different ion currents and the total current were made sequentially for 4 seconds in high sensitivity and 4 seconds in low sensitivity. A period of 64 seconds was required for the entire measurement cycle. Included in the cycle was an operation to correct any DC drift of the zero voltage level in the output signal. One spectrometer produced useless data due to a malfunction. The other detector system experienced intermittent degeneration of the amplifier output, and, consequently, the data were good only during certain periods. This degeneration was not a result of instrument malfunction but of an unexpected spacecraft attitude which oriented the sensor toward the Sun and caused it to overheat. Pressure Gauges Two Redhead (cold cathode) and two Bayard-Alpert (hot filament) ionization vacuum gauges were used to measure the neutral particle density and ambient pressure of the upper atmosphere between and . The pressure gauges were operated for 4-minutes periods when the satellite was within range of a ground telemetry station. The neutral particles were ionized by electron bombardment, and the resulting ion currents were detected and converted to voltages suitable for telemetry. These two types of sensors together were capable of measuring over the pressure range 10.E-4 torr (10.E12 molecules/cc) to 10.E-11 torr. One Bayard-Alpert gauge suffered a loss in sensitivity, and no useful data were obtained from it. The remaining three gauges operated normally. Orbital decay The spacecraft decayed from orbit after 1,325 days on 24 November 1966. See also Explorer 32 Explorer program References External links NASA's Explorer Missions Gunter's Space Page - information on Explorer 17 Space History Notes Spacecraft launched in 1963 Satellites formerly orbiting Earth Explorers Program