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Explorer 4 was a USThe United States of America also referred to as the United States U. America ¹ or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in satelliteFor other uses, please see Satellite (disambiguation A satellite is an object that orbits another object (known as its primary . The term is often used to describe an artificial satellite (as opposed to natural satellites, or moons). Because all objects e launched on July 26, 1958. It was instrumentated by Dr. James van Allen's group. The Department of DefenseThe United States Department of Defense abbreviated DoD or DOD and sometimes called the Defense Department is a civilian Cabinet organization of the United States government. The Department of Defense controls the U. military and is headquartered at The P's Advanced Research Projects Agency had initially planned two satellites for the purposes of studying the Van Allen radiation beltThe Van Allen radiation belt is a torus of energetic charged particles around Earth, trapped by Earth's magnetic field. When the belts "overload", particles strike the upper atmosphere and fluoresce, causing the polar aurora. The presence of a radiation bs and the effects of nuclear explosionsmushroom cloud of the atomic bombing of Nagasaki, Japan, in 1945 lifted nuclear fallout some 60,000 feet (18 km) above the epicenter. A nuclear weapon is a weapon that derives its energy from nuclear reactions and has enormous destructive power a single n 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.