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 scientist at the University of Iowa. He was instrumental in establishing the field of magnetospheric research in space. The Van Allen radiation belts were named afte ...

's group. The

Department of Defense Department of Defence or Department of Defense may refer to: Current departments of defence * Department of Defence (Australia) * Department of National Defence (Canada) * Department of Defence (Ireland) * Department of National Defense (Philipp ...

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 Adv ...

(ARPA) had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of

nuclear explosions A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, tho ...

upon these belts (and the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

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 dynam ...

in general), however Explorer 4 was the only such satellite launched as the other,

Explorer 5 Explorer 5 was a United States satellite with a mass of . It was the last of the original series of Explorer satellites built, designed, and operated by the Jet Propulsion Laboratory. Background Explorer 5 was similar in all respects to Ex ...

, suffered launch failure. Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s and

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s) trapped in the terrestrial radiation belts.

Juno I launch vehicle

The launch vehicle was a

Juno I The Juno I was a four-stage American space launch vehicle, used to launch lightweight payloads into low Earth orbit. The launch vehicle was used between January 1958 to December 1959. The launch vehicle was a member of the Redstone launch veh ...

, 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 unmanned sub-orbital spaceflights in 1956 and 1957 to test re-entry nosecones that were later to be deployed on the more a ...

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 (abbreviated to Sgt. and capitalized when used as a named person's title) is a rank in many uniformed organizations, principally military and policing forces. The alternative spelling, ''serjeant'', is used in The Rifles and other uni ...

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 Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

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 the previous year; the Soviet Union's ...

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

s (E>700 keV) and

s (E>10

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...

), a shielded directional

cesium iodide Caesium iodide or cesium iodide (chemical formula CsI) is the ionic compound of caesium and iodine. It is often used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are hi ...

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

(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, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more specif ...

. 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 Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...

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 launch heads located at Cape Canaveral Space Force Station and the Kennedy Space Center (KSC), Florida. The range ha ...

. 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 Operation Argus was a series of United States low-yield, high-altitude nuclear weapons tests and missile tests secretly conducted from 27 August to 9 September 1958 over the South Atlantic Ocean. The tests were performed by the Defense Nucle ...

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 A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both 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