Explorer 51, also called as AE-C (Atmospheric Explorer-C), was a NASA scientific satellite belonging to series Atmosphere Explorer, being launched on 16 December 1973, at 06:18:00 UTC, from Vandenberg board 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.