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Explorer 55
Explorer 55, also known as AE-E (Atmosphere Explorer-E), was a NASA scientific satellite belonging to the Atmosphere Explorer series launched on 20 November 1975 from Cape Canaveral Air Force Station (CCAFS) aboard a Thor-Delta 2910 launch vehicle.
The purpose of the Explorer 55 (AE-E) mission was to investigate the chemical processes and energy transfer mechanisms that control the structure and behavior of the atmosphere of Earth and ionosphere in the region of high absorption of solar energy at low and equatorial latitudes. The simultaneous sampling at higher latitudes was carried out by the Explorer 54 (AE-D) spacecraft until its failure on 29 January 1976, and then by Explorer 51 (AE-C), until it reentered on 12 December 1978. The same type of spacecraft as Explorer 51 was used, and the payload consisted of the same types of instruments except that the low-energy electron and ultraviolet (UV) nitric oxide experiments were deleted and a backscatter UV spectrometer was added to monitor the ozone content of the atmosphere.
The 2 experiments that were deleted were more appropriate for the high-latitude regions. The perigee swept through more than 6 full latitude cycles and two local time cycles during the first year after launch when the orbit was elliptical and the perigee height was varied between 130 km (81 mi) and 400 km (250 mi). The circularization of the orbit around 390 km (240 mi) was made on 20 November 1976 and the spacecraft was raised to this height whenever it would decay to about 250 km (160 mi).
The Miniature Electrostatic Analyzer (MESA) obtained data on the neutral density of the atmosphere in the altitude range of 120 km (75 mi) to 400 km (250 mi), by the measurements of satellite deceleration due to aerodynamic drag, which is directly proportional to atmospheric density. The instrument consisted of three single-axis accelerometers, mounted mutually at right angles, two in the spacecraft X-Y plane and the other along the Z-axis. The instrument determined the applied acceleration from the electrostatic force required to recenter a proof mass. The output of the device was a digital pulse rate proportional to the applied acceleration. The sample time of each instrument was 0.25 seconds. The measurements allowed the determination of the density of the neutral atmosphere, monitored the thrust of the "Orbit-Adjust Propulsion System" (OAPS), determined the satellite minimum altitude, measured spacecraft roll, and provided some attitude-sensing information. Spacecraft nutations of less than 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 (75 mi) (± 2%) and 280 km (170 mi) (± 10%); and 2.E-5 G between 180 km (110 mi) (± 2%) and 400 km (250 mi) (± 10%). Numbers in parentheses represent errors. There may be a systematic error of up to ± 5% due to drag coefficient uncertainty. The highest measurement altitude was determined assuming the instrument could sense to 0.2% of full scale.
The Backscatter Ultraviolet instrument (BUV) monitored the spatial distribution of atmospheric ozone by measuring the intensity of the UV radiation backscattered from the Earth's atmosphere. To obtain this ozone distribution, the BUV subsystem measured direct solar radiation and backscattered UV radiation from the daytime Sun-illuminated atmosphere. The instrument consisted of a spectrometer (monochromator) and a photometer. The monochromator measured the intensity of UV radiation backscatter and reflected radiation from the Earth's atmosphere in 12 wavelengths (2555 to 3398 A) in which ozone attenuation occurs. The photometer measured the reflected UV radiation in a single wavelength span in which attenuation by ozone does not occur. The BUV had four operating modes.
This experiment was flown to measure, throughout the orbit, the individual concentrations of all thermal ion species in the mass range 1 to 72 atomic mass units (u) and in the ambient density range from 5 to 5.E6 ions/cc. The mass range was normally scanned in 1.7 seconds, but the scan time per range could be increased by command. Laboratory and inflight determination of spectrometer efficiency and mass discrimination permitted direct conversion of measured ion currents to ambient concentrations. Correlation of these measured data with the results from companion experiments, CEP (1975-107A-01) and RPA (1975-107A-04) permitted individual ion concentrations to be determined with high accuracy. The experiment's four primary mechanical components were guard ring and ion-analyzer tube, collector and preamplifier assembly, vent, and main electronics housing. A three-stage Bennett tube with 7- to 5-cycle drift spaces was flown; it was modified to permit ion concentration measurements to be obtained at low altitudes. The balance between ion-current sensitivity and mass resolution in a Bennett spectrometer may be altered by changing appropriate voltages. These voltage changes were controlled independently by ground command for each one of the three mass ranges: 1 to 4, 2 to 18, and 8 to 72.
The capacitance manometer flown on Explorer 55 (AE-E) was primarily an engineering experiment to provide data on spacecraft operations. However, data from this experiment were also correlated with accelerometer and ion gauge data in evaluating satellite drag. The manometer, also referred to as Pressure Sensor B (PSB), provided a direct measure of atmospheric pressure in the region below 200 km (120 mi). The accuracy of the PSB gauge varied from about 10% at 120 km (75 mi) to about 40% at 180 km (110 mi). The PSB consisted of two spherical, thermally controlled chambers, separated by a thin membrane stretched flat and under radial tension. Any deflection of the diaphragm caused by a pressure differential between the two sides caused a change in capacitance between the diaphragm and an adjacent electrode which biased an AC bridge circuit. Air was allowed into one of the chambers through two ports 180° apart and perpendicular to the spacecraft spin axis. Thus the wake-ram pressure differential was sampled twice each spacecraft revolution.
The cold cathode ion gauge was primarily an engineering experiment to provide data on spacecraft operation. However, data from this experiment were correlated with accelerometer and capacitance manometer data to evaluate satellite drag performance. The ion gauge, also referred to as Pressure Sensor A (PSA), measured atmospheric pressure in the region between 120 km (75 mi) and 370 km (230 mi) above the Earth's surface for values of atmospheric pressure between 1.3E-3 and 1.3E-7 mb. The estimated accuracy of the PSA was ± 20%. The cylindrically shaped sensor package consisted of a wedge-shaped orifice, a cathode near ground potential, an anode operating at about 130 VDC, and a permanent magnetic field of about 0.16t (1600 gauss). The gauge contained no primary source of ionizing electrons. The discharge was initiated by field emission and was self-sustaining at a pressure above 1.3E-7 mb. The ion current was collected at the cathode. The sensor was mounted on the spacecraft, with the orifice perpendicular to the spacecraft spin axis, which was normal to the orbital plane. The instrument was operated in two modes, spinning and despun. When the spacecraft was in a spinning mode, the PSA alternately sampled the ram and wake pressure. When the spacecraft was in the despun mode, the PSA paced 30° from the direction of motion. Data from this experiment were not tape-recorded but observed in real-time.
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Explorer 55
Explorer 55, also known as AE-E (Atmosphere Explorer-E), was a NASA scientific satellite belonging to the Atmosphere Explorer series launched on 20 November 1975 from Cape Canaveral Air Force Station (CCAFS) aboard a Thor-Delta 2910 launch vehicle.
The purpose of the Explorer 55 (AE-E) mission was to investigate the chemical processes and energy transfer mechanisms that control the structure and behavior of the atmosphere of Earth and ionosphere in the region of high absorption of solar energy at low and equatorial latitudes. The simultaneous sampling at higher latitudes was carried out by the Explorer 54 (AE-D) spacecraft until its failure on 29 January 1976, and then by Explorer 51 (AE-C), until it reentered on 12 December 1978. The same type of spacecraft as Explorer 51 was used, and the payload consisted of the same types of instruments except that the low-energy electron and ultraviolet (UV) nitric oxide experiments were deleted and a backscatter UV spectrometer was added to monitor the ozone content of the atmosphere.
The 2 experiments that were deleted were more appropriate for the high-latitude regions. The perigee swept through more than 6 full latitude cycles and two local time cycles during the first year after launch when the orbit was elliptical and the perigee height was varied between 130 km (81 mi) and 400 km (250 mi). The circularization of the orbit around 390 km (240 mi) was made on 20 November 1976 and the spacecraft was raised to this height whenever it would decay to about 250 km (160 mi).
The Miniature Electrostatic Analyzer (MESA) obtained data on the neutral density of the atmosphere in the altitude range of 120 km (75 mi) to 400 km (250 mi), by the measurements of satellite deceleration due to aerodynamic drag, which is directly proportional to atmospheric density. The instrument consisted of three single-axis accelerometers, mounted mutually at right angles, two in the spacecraft X-Y plane and the other along the Z-axis. The instrument determined the applied acceleration from the electrostatic force required to recenter a proof mass. The output of the device was a digital pulse rate proportional to the applied acceleration. The sample time of each instrument was 0.25 seconds. The measurements allowed the determination of the density of the neutral atmosphere, monitored the thrust of the "Orbit-Adjust Propulsion System" (OAPS), determined the satellite minimum altitude, measured spacecraft roll, and provided some attitude-sensing information. Spacecraft nutations of less than 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 (75 mi) (± 2%) and 280 km (170 mi) (± 10%); and 2.E-5 G between 180 km (110 mi) (± 2%) and 400 km (250 mi) (± 10%). Numbers in parentheses represent errors. There may be a systematic error of up to ± 5% due to drag coefficient uncertainty. The highest measurement altitude was determined assuming the instrument could sense to 0.2% of full scale.
The Backscatter Ultraviolet instrument (BUV) monitored the spatial distribution of atmospheric ozone by measuring the intensity of the UV radiation backscattered from the Earth's atmosphere. To obtain this ozone distribution, the BUV subsystem measured direct solar radiation and backscattered UV radiation from the daytime Sun-illuminated atmosphere. The instrument consisted of a spectrometer (monochromator) and a photometer. The monochromator measured the intensity of UV radiation backscatter and reflected radiation from the Earth's atmosphere in 12 wavelengths (2555 to 3398 A) in which ozone attenuation occurs. The photometer measured the reflected UV radiation in a single wavelength span in which attenuation by ozone does not occur. The BUV had four operating modes.
This experiment was flown to measure, throughout the orbit, the individual concentrations of all thermal ion species in the mass range 1 to 72 atomic mass units (u) and in the ambient density range from 5 to 5.E6 ions/cc. The mass range was normally scanned in 1.7 seconds, but the scan time per range could be increased by command. Laboratory and inflight determination of spectrometer efficiency and mass discrimination permitted direct conversion of measured ion currents to ambient concentrations. Correlation of these measured data with the results from companion experiments, CEP (1975-107A-01) and RPA (1975-107A-04) permitted individual ion concentrations to be determined with high accuracy. The experiment's four primary mechanical components were guard ring and ion-analyzer tube, collector and preamplifier assembly, vent, and main electronics housing. A three-stage Bennett tube with 7- to 5-cycle drift spaces was flown; it was modified to permit ion concentration measurements to be obtained at low altitudes. The balance between ion-current sensitivity and mass resolution in a Bennett spectrometer may be altered by changing appropriate voltages. These voltage changes were controlled independently by ground command for each one of the three mass ranges: 1 to 4, 2 to 18, and 8 to 72.
The capacitance manometer flown on Explorer 55 (AE-E) was primarily an engineering experiment to provide data on spacecraft operations. However, data from this experiment were also correlated with accelerometer and ion gauge data in evaluating satellite drag. The manometer, also referred to as Pressure Sensor B (PSB), provided a direct measure of atmospheric pressure in the region below 200 km (120 mi). The accuracy of the PSB gauge varied from about 10% at 120 km (75 mi) to about 40% at 180 km (110 mi). The PSB consisted of two spherical, thermally controlled chambers, separated by a thin membrane stretched flat and under radial tension. Any deflection of the diaphragm caused by a pressure differential between the two sides caused a change in capacitance between the diaphragm and an adjacent electrode which biased an AC bridge circuit. Air was allowed into one of the chambers through two ports 180° apart and perpendicular to the spacecraft spin axis. Thus the wake-ram pressure differential was sampled twice each spacecraft revolution.
The cold cathode ion gauge was primarily an engineering experiment to provide data on spacecraft operation. However, data from this experiment were correlated with accelerometer and capacitance manometer data to evaluate satellite drag performance. The ion gauge, also referred to as Pressure Sensor A (PSA), measured atmospheric pressure in the region between 120 km (75 mi) and 370 km (230 mi) above the Earth's surface for values of atmospheric pressure between 1.3E-3 and 1.3E-7 mb. The estimated accuracy of the PSA was ± 20%. The cylindrically shaped sensor package consisted of a wedge-shaped orifice, a cathode near ground potential, an anode operating at about 130 VDC, and a permanent magnetic field of about 0.16t (1600 gauss). The gauge contained no primary source of ionizing electrons. The discharge was initiated by field emission and was self-sustaining at a pressure above 1.3E-7 mb. The ion current was collected at the cathode. The sensor was mounted on the spacecraft, with the orifice perpendicular to the spacecraft spin axis, which was normal to the orbital plane. The instrument was operated in two modes, spinning and despun. When the spacecraft was in a spinning mode, the PSA alternately sampled the ram and wake pressure. When the spacecraft was in the despun mode, the PSA paced 30° from the direction of motion. Data from this experiment were not tape-recorded but observed in real-time.
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