Explorer 4
Encyclopedia
Explorer 4 was a US
satellite
launched on July 26, 1958. It was instrumented 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 belt
s and the effects of nuclear explosions
upon these belts (and the Earth
's magnetosphere
in general), however Explorer 4 was the only such satellite launched.
Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (proton
s and electron
s) 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-Müller tube
s 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.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
launched on July 26, 1958. It was instrumented by Dr. James van Allen
James Van Allen
James Alfred Van Allen was an American space scientist at the University of Iowa.The Van Allen radiation belts were named after him, following the 1958 satellite missions in which Van Allen had argued that a Geiger counter should be used to detect charged particles.- Life and career :* September...
's group. The Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...
's Advanced Research Projects Agency had initially planned two satellites for the purposes of studying the Van Allen radiation belt
Van Allen radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...
s and the effects of nuclear explosions
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
upon these belts (and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
in general), however Explorer 4 was the only such satellite launched.
Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) 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-Müller tube
Geiger-Müller tube
A Geiger–Müller tube is the sensing element of a Geiger counter instrument that can detect a single particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in...
s and the caesium iodide
Caesium iodide
Caesium iodide is an ionic compound often used as the input phosphor of an x-ray image intensifier tube found in fluoroscopy equipment....
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
Orbital decay
Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar...
from orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
after 454 days on October 23, 1959.