|Mission type||Magnetospheric research|
|Mission duration||1,876 days (5 years, 1 month and 21 days)|
|Manufacturer||Goddard Space Flight Center|
|Launch mass||212.0 kilograms (467.4 lb)|
|Start of mission|
|Launch date||July 1, 1966, 16:02:25UTC|
|Launch site||Cape Canaveral LC-17A|
|End of mission|
|Last contact||September 21, 1971|
|Perigee altitude||265,679 kilometers (165,085 mi)|
|Apogee altitude||480,762 kilometers (298,732 mi)|
|Argument of perigee||119.2000 degrees|
|Mean anomaly||21.7899 degrees|
|Epoch||May 12, 1971, 12:00:00 UTC|
Explorer 33 (also known as AIMP-D, IMP-D, and AIMP 1) was a spacecraft in the Explorer program launched by NASA on July 1, 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.
When it was launched, AIMP-D achieved the highest orbit of any satellite up until that time, with an apogee of 450,000 kilometers (280,000 mi) and a perigee of 50,000 km (31,000 mi) 
Originally intended for a lunar orbit, mission controllers worried that the spacecraft's trajectory was too fast to guarantee lunar capture. Consequently, mission managers opted for a backup plan of placing the craft into an eccentric Earth orbit with a perigee of 265,679 km and an apogee of 480,762 km — still reaching distances beyond the Moon's orbit.
Despite not attaining the intended lunar orbit, the mission met many of its original goals in exploring solar wind, interplanetary plasma, and solar X-rays. Principal investigator James Van Allen used electron and proton detectors aboard the spacecraft to investigate charged particle and X-ray activity. Astrophysicists N. U. Crooker, Joan Feynman, and J. T. Gosling used data from Explorer 33 to establish relationships between the Earth's magnetic field and the solar wind speed near Earth.
The first of Explorer 33's predecessors in the Interplanetary Monitoring Platform series, Explorer 18 (IMP-A), had been the first spacecraft to fly with integrated circuits onboard. AIMP-D advanced the state of the art again when it was the first spacecraft to use the MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor), which was adopted by NASA for the IMP program in 1964. The use of MOSFETs was a major step forward in spacecraft electronics design. The MOSFET blocks were manufactured by General Microelectronics, which had NASA as its first MOS contract shortly after it had commercialized MOS technology in 1964.
MOSFETs had first been demonstrated in 1960 and publicly revealed in 1963. Metal-oxide-semiconductor technology simplified semiconductor device fabrication and manufacturing, enabling higher transistor counts on integrated circuit chips. This resolved a growing problem facing spacecraft designers at the time, the need for greater on-board electronic capability for telecommunications and other functions. The Goddard Space Flight Center used MOSFETs in building block circuits, with MOSFET blocks and resistors accounting for 93% of the AIMP-D's electronics. 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-D, despite the satellite having twice the electrical complexity of IMP-A. While IMP-A through IMP-C had made some use of integrated circuits, the encoders still primarily used discrete transistors (one per package). AIMP-D's design put 4,200 semiconductors into 700 packages, reducing the number of individual components that had to be used and the amount of space they occupied. Components were combined into cordwood modules.
AIMP-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. 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. This cycle was also interleaved with analog transmissions for certain experiments.