Explorer 33

Explorer 33, also known as IMP-D and AIMP-1, was a spacecraft in the Explorer program launched by NASA on 1 July 1966 on a mission of scientific exploration. It was the fourth satellite launched as part of the Interplanetary Monitoring Platform series, and the first of two "Anchored IMP" spacecraft to study the environment around Earth at lunar distances, aiding the Apollo program. It marked a departure in design from its predecessors, IMP-A (Explorer 18) through IMP-C (Explorer 28). Explorer 35 (AIMP-E, AIMP 2) was the companion spacecraft to Explorer 33 in the Anchored IMP program, but Explorer 34 (IMP-F) was the next spacecraft to fly, launching about two months before AIMP-E, both in 1967. Explorer 33 (IMP-D) was a spin-stabilized (spin axis parallel to the ecliptic plane, spin period varying between 2.2 and 3.6 seconds) 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 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, across and high. Four n/p solar cell arrays, producing an average of 43 watts, extended from the main bus, along with two magnetometer booms. Four whip antennas are mounted on top of the spacecraft. A thrust retrorocket (Thiokol TE-M-458) was mounted on top of the bus. Power was stored in silver-cadmium batteries (Ag-Cd). Communication (PFM-PM telemetry) was via a 7-watts transmitter and a digital data processor. 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.