Explorer 4


Spacecraft n: 15

USA satellite n: 11

Launch data:

Designation00009 / 1958Eps 1 / 58005A
Launch date26 Jul 1958 - 15:00:57 UT
Launch siteCC, LC5
Launch vehicleJupiter C (Juno 1)
MissionScientific: ionosphere study
Earth orbit on 26 Jul 1958:
Perigee/Apogee263 x 2213 km
Eccentricity 0.128
Period110,18 min


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 cesium 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.

Spacecraft data:

Prime contractor  Jet Propulsion Laboratory (JPL)
Operator  NASA
Mass at launch  17,5 kg
Payload 8 kg
Basic shape  Cylinder
Dimension  2.03 long 0.15 dia
Equipment  Cosmic-ray detection package, temperature sensors
micrometeorite impact microphone, micrometeorite erosion gauges

End of life

Decay23 Oct 1959
Lifetime454 days

Ref.: #1, #2, #7, #14, #15, #27, #206 - update: 10.07.04 Home