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Explorer 7



Explorer 7 (1959 Iota) was designed to measure solar X-ray and Lyman-alpha flux, trapped energetic particles, and heavy primary cosmic rays (Z>5). Secondary objectives included collecting data on micrometeoroid penetration and molecular sputtering and studying the Earth-atmosphere heat balance.

Spacecraft and Subsystems

The spin-stabilized satellite's external structure consisted of two truncated conical fiberglass shells joined by a cylindrical aluminum center section. The spacecraft was 76 cm (30 in.) wide at its equator and about 76 cm high with a payload mass of about 41.5 kg. The spacecraft was powered by approximately 3000 solar cells mounted on both the upper and lower shells. Additional power was provided by 15 rechargeable nickel-cadmium batteries that were positioned on its equator near the outer skin as an aid in maintaining a proper spin rate. Two crossed dipole (1 W, 20 MHz) telemetry antennas projected outward from the center section, and a 108-MHz antenna used for tracking was mounted on the bottom of the lower shell.

Located around the periphery of the center section were five bolometers for thermal radiation measurements and three cadmium sulfide micrometeoroid detector cells. A cylindrical ion chamber (lithium flouride window) and a beryllium window X-ray chamber were located on opposite sides of the upper cone, and a cosmic-ray Geiger counter was located on the very top. A primary cosmic-ray ionization chamber was located within the center portion of the upper cone. Communications and tracking were provided by a 108 MHz transmitter at 15 mW designed to operate for 2 months and a 20 MHz transmitter at 600 mW powered by the solar cells designed for cut-off after about one year.

The Juno II launch vehicle was a four-stage rocket with a total liftoff mass of approximately 55,000 kg (121,000 lbs). The first stage was a modified Army Jupiter IRBM with a diameter of 267 cm (8.75 ft). The second stage comprised 11 scaled-down solid Sergeant rockets in a cluster. The third stage was a cluster of three scaled-down Sergeants, and the fourth stage was a single Sergeant rocket. Total height was 23.2 m (76 ft).

Mission Profile

Explorer 7 was launched on a Juno II at 15:31 UT (11:31 a.m. EDT) on 13 October 1959 from the Atlantic Missile Range into a 573 x 1073 km orbit. On 16 June 1960, NASA announced one of the four frequency modulated subcarriers on the second transmitter had become erratic and the information it was transmitting on 3 of the 7 experiments was no longer intelligible. The tracking beacon ceased transmitting on 5 December 1959. As of September 1960 the orbit was 554 x 1083 km (344 x 673 miles) with an inclination of 50.3 degrees and a period of 101.2 minutes. Useful real-time data were transmitted from launch through February 1961 and intermittently until August 24, 1961. The spacecraft provided significant geophysical information on radiation and magnetic storms, demonstrated methods of controlling internal temperatures, recorded the first micrometeorite penetration of a sensor in flight, and detected large scale weather patterns.

Image courtesy of the University of Iowa

Alternate Names

  • 00022
  • 1959 Iota 1
  • Explorer7
  • S 1A

Facts in Brief

Launch Date: 1959-10-13
Launch Vehicle: Juno
Launch Site: Cape Canaveral, United States
Mass: 41.5 kg

Funding Agency

  • Department of Defense-Department of the Army (United States)


  • Space Physics
  • Earth Science

Additional Information

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



NameRoleOriginal AffiliationE-mail
Mr. Paul ButlerProgram ManagerNASA Goddard Space Flight Center
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center
Explorer 7 diagram

Explorer 7 Diagram.

Explorer 7 replica

Replica of Explorer 7 at the Smithsonian Air and Space Udvar-Hazy Center.

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