Explorer 4, launched July 26, 1958 from Cape Canaveral, was the first of two planned satellites from the Department of Defense's Advanced Research Projects Agency (ARPA) to study the Van Allen radiation belts. It reached a height of 163 miles. Soon after orbit insertion, Explorer 4 developed a tumbling motion with a period about 6 seconds, which made interpretation of the data collected from the satellite very difficult.
Explorer 4 performed until the low-power and plastic scintillator detector failed on September 3, 1958. The Geiger-Muller tubes and the cesium/iodide crystal detectors operated until September 19. The high-power transmitter worked until October 5, likely due to the exhaustion of the power batteries.
Explorer re-entered Earth's atmosphere October 23, 1959.
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