi).
Explorer 39 was an inflatable sphere, 3.6 m in diameter. It was orbited to make atmospheric density determinations. The satellite 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 136.62 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.