AE-A

Description

Atmospheric Explorer A (AE-A, or Explorer 17) was a spin-stabilized sphere 0.95 m in diameter. The spacecraft was vacuum sealed in order to prevent contamination of the local atmosphere. Explorer 17 carried four pressure gauges (two cold-cathode ion gauges and two hot-cathode ion gauges) for the measurement of total neutral particle density, two mass spectrometers for the measurement of certain neutral particle concentrations, and two electrostatic probes for ion concentration and electron temperature measurements.

The primary objectives of AE-A were to determine the diurnal and spatial variation of electron density and temperature, and of the neutral parameters: density, composition, pressure, and temperature in the regions between 250 and 900 km altitude; to compare values of direct density measurements with the values calculated from satellite drag observations; and to advance the state of the art of direct atmospheric density measurements by study of detector outgassing characteristics and degradation and reliability over extended time periods and interaction between spacecraft instruments and components.

Spacecraft and subsystems

Atmospheric Explorer A was a 95 cm diameter sphere composed of 0.06 cm thick stainless steel. The sphere was vacuum-tight and pressure sealed. Total mass was 185.6 kg. The spacecraft was spin-stabilized at about 90 rpm with a nutation damper to ensure rotation about the spin axis. The experiments were held inside the sphere with ports in the sphere wall to allow the introduction of atmospheric gases. The detectors were all sealed under vacuum and not opened until orbit.

Attitude knowledge for spin axis orientation was supplied by a digital solar aspect sensor, slit fan sensor, a Sun-Moon switch, and visible and infrared Earth horizon sensors. Power of approximately 134 W was supplied by 68 kg of Ag/Zn batteries with a capacity of 60 to 75 hours, or about 800 experiment turn-ons. No active thermal control was used.

Communications were via four canted turnstile RF antennas protruding from the bottom of the craft and two 500 mW transmitters, telemetry at 136.32 MHz and a beacon at 136.56 MHz, and two command receivers. There was no tape recorder for data storage, data were transmitted in real time in four-minute turn-on periods when the satellite was over a Minitrack station. Data was handled by a 40 digital channel solid-state pulse code modulation / phase modulation (PCM/PM) telemetry system. It could transmit up to twenty 9-bit words per second on each channel. (This is slightly less than the published data rate of 8640 bits/sec, possibly 24 words/sec were transmitted.) Three of the channels were required for synchronization with the Minitrack/STADAN ground network.

Mission Profile

AE-A was launched on 3 April 1963 into a 255 x 916 km altitude orbit with an inclination of 57.6 degrees. The satellite perigee was initially at 40 N, it moved north to 58 N and then south to 20 S over the lifetime of the satellite. Battery power was exhausted on July 10, 1963, after the satellite was commanded on over 650 times. Three of the four pressure gauges and both electrostatic probes operated normally. One spectrometer malfunctioned, and the other operated intermittently. The mission discovered a belt of neutral helium around the Earth. The orbit decayed and AE-A reentered the Earth's atmosphere on 24 November 1966.