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.

[2] Explorer 33 (IMP-D) is 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.

Four n/p solar cell arrays that produced an average of 43 watts, extend from the main bus, along with two 183 cm (72 in) magnetometer booms.

The instrument package included a circuit for spin-demodulating the outputs from the sensors in the spin plane.

On rare occasions (less than 10), a GM tube would produce a high, spurious count rate for a period of several hours.

This effect apparently was produced only during periods of extremely high particle and X-ray fluxes.

[6] This experiment consisted of a 10.2 cm (4.0 in), Neher-type ionization chamber and two Lionel type 205 HT Geiger–Müller tubes (GM).

[7] A wide-aperture, multi-grid potential analyzer was used to observe the intensity of the electron and ion components of the low-energy plasma in interplanetary space and near Earth.

[8] A split-collector Faraday cup mounted on the spacecraft equator was used to study the directional intensity of solar wind ions and electrons.

Twenty-seven directional current samples from the two collectors were taken in the energy per charge (E/Q) window from 80 to 2850 eV.

The Thor-Delta E1 second and third stages both delivered too much thrust, resulting in an excess velocity of about 21.3 m/s (70 ft/s) towards the Moon.

This was too much for the retrorocket to overcome to put the spacecraft into the intended lunar orbit (1,300 × 6,440 km (810 × 4,000 mi) with 175°.

Metal–oxide–semiconductor technology simplified semiconductor device fabrication and manufacturing, enabling higher transistor counts on integrated circuit chips.

[10] This resolved a growing problem facing spacecraft designers at the time, the need for greater on-board electronic capability for telecommunications and other functions.

MOS technology allowed for a substantial increase in the overall number of transistors and communication channels, from 1,200 transistors and 175 channels on the first three IMP spacecraft up to 2,000 transistors and 256 channels on the AIMP-D. MOS technology also greatly reduced the number of electrical parts required on a spaceship, from 3,000 non-resistor parts on IMP-A down to 1,000 non-resistor parts on the AIMP-1, despite the satellite having twice the electrical complexity of IMP-A.

[16][18] While IMP-A through IMP-C had made some use of integrated circuits, the encoders still primarily used discrete transistors (one per package).

AIMP-1's design put 4,200 semiconductors into 700 packages, reducing the number of individual components used and the amount of space they occupied.

[10] AIMP-1 (IMP-D) improved upon its predecessors' Digital Data Processors (DDPs) and had an Optical Aspect Computer capable of operating in different power-saving modes to reduce load on the satellite's batteries and solar panels.

[19] As in previous IMP spacecraft, experiments stored data into accumulators which were then read out on a repeating cycle and encoded into pulse-frequency modulation (PFM) signals to be sent to ground stations.