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Explorer 4



Explorer 4 (1958 Epsilon) was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts and to observe the effects of the Project Argus A-bomb detonations. An unexpected tumble motion of the satellite made the interpretation of the detector data very difficult.

Spacecraft and Subsystems

Explorer 4 was identical in size to the earlier Explorers but was over 3 kg heavier, primarily due to a larger instrument payload. The satellite was a 2.03 m long, 0.152 m diameter cylinder and nosecone that comprised the fourth stage of the Jupiter-C launch vehicle. The on-orbit mass (after fuel burnout) was 17.43 kg. The spacecraft body was made of stainless AISI-410 steel, 0.058 cm thick. The surface was sandblasted, no aluminum oxide striping was used as on earlier Explorers.

The base of the cylinder held the Sergeant solid-fuel rocket motor. The Mallory mercury batteries for the low power transmitter were in the upper part of the nose cone. Below these was the low power (10 mW, 108.00 MHz) transmitter for the carrier and sub-carrier signals, which used the stainless steel nose cone as an antenna.

Below the nose cone was the detector deck, holding the instrumentation for the radiation experiments, the command receiver, for interrogations, high power playback transmitter (25-30 mW, 108.03 MHz) for interrogation response, cosmic ray experiment electronics, and Mallory mercury batteries for the high power transmitter. The lower spacecraft body was used as the antenna for the high power transmitter. A heat radiation shield was mounted between the payload and the rocket motor. Temperature gauges were mounted at various locations in the spacecraft.

The radiation experiment comprised four detectors, two Geiger-Mueller counters and two scintillation counters. One of the Geiger-Mueller counters was unshielded and one was shielded with 1.6 g/square cm lead to screen out lower energy particles and radiation. One scintillator was a CsI crystal with a 0.8 mg/square cm nickel foil window, the other was a plastic scintillator with a 0.14 g/square cm aluminum window. All radiation experiments were within the spacecraft wall, which provided a protection of 1.2 g/square cm iron.

Juno 1 Launch Vehicle

The launch vehicle was a Juno 1, a variant of the three-stage Jupiter-C 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 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.

Mission Profile

Explorer 4 was launched on 26 July 1958 at 15:00:07 UT from the Cape Canaveral Missile Test Center of the Atlantic Missile Range. The spacecraft was injected into an initial 263 x 2213 km orbit with an inclination of 50.33 degrees and a period of 110.2 minutes at 15:07 UT. This was a much higher inclination and apogee than previous Explorers 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.

Explorer 4 was in orbit and operational during the three Project Argus launches 27 August to 6 September, part of the mission objective was to observe the effects of these high-altitude A-bomb detonations on the space environment. 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. The spacecraft decayed from orbit after 454 days on October 23, 1959.

Alternate Names

  • 00009
  • 1958 Epsilon 1
  • Explorer4

Facts in Brief

Launch Date: 1958-07-26
Launch Vehicle: Jupiter C (Juno I)
Launch Site: Cape Canaveral, United States
Mass: 17.43 kg

Funding Agencies

  • Department of Defense-Department of the Army (United States)
  • NASA Jet Proplusion Laboratory (United States)


  • Space Physics

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Mr. J. FroelichProject ManagerNASA Jet Propulsion Laboratory
Dr. Albert R. HibbsProject ScientistNASA Jet Propulsion Laboratory
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