A series of spacecraft in NASA's Explorer Program that mainly studied the solar wind and the Earth's magnetosphere. They were launched over ten years, beginning with IMP-1 (IMP-A) in November 1963 and ending with IMP-8 (IMP-J), launched in October 1973. The latter carried 12 experiments, including a charged-particle measurements experiment (CPME) and a magnetic-field experiment. It also used an electrostatic analyser and solid-state particle telescope to determine the composition and energy of low-energy particles observed during solar flares. The project was closed down in November 2001.