# Interplanetary Monitoring Platform

Interplanetary Monitoring Platform was a program managed by the NASA Goddard Space Flight Center in Greenbelt, Maryland, as part of the Explorers program, with the primary objectives of investigation of interplanetary plasma and the interplanetary magnetic field. The orbiting of IMP satellites in a variety of interplanetary and earth orbits allowed study of spatial and temporal relationships of geophysical and interplanetary phenomena simultaneously by several other NASA satellites.

## Satellites

| Launch Date | Launch Place | Satellite | Satellite | Launch mass | Decay Date | Notes |
| Launch Date | Launch Place | Explorer | IMP | Launch mass | Decay Date | Notes |
| 27 November 1963, 02:30 UTC | Cape Canaveral LC-17B | Explorer 18 | IMP-A | 138 kg (304 lb) | December 30, 1965 | IMP 1. First use of integrated circuits in a spacecraft. First satellite in IMP-A/-B/-C design series. |
| 4 October 1964, 03:45 UTC | Cape Canaveral LC-17A | Explorer 21 | IMP-B | 138 kg (304 lb) | January 1, 1966 | IMP 2. Second satellite in IMP-A/-B/-C design series. |
| 29 May 1965, 12:00 UTC | Cape Canaveral LC-17B | Explorer 28 | IMP-C | 128 kg (282 lb) | 4 July 1968 | IMP 3. Third satellite in IMP-A/-B/-C design series. |
| 1 July 1966, 16:02 UTC | Cape Canaveral LC-17A | Explorer 33 | IMP-D / AIMP-D | 212 kg (467 lb) | In orbit | AIMP 1. First use of MOSFET integrated circuits in a spacecraft, similar design to IMP-E. Originally intended to orbit the Moon, but placed in an elliptical high orbit instead. |
| 24 May 1967, 14:05 UTC | Vandenberg SLC-2E | Explorer 34 | IMP-F | 163 kg (359 lb) | May 3, 1969 | IMP 4. Similar design to IMP-G. |
| 19 July 1967, 14:19 UTC | Cape Canaveral LC-17B | Explorer 35 | IMP-E / AIMP-E | 104 kg (229 lb) | After June 24, 1973 | AIMP 2, similar design to IMP-D. Positioned in Selenocentric orbit. |
| 21 June 1969, 08:47 UTC | Vandenberg SLC-2W | Explorer 41 | IMP-G | 175 kg (386 lb) | December 23, 1972 | IMP 5. Similar design to IMP-F. |
| 13 March 1971, 16:15 UTC | Cape Canaveral LC-17A | Explorer 43 | IMP-I | 635 kg (1,400 lb) | October 2, 1974 | IMP 6. First spacecraft in IMP-I/-H/-J series. |
| 23 September 1972, 01:20 UTC | Cape Canaveral LC-17B | Explorer 47 | IMP-H | 390 kg (860 lb) | In orbit | IMP 7. Second spacecraft in IMP-I/-H/-J series. |
| 26 October 1973, 02:26 UTC | Cape Canaveral LC-17B | Explorer 50 | IMP-J | 410 kg (900 lb) | In orbit | IMP 8. Last satellite IMP, remained in service until 2006. Third spacecraft in IMP-I/-H/-J series. |

## Technology

The IMP program was the first space program to use integrated circuit (IC) chips, which it first launched into space with the IMP-A (Explorer 18) in 1963. This predates the use of IC chips in the Apollo Guidance Computer, used for the Apollo program.
The MOSFET (metal–oxide–semiconductor field-effect transistor, or MOS transistor) was adopted by NASA for the IMP program in 1964. The use of MOSFETs was a major step forward in spacecraft electronics design.
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. The Goddard Space Flight Center used MOSFETs in building block circuits, with MOSFET blocks and resistors accounting for 93% of the IMP-D's electronics. 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 MOSFET blocks were manufactured by General Microelectronics, which had NASA as its first MOS contract shortly after it had commercialized MOS technology in 1964.

## Applications

IMPs were used to study the magnetic fields, solar wind and cosmic rays outside the magnetic field of the Earth. It was closely related to the development of the Apollo program. The IMP program consisted of a network of eleven satellites designed to collect data on space radiation in support of the Apollo program. The IMP satellites investigated plasma (ionized gas), cosmic rays, and magnetic fields in interplanetary and cislunar space, from various solar and terrestrial orbits. Data gathered by IMP spacecraft and satellites were used to support the Apollo program, enabling the first human Moon landing with the Apollo 11 mission in 1969.