The Atmospheric Explorer series were three similar spacecraft which orbited the Earth from 1973 to 1981. AE-C was launched by Delta on Dec. 16, 1973 in a highly elliptical orbit of 160 km to 4300 km altitude, with an inclination of 68 degrees. After about a year its orbit was circularized and maintained within 200km to 400km throughout the rest of its lifetime. AE-C returned to Earth on Dec. 11, 1978. The ill-fated AE-D was usable from Oct. 6, 1975 until Jan. 29, 1976. AE-E lasted from Nov. 19, 1975 to June 10, 1981, with an altitude profile similar to AE-C, but at 19 degrees inclination. The three spacecraft were instrumented primarily for the analysis of the Earth's atmosphere and particles and fields in the magnetosphere.