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Explorer 33



Explorer 33 (IMP-D) was a spin-stabilized (spin axis parallel to the ecliptic plane, spin period varying between 2.2 and 3.6 s) spacecraft instrumented for studies of interplanetary plasma, energetic charged particles (electrons, protons, and alphas), magnetic fields, and solar X rays at lunar distances. The spacecraft failed to achieve lunar orbit but did achieve mission objectives. Explorer 33 was also known as Interplanetary Monitoring Platform D (IMP-D) or Anchored Interplanetary Monitoring Platform 1 (AIMP-1).

Explorer 33 was planned to be the first U.S. spacecraft to go into lunar orbit. The science objectives were to study the near-lunar magnetic field, ionosphere, solar plasma flux, energetic particle population, cosmic dust, and variations of the gravitational field from lunar orbit. After failing to achieve the intended lunar orbit, it made measurements from a highly elliptical Earth orbit of the interplanetary magnetic and radiation environment.

Spacecraft and Subsystems

Explorer 33 was similar in design to Explorer 28. The spacecraft had a mass of 93.4 kg. The main body of the spacecraft was an octagonal prism, 71 cm (28 in) across and 20.3 cm (8 in) high. Four n/p solar cell arrays, producing an average of 43 W, extended from the main bus, along with two 183 cm (6 ft) magnetometer booms. Four whip antennas are mounted on top of the spacecraft. A 35.8 kgf thrust retrorocket (Thiokol TE-M-458) was mounted on top of the bus. Power was stored in silver-cadmium (Ag-Cd) batteries. Communication (PFM-PM telemetry) was via a 7-W transmitter and a digital data processor.

The scientific payload comprised seven experiments: two flux-gate magnetometers, an energetic particles experiment, an electron and proton experiment, a thermal ion and electron experiment, a plasma probe, and a solar cell damage experiment.

Mission Profile

Explorer 33 launched on 1 July 1966 from Cape Kennedy, Florida. The Thor Delta E-1 second and third stages both delivered too much thrust, resulting in an excess velocity of about 21.3 m/s (70 fps) towards the Moon. This was too much for the retrorocket to overcome to put the spacecraft into the intended lunar orbit (1300 x 6440 km with 175 deg. inclination). Instead, the retrorockets were used to put Explorer 33 into a highly elliptical initial Earth orbit of 449,174 x 30,550 km with an inclination of 28.9 deg and an apogee beyond lunar orbit. It came within 35,000 km of the Moon on its first orbit, and came within 40,000 to 60,000 km on subsequent approaches in Septemeber, November, and December, 1966. All experiments operated successfully until September, 1971.

The initial apogee occurred at about 1600 h local time. Over the first 3-yr period, perigee varied between 6 and 44 earth radii. Apogee varied between 70 and 135 earth radii, and the inclination with respect to the equator of the Earth varied between 7 and 60 deg. Periods of principal data coverage (essentially 100%) are July 1, 1966 (launch), to January 14, 1970; February 21, 1970, to March 6, 1970; July 31, 1970, to September 14, 1970; January 15, 1971, to February 28, 1971; March 23, 1971, to May 31, 1971; and August 23, 1971, to September 15, 1971. No data were obtained after September 21, 1971.

In January 1967 an electronic "screwdriver" was used remotely by engineers at Goddard Space Flight Center to restore power to the temporarily blacked out bus, the most remote repair to a spacecraft achieved to that time.

Alternate Names

  • 02258
  • AIMP 1
  • Anchored IMP 1
  • Explorer33
  • IMP-D

Facts in Brief

Launch Date: 1966-07-01
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 93.4 kg
Nominal Power: 43 W

Funding Agency

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


  • Planetary Science
  • Space Physics
  • Solar Physics
  • Astronomy

Additional Information

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



NameRoleOriginal AffiliationE-mail
Dr. Alois W. SchardtGeneral ContactNASA Headquarters
Mr. John J. BrahmGeneral ContactNASA Goddard Space Flight Center
Mr. Jerome BarskyGeneral ContactNASA Goddard Space Flight Center
Mr. Paul L. HeffnerGeneral ContactNASA Goddard Space Flight Center
Dr. Norman F. NessProject ScientistNASA Goddard Space Flight Center
Mr. Paul G. MarcotteProject ManagerNASA Goddard Space Flight Center
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