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Explorer 4 | font size="+1"> Explorer IV | | olspan="2" bgcolor="#000000" align="center" font size="-1" color="#ffffff"| | | b> Organization: | Army Ballistic Missile Agency | | b> Major Contractors: | Jet Propulsion Laboratory | | b> Mission Type: | Earth Science | | b> Satellite of: | Earth | | b> Launch: | July 26, 1958 at 15:07:00 UTC | | b> Launch Vehicle: | Jupiter-C | | b> Decay: | October 23, 1959 | | b> Mission Duration: | 71 days | | b> Mass: | 25.5 kg | | b> NSSDC ID: | 1958-005A | | b> Webpage: | NASA NSSDC Master Catalog | | olspan="2" cellspacing="0" cellpadding="2" bgcolor="#skyblue"|Orbital elements | | b> Semimajor Axis: | 7,616.2 km | | b> Eccentricity: | .127936 | | b> Inclination: | 50.3 | | b> Orbital Period: | 110.2 minutes | | b> Apogee: | 2,213 km | | b> Perigee: | 263 km | | b> Orbits: | ~5,930 | | olspan="2" cellspacing="0" cellpadding="2" bgcolor="#skyblue"|Instruments | | b> Geiger-Mueller detector : | Measurements of charged particles | | b> Caesium iodide crystal detectors : | Measurements of charged particles | | b> Plastic scintillation counter : | Measurements of charged particles | Explorer 4 was a US satellite launched on July 26, 1958. It was instrumentated by Dr. James van Allen's group. The Department of Defense's Advanced Research Projects Agency had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of nuclear explosions upon these belts (and the Earth's magnetosphere in general), however Explorer IV was the only such satellite launched. Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts. An unexpected tumble motion of the satellite made the interpretation of the detector data very difficult. The low-power transmitter and the plastic scintillator detector failed September 3, 1958. The two Geiger-Mueller tubes and the caesium iodide crystal detectors continued to operate normally until September 19, 1958. The high-power transmitter ceased sending signals on October 5, 1958. It is believed that exhaustion of the power batteries caused these failures. The spacecraft decayed from orbit after 454 days on October 23, 1959. See also External links
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