The Geotail mission was a collaboration between NASA and the Institute of Space and Astronautical Science (ISAS) of Japan. Its primary objective was to study the dynamics of Earth's magnetotail over a wide range of distances, extending from the near-Earth region to as far as about 200 Earth radii from the planet. With an elongated orbit, Geotail sailed through the invisible boundaries of the magnetosphere, gathering data on the physical process at play there to help understand how the flow of energy and particles from the Sun reach Earth. Geotail made many scientific breakthroughs, including helping scientists understand how quickly material from the Sun passes into the magnetosphere, the physical processes at play at the magnetosphere’s boundary, and identifying oxygen, silicon, sodium, and aluminum in the lunar atmosphere.