Explorer 4 was an American satellite launched on 26 July 1958. It was instrumented by Dr.

James van Allen James Alfred Van Allen (September 7, 1914August 9, 2006) was an American space physicist at the University of Iowa. He was instrumental in establishing the field of magnetospheric research in space. The Van Allen radiation belts were named af ...

's group. The

Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an executive department of the U.S. federal government charged with coordinating and supervising the six U.S. armed services: the Army, Navy, Marines, Air Force, Space Force, ...

Advanced Research Projects Agency The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...

(ARPA) had initially planned two satellites for the purposes of studying the

Van Allen radiation belt The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...

s and the effects of nuclear explosions upon these belts (and the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

in general), however Explorer 4 was the only such satellite launched as the other, Explorer 5, suffered launch failure. 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.

Juno I launch vehicle

The launch vehicle was a Juno I, a variant of the three-stage

Jupiter-C The Jupiter-C was an American research and development vehicle developed from the Jupiter-A. Jupiter-C was used for three Uncrewed vehicle, uncrewed sub-orbital spaceflights in 1956 and 1957 to test Re-entry vehicle, re-entry nosecones that were ...

with an added fourth propulsive stage, which in this case was the Explorer 4. The first stage was an upgraded Redstone liquid-fueled rocket. The second stage comprised a cluster of eleven

Sergeant Sergeant (Sgt) is a Military rank, rank in use by the armed forces of many countries. It is also a police rank in some police services. The alternative spelling, ''serjeant'', is used in The Rifles and in other units that draw their heritage f ...

solid-fuel rocket motors and the third stage held three Sergeants. The booster was equipped to spin the fourth stage in increments, leading to a final rate of 750 rpm about its long axis.

Instrument

Charged Particle Detector

The purpose of this experiment was to extend the first measurements of the trapped radiation belt discovered with

Explorer 1 Explorer 1 was the first satellite launched by the United States in 1958 and was part of the U.S. participation in the International Geophysical Year (IGY). The mission followed the first two satellites, both launched by the Soviet Union duri ...

and Explorer 3 and to provide measurements of artificially injected electrons from the three high-altitude Argus nuclear detonations. Four separate radiation detectors were used in the experiment: a shielded directional plastic scintillation counter sensitive to electrons (E>700 keV) and protons (E>10 MeV), a shielded directional caesium iodide scintillation counter sensitive to electrons (E>20 keV) and protons (E>400 keV), an omnidirectional Anton type 302 Geiger–Müller (GM) counter sensitive to electrons (E>3 MeV) and protons (E>30 MeV), and a shielded omnidirectional Anton type 302 Geiger-Müller tube sensitive to electrons (E>5 MeV) and protons (E>40 MeV). The plastic scintillation counter and the caesium iodide (CsI) scintillation counter were each viewed by a separate

Photomultiplier tube Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible light, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum t ...

. These detectors were mounted orthogonally to the longitudinal axis of the satellite with apertures facing in opposite directions. The two GM counters were located side by side along the satellite longitudinal axis.

Mission

Explorer 4 was launched on 26 July 1958 at 15:00:07 GMT from the Cape Canaveral Missile Test Center of the

Atlantic Missile Range The Eastern Range (ER) is an American rocket range (Spaceport) that supports missile and rocket launches from the two major List of rocket launch sites, launch heads located at Cape Canaveral Space Force Station and the Kennedy Space Center ( ...

. The spacecraft was injected into an initial orbit with an inclination of 50.30° and a period of 110.20 minutes at 15:07 GMT. This was a much higher inclination and apogee than previous Explorer to allow it to sample more area at higher altitudes. Soon after orbit insertion, the spacecraft developed an end-over-end tumbling motion with a period of about 6 seconds, which affected the measurements and signal level throughout the mission. The mission remained secret from the public for six months. The satellite telemetry was analyzed for three Operation Argus nuclear weapons tests at high altitude. Explorer 4 was in orbit and operational during the three Project Argus launches 27 August 1958 to 6 September 1958, part of the mission objective was to observe the effects of these high-altitude A-bomb detonations on the space environment. 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 on 3 September 1958. The two Geiger-Müller tubes and the caesium iodide crystal detectors continued to operate normally until 19 September 1958. The high-power transmitter ceased sending signals on 5 October 1958. It is believed that exhaustion of the power batteries caused these failures. The spacecraft decayed from orbit after 454 days on 23 October 1959.

References

External links