Explorer 39
Explorer 39 satellite
NamesAD-C
Air Density experiment-C
Mission typeAir density
OperatorNASA
COSPAR ID1968-066A
SATCAT no.03337
Spacecraft properties
SpacecraftExplorer XXXIX
Spacecraft typeExplorer
BusAD
ManufacturerLangley Research Center
Launch mass9.4 kg (21 lb)
Dimensions3.6 m (12 ft) diameter
PowerSolar cells and
rechargeable batteries
Start of mission
Launch date8 August 1968, 20:12:00 GMT[1]
RocketScout B (S-165C)
Launch siteVandenberg, SLC-5
ContractorVought
Entered service8 August 1968
End of mission
Last contactJune 1971
Decay date22 June 1981
Orbital parameters
Reference systemGeocentric orbit[2]
RegimeLow Earth orbit
Perigee altitude670 km (420 mi)
Apogee altitude2,538 km (1,577 mi)
Inclination80.60°
Period118.20 minutes
Instruments
Nonsystematic Changes of Air Density
Systematic Changes of Air Density
Explorer program
 

Explorer 39, also known as AD-C (Air Density experiment-C), was a NASA scientific satellite belonging to series Air Density. It was launched on 8 August 1968, join with Explorer 40, from Launch Complex 5 of the Vandenberg Air Force Base, through a Scout launch vehicle.[3]

Spacecraft

Explorer 39 was an inflatable sphere, 3.6 m (12 ft) 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.[3]

Launch

Explorer 39 orbited the Earth once every 118.20 minutes, at an inclination of 80.60°. Its perigee was 670 km (420 mi) and apogee was 2,538 km (1,577 mi).[2]

Mission

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 Smithsonian Astrophysical Observatory (SAO) Baker-Nunn camera network for tracking. Explorer 39 has an expected orbital lifetime of 50 years.[3]

Experiments

Nonsystematic Changes of Air Density

This experiment was designed to determine nonsystematic upper atmospheric density changes. The data are derived from studies of the drag on a 3.6 m (12 ft) in diameter low-density sphere caused by short-term differences in solar activity. Density values near perigee were deduced from sequential observations of the spacecraft position using optical (Baker-Nunn camera network) and radio and/or radar tracking techniques. This experiment has determined reasonable density values, and it is capable of yielding long-term atmospheric density values, as Explorer 39 has an expected orbital lifetime of 50 years.[4]

Systematic Changes of Air Density

This experiment was designed to determine systematic changes of air density as a function of altitude, latitude, and time of day, by measuring the drag on a 3.6 m (12 ft) in diameter low-density sphere with ground tracking.[5]

Decay

Explorer 39 reentered the atmosphere on 22 June 1981.[2]

See also

References

  1. ↑ "Launch Log". Jonathan's Space Report. 21 July 2021. Retrieved 13 November 2021.
  2. 1 2 3 "Trajectory: Explorer 39 (AD-C) 1968-066A". NASA. 28 October 2021. Retrieved 13 November 2021. Public Domain This article incorporates text from this source, which is in the public domain.
  3. 1 2 3 "Display: Explorer 39 (AD-C) 1968-066A". NASA. 28 October 2021. Retrieved 13 November 2021. Public Domain This article incorporates text from this source, which is in the public domain.
  4. ↑ "Experiment: Nonsystematic Changes of Air Density". NASA. 28 October 2021. Retrieved 13 November 2021. Public Domain This article incorporates text from this source, which is in the public domain.
  5. ↑ "Experiment: Systematic Changes of Air Density". NASA. 28 October 2021. Retrieved 13 November 2021. Public Domain This article incorporates text from this source, which is in the public domain.
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