ISEE-1

The International Sun-Earth Explorer 1 (ISEE-1, or Explorer 56) was a 340-kg space probe used to study magnetic fields near the Earth. ISEE-1 was a spin-stabilized spacecraft and based on the design of the prior IMP (Interplanetary Monitoring Platform) series of spacecraft. ISEE-1 and ISEE-2 were launched on October 22, 1977, and they re-entered on September 26, 1987.

Mission
The space probe was part of a program consisting of three spacecraft: a mother/daughter pair (ISEE-1 and ISEE-2) and the ISEE-3 spacecraft (later renamed to International Cometary Explorer). The program was a cooperative mission between NASA and ESRO (later ESA) designed to study the interaction between the Earth's magnetic field and the solar wind. At least 32 institutions were involved, and the focus was on understanding magnetic fields. ISEE-1 (a.k.a. Explorer 56) and ISEE-3 were built by NASA, while ISEE-2 was built by ESA. All three had complimentary instruments supported by the same group of over 100 scientists.

Orbit
ISEE-1 and ISEE-2 remained near the Earth. ISEE-3 was the first spacecraft to be placed in a halo orbit at the Earth-Sun Lagrangian points and it was later launched into a heliocentric orbit.