Interplanetary Monitoring Platform

[5] The IMP-D (Explorer 33), launched in 1966, was the first spacecraft to use the MOSFET, which had been first demonstrated in 1960 and publicly revealed in 1963.

MOS technology's simplicity of semiconductor device fabrication and manufacturing enabled 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.

MOS technology greatly increased the number of on-board 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 IMP-D. MOS technology also greatly reduced the number of electrical parts required on a spaceship, from 3,000 non-resistor parts on the IMP-A (Explorer 18) down to 1,000 non-resistor parts on the IMP-D, despite the IMP-D having twice the electrical complexity of the IMP-A.

The IMP satellites investigated plasma (ionized gas), cosmic rays, and magnetic fields in interplanetary and cislunar space, from various solar and terrestrial orbits.