Explorer 54
NASA satellite studying atmospheric chemistry and energy transfer.
Explorer 54, also designated AE-D (Atmosphere Explorer-D), was a NASA scientific satellite launched on 6 October 1975 from Vandenberg Air Force Base aboard a Thor-Delta 2910 launch vehicle. It belonged to the Atmosphere Explorer series and was designed to study the Earth's atmosphere and ionosphere.
Quick Facts
- Names list
- AE-D; Atmosphere Explorer-D
- Operator
- NASA
- Satcat
- 08353
- Mission duration
- 4 months (achieved)
- Spacecraft
- Explorer LIV
- Manufacturer
- Goddard Space Flight Center
- Launch mass
- 681 kg / 1,501 lb
- Launched
- 6 October 1975, 09:00:50 UTC
Facts from the source article.
Mission
Explorer 54 continued investigations begun by Explorer 51 (AE-C) into chemical processes and energy transfer mechanisms controlling the Earth's atmosphere and ionosphere in the region of high solar energy absorption. It sampled high latitude regions while Explorer 55 (AE-E) sampled equatorial and low latitudes. The spacecraft and payload were similar to Explorer 55, except for the deletion of the extreme solar ultraviolet monitor and the Bennett ion mass spectrometer.
Experiments
The Miniature Electrostatic Analyzer (MESA) measured neutral atmospheric density between 120 km and 400 km by detecting satellite deceleration from aerodynamic drag. It used three single-axis accelerometers mounted mutually at right angles, two in the spacecraft X-Y plane and one in the Z-axis. The instrument determined acceleration from the electrostatic force needed to recenter a proof mass, outputting a digital pulse rate proportional to acceleration. Each measurement sampled for 0.25 seconds. The data allowed determination of neutral density, monitoring of the Orbit-Adjust Propulsion System (OAPS) thrust, satellite minimum altitude, spacecraft roll, and some attitude information. Nutations below 0.01° were monitored. The instrument had three sensitivity ranges: 8.E-3 Earth's gravity (G) in OAPS monitor mode; 4.E-4 G between 120 km (± 2%) and 280 km (± 10%); and 2.E-5 G between 180 km (± 2%) and 400 km (± 10%). A systematic error of up to ± 5% from drag coefficient uncertainty was possible. The highest measurement altitude assumed the instrument could sense to 0.2% of full scale. The capacitance manometer, also called Pressure Sensor B (PSB), was primarily an engineering experiment providing data on spacecraft operations, but its data were correlated with accelerometer and ion gauge data for drag evaluation. It directly measured atmospheric pressure below 200 km, with accuracy varying from about 10% at 120 km to about 40% at 180 km. The PSB consisted of two spherical, thermally controlled chambers separated by a thin membrane under radial tension. Deflection of the diaphragm from a pressure differential changed capacitance between the diaphragm and an adjacent electrode, biasing an AC bridge circuit. Air entered one chamber through two ports 180° apart and perpendicular to the spacecraft spin axis, sampling the wake-ram pressure differential twice each revolution.
End of mission
Explorer 54 operated for just under four months before its solar panels failed on 29 January 1976. Its polar orbit allowed it to sample every latitude, with the perigee sweeping through all latitudes in three months and all local times in four. Despite the early shutdown, all perigee altitude regions were sampled. Telemetry stopped, and the spacecraft reentered the atmosphere about a month later. To keep up coordinated studies with Explorer 55, Explorer 51 was turned back on 28 February 1976 to take its place.
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