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.