Explorer S-45A (satellite)

The Explorer S-45A, also known as SEPO (Space Environment and Plasma Observation) or Injun 5, was a NASA satellite launched on August 8, 1968, from Vandenberg Air Force Base via a Thor-Agena D rocket. It was part of the Explorer program, a long-running series of United States space exploration missions. The primary scientific objective of Explorer S-45A was to study the Earth's magnetosphere, ionosphere, and the radiation belts, focusing particularly on wave-particle interactions, auroral phenomena, and the distribution of charged particles. The satellite carried a suite of instruments, including detectors for measuring charged particle fluxes, plasma density, and electromagnetic waves.

Explorer S-45A was placed into a highly elliptical, polar orbit which provided comprehensive coverage of the Earth's radiation belts and allowed for repeated passes through the auroral zones. This orbit facilitated the acquisition of data on the dynamic processes occurring in these regions. The data collected by the satellite contributed significantly to our understanding of space weather and its effects on spacecraft and terrestrial systems.

The spacecraft was spin-stabilized and powered by solar cells. It operated successfully for several years, returning valuable scientific data until its eventual decommissioning. The data gathered by Explorer S-45A continues to be analyzed and used by researchers studying space physics and the Earth's space environment. The mission is considered a significant contribution to the exploration of the magnetosphere and the understanding of wave-particle interactions in space.