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Explorer 5



Explorer 5 was similar in all respects to Explorer 4 and was designed with the same basic science objectives, 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. Explorer 5 launched but failed to reach orbit on August 24, 1958.

Spacecraft and Subsystems

Explorer 5 was a 2.03 m long, 0.15 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 5 launched on 24 August 1958 at 6:17:22 UT from the Cape Canaveral Missile Test Center of the Atlantic Missile Range. The spacecraft was not oriented correctly when the second stage fired about 3 minutes after liftoff, preventing it from achieving orbit.

The Explorer 5 failure occurred immediately after separation of the first stage Redstone booster from the second stage. Under normal operations, when either the propellant or oxygen is depleted in the Redstone, about 155 seconds after liftoff, the propellant main valves to the combustion chamber are shut off. Five seconds after this a timer activates six explosive bolts holding the booster to the rest of the rocket. These release the Redstone from the second stage, and coil springs push them apart in preparation for the subsequent second stage firing. In this case for some reason the Redstone continued to exert thrust after separation, causing it to catch up to the back of the second stage and collide with it approximately 12 seconds after separation. This changed the orientation of the rocket, resulting in the second stage firing in the wrong direction, so that an orbital trajectory was not achieved and the spacecraft reentered the atmosphere and fell back to Earth.

Alternate Names

  • 1958-F11
  • Explorer5

Facts in Brief

Launch Date: 1958-08-24
Launch Vehicle: Jupiter C (Juno I)
Launch Site: Cape Canaveral, United States
Mass: 17.43 kg

Funding Agency

  • Department of Defense-Department of the Air Force (United States)


  • Space Physics

Additional Information

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



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