Explorer 51, also known as AE-C (Atmospheric Explorer-C), was a NASA scientific satellite belonging to the Atmosphere Explorer series launched on 16 December 1973, at 06:18:00 UTC, from Vandenberg aboard a Delta 1900 launch vehicle.
Spacecraft The AE-C spacecraft was a multi-sided polyhedron with a diameter of approximately . It weighed including of instrumentation. The initial elliptical orbit was altered many times in the first year of life by means of an onboard propulsion system employing a thruster. The purpose of these changes was to alter the perigee height to . After this period, the orbit was circularized and was raised periodically to about when it would decay to altitude. During the first year, the latitude of perigee moved from about 10° up to 68° north and then down to about 60° south. During this period about two cycles through all local times were completed. The spacecraft could be operated in either of two modes: spinning at a nominal 4 rpm or despun to 1 revolution per orbit. The spin axis was perpendicular to the orbit plane. Power was supplied by a solar cell array. The spacecraft used a PCM telemetry data system that operated in real-time or in a tape recorder mode. The payload included instrumentation for the measurement of solar ultraviolet; the composition of positive ions and neutral particles; the density and temperature of neutral particles, positive ions and electrons; the measurement of airglow emissions, photoelectron energy spectra, and proton and electron fluxes up to 25 keV.
Mission The purpose of the Explorer 51 mission was to investigate the thermosphere, with emphasis on the energy transfer and processes that govern its state. The study of photochemical processes accompanying the absorption of solar UV radiation in the Earth's atmosphere was accomplished by making closely coordinated measurements of reacting constituents and the solar input.
The data from Explorer 51 served, among other things, to obtain the angular load distribution around the satellite and compare it with the data of Explorer 32 and model the hydroxyl ion emissions in the Earth's atmosphere.
Instruments The satellite carried instruments to measure ultraviolet solar radiation, temperature, composition and density of the positive ions, of the neutral particles and of the electrons, to measure atmospheric glow emissions, the energy spectra of the photoelectrons and the protons and electrons flows with energies up to 25 keV.
Experiments Miniature Electrostatic Accelerometer (MESA) The miniature electrostatic analyzer (MESA) obtained data on the neutral density of the atmosphere in the altitude range of from the measurements of satellite deceleration due to aerodynamic drag. 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 measurements allowed 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 (± 2%) and (± 10%); and 2.E-5 G between (± 2%) and (± 10%). Numbers in parentheses represent errors; in addition, 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.
Bennett Ion-Mass Spectrometer (BIMS) This experiment was flown to measure, throughout the Explorer 51 (AE-C) orbit, the individual concentrations of all thermal ion species in the mass range of 1 to 72 atomic mass units (u), and in the ambient density range from 8.E1 to 5.E6 ions/cc. Any combination of the following three mass ranges, expressed in units, were selected by ground command: range A, -1 to 4, range B, -2 to 18, range C, -8 to 72. Each range was normally scanned in 1.7 seconds (approximately along orbit). Normal operation consisted in sequence ABCABC (1 to 72 units in 5.1 seconds). Laboratory and inflight determination of spectrometer efficiency and mass discrimination permitted direct conversion of measured ion currents to ambient concentrations. The experiment's four primary mechanical components were the guard ring and ion-analyzer tube, collector and preamplifier assembly, vent, and main electronics housing. The guard ring was normally at ground potential, but it could be placed at -6 volts by command if desirable, e.g., if the spacecraft acquired a positive charge. A three-stage Bennett tube with 7 to 5-cycle drift spaces was flown and was modified to permit ion concentration measurements to be obtained at low altitudes. The frequency of the 30-volt peak-to-peak Radio Frequency (RF) voltage varied with the mass range measured: range A, -10 MHz, range B, -5 MHz, and range C, -2.5 MHz. Primary analog instrument output was a compressed ion current spectrum which displayed the full dynamic range of the amplifier system on a single telemetry channel. Onboard data processing provided a readout of primary experiment data in the form of two digital words for each peak in the ion spectrum. One 8-bit word indicated peak amplitude (current) and the other 8-bit word identified sweep position, i.e., species identification.
Capacitance Manometer The capacitance manometer 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), measured atmospheric pressure in the region below . The accuracy of the PSB gauge varied from about 10% at to about 40% at . 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 was measured by an Alternating current (AC) bridge circuit. Air was permitted into one of the chambers through two ports 180° apart and perpendicular to the spacecraft spin axis. Thus, the wave-ram pressure differential was sampled twice for each spacecraft revolution.
Closed-Source Neutral Mass Spectrometer This experiment measured in situ the spatial distribution and temporal changes of the concentrations of the neutral atmospheric species. In addition, new insight into in situ measurement techniques was obtained from comparisons of these measurements with those obtained from other on-board experiments; namely, open source spectrometer (1973-101A-07), solar Extreme ultraviolet (EUV) spectrophotometer (1973-101A-06), and density accelerometer (1973-101A-02). The mass-spectrometer sensor had a gold-plated stainless steel thermalizing chamber and ion source, a hyperbolic-rod quadrupole analyzer, and an off-axis electron multiplier. Five different sequences of mass selections were available and, expressed in atomic mass units (u), were: (a) geophysical -1, 2, 4, total, 16, 28, 32, selected, 40, (b) analytical -12, 14, 18, 20, 22, 30, 44, calibrate, zero, (c) individual -selected, selected, selected, ... (any mass 1 to 44), (d) sweep digital -1, 2, 3, 4, 5, ... 45 (in 3/16 units steps), (e) sweep analog -2, 3, 4, 5, ... 45 (continuous). Five operational formats were available and selected by ground command, and each one contained a different combination of the five mass selection sequences listed above. When operating in the "normal format", the analyzer measured all masses in the range 1 to 44 with emphasis on hydrogen, helium, oxygen, nitrogen and argon. Another format was optimized for minor constituent studies of any individual gas species in the measured range. Spatial resolution was determined primarily by the mode of spacecraft operation. In orbit, the pre-sealed spectrometer was opened, and the atmospheric constituents passed through a knife-edged orifice into the thermalization chamber and ion source. Selected ions left the quadrupole analyzer through a weak focusing lens and were accelerated into a 14-stage electron multiplier, where they were turned 90° to strike the first dynode. For each impacting ion, the multiplier output was a pulse of 2.E6 electrons. These output pulses constituted the measurement, and the count rate was proportional to the chamber density of the selected species. These density values were then converted to ambient concentrations. The analyzer normally operated at a resolution of 1 unit (u) over the mass range. Pulses occurring during 0.015-second integration intervals were accumulated in a 16-bit counter. Multiple integration periods (up to 16) were assigned to each measurement for less dense atmospheric species. Automatically selected ranges of ionizing electron currents were used. The overall dynamic range of the measurements was greater than 1.E7.
Cold Cathode Ion Gauge The cold cathode ion gauge flown on Explorer 51 (AE-C) was primarily an engineering experiment to provide data on spacecraft operation. However, data from this experiment was 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 and above the Earth's surface for values of atmospheric pressure between 1.3E-3 to 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 1300 VDC, and a permanent magnetic field of about 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 could be operated in two modes, spinning or 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 faced 30° from the direction of motion. Data from this experiment were not tape-recorded but were observed in real-time.
Cylindrical Electrostatic Probes (CEP)