International sun-Earth explorer: a three-spacecraft program.

The International Sun-Earth Explorer is a three-spacecraft program of the National Aeronautics and Space Administration and the European Space Agency aimed at securing a more quantitative knowledge of the structure and stability of the magnetosphere. One spacecraft (ISEE-C) makes observations in the solar wind upstream of the earth, while the other two (ISEE-A and ISEE-B), in the same highly eccentric orbit but separated by a relatively small variable distance, observe inside the magnetosphere. This international program is concurrent with the International Magnetospheric Study, to which ISEE-A and ISEE-C form a large U.S. contribution. The scientific aims and technological methods are discussed.