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The Third Interplanetary Monitoring Platform (IMP 3, or IMP C), also designated Explorer 28, was a solar-cell and chemical-battery powered spacecraft instrumented for interplanetary and distant magnetospheric studies of energetic particles, cosmic rays, magnetic fields, and plasmas. The primary objective was to study in detail the radiation environment of cislunar space, and to monitor this region over a significant portion of a solar cycle.

Secondary objectives were to develop solar-flare prediction capability for Project Apollo, develop relatively inexpensive spin-stabilized spacecraft for interplanetary investigations, study the quiescent properties of the interplanetary magnetic field and its dynamical relationships with particle fluxes from the Sun, and to extend our knowledge of solar-terrestrial relationship.

Spacecraft and Subsystems

The IMP 3 spacecraft main bus was a squat octagonal prism, 71 cm (28 inches) across and 20 cm (8 inches) high. Four solar panel paddles extended radially, 90 degrees apart, from four of the eight vertices. Each paddle was 71 cm long by 51 cm wide. A rubidium vapor magnetometer contained in a spherical compartment (approximately 33 cm in diameter) was mounted on a tower in the center of the octagon upper deck, standing 181.9 cm (71.6 inches) above the deck. Two fluxgate magnetometer booms extended 430 cm (14 ft) at an angle from the bottom deck. Four turnstile antennas, 41 cm (16 inches) long, extended at an angle from the top deck.

The spacecraft hull was fabricated from nylon honeycomb and fiberglass. Total spacecraft mass was 59 kg. Data was communicated and stored using pulsed frequency modulation (PFM) telemetry, a 4-W output transmitter at 136.125 MHz, and a PFM encoder with a digital data processor. It was spin-stabilized. Power of 39 W was supplied by 6144 n-on-p solar cells in the paddle arrays and silver-cadmium batteries. Total spacecraft mass was 59 kg.

The scientific payload had a total mass of 20 kg, most of the instruments and electronics are mounted on the periphery of the spacecraft bus. The experiments on IMP-C are a cosmic ray detector, cosmic ray range vs. energy loss experiment, fluxgate magnetometer, ion chamber and Geiger-Mueller counters, a Faraday cup, retarding potential analyzer, and a solar wind proton analyzer.

Mission Profile

IMP 3 launched from Cape Kennedy on 29 May 1965 at 12:00:00 UT on a three stage Thor-Delta. The spacecraft was placed into an initial elliptical 195 x 263,604 km (121 x 163,831 mile) altitude Earth orbit at an inclination of 34 degrees with a period of 8550 minutes. This was higher than the planned orbit apogee of about 210,000 km due to a longer than planned third stage burn.Initial spacecraft parameters included a local time of apogee of 2020 h, a spin rate of 23.7 rpm, and a spin direction of 64.9-deg right ascension and -10.9-deg declination. Each normal telemetry sequence was 81.9 s in duration and consisted of 795 data bits. After every third normal telemetry sequence there was an 81.9-s interval of rubidium vapor magnetometer analog data transmission. Data were transmitted continuously on the 136.125 MHz band.

By August 1966 the spacecraft orbit had evolved to a 26,665 x 234,852 km altitude orbit with an inclination of 50 degrees. Performance was essentially normal until late April 1967, then intermittent until May 12, 1967, after which no further data were acquired. The spacecraft orbit decayed on 4 July 1968.

The Solar Wind Protons and Plasma, Faraday Cup experiments failed at launch and returned no data.

Alternate Names

  • 01388
  • Explorer 28
  • IMP 3
  • S 74B

Facts in Brief

Launch Date: 1965-05-29
Launch Vehicle: Thor-Delta
Launch Site: Cape Canaveral, United States
Mass: 59 kg
Nominal Power: 38 W

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Space Physics

Additional Information

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



NameRoleOriginal AffiliationE-mail
Mr. C. J. CrevelingGeneral ContactNASA Goddard Space Flight Center
Mr. Everett J. PyleGeneral ContactNASA Goddard Space Flight Center
Mr. Paul ButlerProject ManagerNASA Goddard Space Flight Center
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center
Ms. Della StewartGeneral ContactNASA Goddard Space Flight Center

Selected References

  • Lowrey, B. E., The decline and fall of IMP 3, second report, NASA-GSFC, TM-X-63164, X-643-68-117, Greenbelt, MD, Feb. 1968.
  • Carr, F. A., Interim flight report, Interplanetary Monitoring Platform IMP 3 - Explorer 28, NASA-GSFC, X-724-66-121, Greenbelt, MD, Mar. 1966.
  • Carr, F. A., Interplanetary monitoring platform IMP III - Explorer XXVIII Interim flight report no. 2, NASA-GSFC, X-724-66-354, Greenbelt, MD, July 1966.

IMP-C Diagram

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