Explorer 51

Did You Know?

• The spacecraft carried instrumentation for measuring proton and electron fluxes up to 25 keV.
• The payload included instruments for measuring airglow emissions and photoelectron energy spectra.

Spacecraft

The AE-C spacecraft was a multi-sided polyhedron with a diameter of approximately 140 cm and a mass of 658 kg, including 85 kg of instrumentation. Its initial elliptical orbit was altered many times in the first year using an onboard propulsion system with a 1.6 kg thruster, adjusting the perigee height to 129 km. After that, the orbit was circularized and periodically raised to about 390 km, then allowed to decay to 250 km. During the first year, the latitude of perigee moved from about 10° up to 68° north and then down to about 60° south, completing about two cycles through all local times. The spacecraft could spin at a nominal 4 rpm or be despun to 1 revolution per orbit, with the spin axis perpendicular to the orbit plane. Power came from a solar cell array, and a PCM telemetry data system operated in real-time or tape recorder mode.

Mission

The mission aimed to investigate the thermosphere, focusing on energy transfer and the processes governing its state. It studied photochemical processes accompanying the absorption of solar UV radiation in Earth's atmosphere by making closely coordinated measurements of reacting constituents and solar input. Data from Explorer 51 helped obtain the angular load distribution around the satellite, compare it with Explorer 32 data, and model hydroxyl ion emissions in the atmosphere.

Experiments

The miniature electrostatic analyzer (MESA) measured neutral atmospheric density between 120 and 400 km altitude by detecting satellite deceleration from aerodynamic drag. It contained three single-axis accelerometers mounted mutually at right angles, two in the X-Y plane and one along the Z-axis. The instrument determined acceleration from the electrostatic force needed to recenter a proof mass, outputting a digital pulse rate proportional to the applied acceleration. It measured neutral density, monitored the orbit-adjust propulsion system (OAPS) thrust, determined satellite minimum altitude, measured spacecraft roll, and provided attitude-sensing information, including monitoring nutations below 0.01°. 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%), with a possible systematic error of up to ±5% from drag coefficient uncertainty.