Explorer 4
Explorer 4
Mission typeEarth science
OperatorArmy Ballistic Missile Agency
Harvard designation1958 Epsilon 1
COSPAR ID1958-005A
SATCAT no.00009
Mission duration71 days
Spacecraft properties
ManufacturerJet Propulsion Laboratory
Launch mass25.50 kilograms (56.2 lb)
Start of mission
Launch dateJuly 26, 1958, 15:00:57 (1958-07-26UTC15:00:57Z) UTC
RocketJuno I
Launch siteCape Canaveral LC-5
End of mission
Last contactOctober 5, 1958 (1958-10-06)
Decay dateOctober 23, 1959
Orbital parameters
Reference systemGeocentric
RegimeMedium Earth
Semi-major axis7,616.2 kilometers (4,732.5 mi)
Perigee altitude263 kilometers (163 mi)
Apogee altitude2,213 kilometers (1,375 mi)
Inclination50.29999923706055 degrees
Period110.20 minutes
RAAN82.42 degrees
Argument of perigee57.72 degrees
Mean anomaly303.52 degrees
Mean motion15.52
Epoch2 October 1959, 06:53:14 UTC
Revolution no.6070
Explorer program

Explorer 4 was an American 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 belts 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 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.


Launched from a Juno I rocket, the mission remained secret from the public for six months.[1]

The satellite telemetry was analyzed for three Operation Argus nuclear weapons tests at high altitude.

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 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.[2]

See also


  1. ^ Herlihy, Ed (Narrator). Project Argus — “Greatest Experiment”: 3 A-Blasts In Space (video). Universal International News. Event occurs at 29s. Retrieved September 9, 2012. “To monitor the radiation shell in outer space, the satellite Explorer 4 was launched. And all of this in a secrecy not broken for six months.”
  2. ^ "Explorer 4". NASA. Retrieved 9 October 2021.