The Explorer-class mother spacecraft, International Sun-Earth Explorer 1, was part of the mother/daughter/heliocentric mission (ISEE 1, ISEE 2, ISEE 3). The purposes of the mission were: to investigate solar-terrestrial relationships at the outermost boundaries of the Earth's magnetosphere, to examine in detail the structure of the solar wind near the Earth and the shock wave that forms the interface between the solar wind and the Earth's magnetosphere, to investigate motions of and mechanisms operating in the plasma sheets, and to continue the investigation of cosmic rays and solar flare effects in the interplanetary region near 1 AU. The three spacecraft carried a number of complementary instruments for making measurements of plasmas, energetic particles, waves, and fields. The mission thus extended the investigations of previous IMP spacecraft.
ISEE 1 re-entered the Earth's atmosphere during orbit 1518 on September 26, 1987. Seventeen of 21 on-board experiments were operational at the end.