Interplanetary Monitoring Platform

• 1. Introduction

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  The Interplanetary Monitoring Platform program asked a question that had never been answered: what exactly fills the space between Earth and the planets? In the early 1960s, NASA's Goddard Space Flight Center in Greenbelt, Maryland, set out to find answers by building a network of eleven satellites. Their targets were interplanetary plasma, the solar wind, cosmic rays, and magnetic fields stretching through cislunar and interplanetary space.

  What makes the IMP program remarkable is not only what it found, but what it invented along the way. The program's first satellite, launched in November 1963, carried a technology so new that it had barely been heard of outside research laboratories. That technology would go on to shape every piece of electronics on Earth. The question the IMP story raises is why a program designed to map invisible magnetic fields became one of the pivotal proving grounds for the microelectronics revolution.

• 2. A Network in Orbit

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  Explorer 18, known within the program as IMP-A, lifted off from Cape Canaveral's Launch Complex 17B on the 27th of November 1963 at 02:30 UTC. It weighed 138 kilograms and was the first of a three-satellite design series alongside IMP-B and IMP-C. Explorer 21 followed from LC-17A on the 4th of October 1964, and Explorer 28 launched on the 29th of May 1965, completing that first family.

  The program then developed a second generation. Explorer 33, designated IMP-D, launched on the 1st of July 1966 and weighed 212 kilograms, nearly 60 percent heavier than the original IMP-A design. It was originally intended to orbit the Moon, but was placed in an elliptical high Earth orbit instead. Explorer 35, the companion IMP-E, did reach Selenocentric orbit after its launch in July 1967, making it one of only two satellites in the program designated as Anchored Interplanetary Monitoring Platforms.

  The program's final design series ran to three spacecraft: Explorer 43, Explorer 47, and Explorer 50, designated IMP-I, IMP-H, and IMP-J. Explorer 50, the eighth and last IMP satellite, outlasted all its predecessors and remained in service until 2006. Both Explorer 33 and Explorer 47 and Explorer 50 are still in orbit today.

• 3. First Chips in Space

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  Explorer 18's departure from Cape Canaveral in 1963 carried more than scientific instruments. It was the first spacecraft anywhere to fly integrated circuit chips. This milestone predates the use of integrated circuits in the Apollo Guidance Computer, the celebrated computing heart of the Moon missions.

  The integrated circuit had only recently moved from laboratory curiosity to manufacturable component. Placing one aboard a spacecraft in 1963 was not a routine engineering decision. Doing so first, before the Apollo program adopted the technology, positioned the IMP program as the opening chapter of space-based microelectronics.

• 4. The MOSFET Breakthrough

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  NASA adopted a second, more consequential chip technology for the IMP program in 1964: the MOSFET, the metal-oxide-semiconductor field-effect transistor. The MOSFET had first been demonstrated in 1960 and publicly revealed in 1963. Explorer 33, launched in 1966, became the first spacecraft anywhere to carry them.

  The Goddard Space Flight Center built IMP-D's electronics using MOSFET blocks combined with resistors. Those two components together accounted for 93 percent of the spacecraft's entire electronics package. The manufacturer was General Microelectronics, which had commercialized MOS technology in 1964 and counted NASA as its first MOS contract customer.

  The numbers behind the upgrade are striking. The first three IMP spacecraft carried 1,200 transistors and 175 communication channels. IMP-D carried 2,000 transistors and 256 channels, despite requiring only 1,000 non-resistor electrical parts, compared to the 3,000 non-resistor parts on IMP-A. IMP-D achieved twice the electrical complexity of IMP-A while using a third fewer physical components. The core reason was MOS technology's simpler manufacturing process, which allowed more transistors to be packed onto each chip. That manufacturing advantage solved a problem spacecraft designers were running into everywhere: the demand for more on-board computing and telecommunications capability was outrunning what conventional electronics could deliver within acceptable size and power limits.

• 5. Supporting Apollo

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  Beyond their role as technology testbeds, the IMP satellites served a direct operational purpose for the Apollo program. The eleven-satellite network collected data on space radiation, solar wind, and magnetic fields along the routes astronauts would travel to and from the Moon.

  IMP spacecraft measured conditions in cislunar space, the region between Earth and the Moon, as well as in the broader interplanetary environment. Their measurements of plasma, cosmic rays, and magnetic field strength fed into the planning and safety assessment for human spaceflights. The data gathered by the IMP network contributed to enabling the Apollo 11 mission, which achieved the first human Moon landing in 1969. That mission flew in the same year Explorer 28, the third IMP satellite, finally decayed from orbit after more than three years of service.