The Geotail mission was a joint project of Japan’s Institute of Space and Astronautical Science (ISAS) and later, from 2003, the Japan Aerospace Exploration Agency (JAXA) and NASA. The mission was part of the International Solar Terrestrial Physics (ISTP) project, which also included the Wind, Polar, SOHO, and Cluster missions.

Geotail’s goal was to study the structure and dynamics of the long tail region of Earth’s magnetosphere, which is created on the nightside of Earth by the solar wind. During active periods, the tail couples with the near-Earth magnetosphere, and often releases energy that is stored in the tail, activating auroras in the polar ionosphere.

Surrounding Earth is a giant magnetic field called the magnetosphere. Its shape is defined not only by the planet’s north and south magnetic poles, but also by a steady stream of particles coming in from the sun called the solar wind. The magnetosphere is buffeted by this wind and can change shape dramatically when the sun lets loose an immense cloud of gas known as a coronal mass ejection. Credit: NASA’s Scientific Visualization Studio

Although technically not a deep space or planetary mission, Geotail, in its extremely elliptical orbit, performed numerous lunar flybys, some closer than the distance at which the Soviet Luna 3 took the first pictures of the far side of the Moon.

The spin-stabilized spacecraft (20 rpms) was designed with a pair of 328-foot (100-meter) tip-to-tip antennae and two 20-foot (6-meter)-long masts.

On its fifth orbit around Earth, near apogee, on Sept. 8, 1992, the spacecraft flew by the Moon at a range of about 7,900 miles (12,647 kilometers). The flyby raised apogee from 261,174 miles (426,756 kilometers) to about 540,000 miles (869,170 kilometers). The flybys continued almost every month and ultimately raised the spacecraft’s apogee to 870,000 miles (1.4 million kilometers). During these orbits, Geotail observed the magnetotail’s far region (from 80 to 220 times the radius of Earth or “Re”).

UC Berkeley Space Science Lab played an integral part in all of these US Missions. The Geotail mission carried an EFD or Electric Fields Detector spearheaded by our own Dr. Forrest Mozer. who was the United States Principal Investigator of that Instrument and Dr. Mozer designed the EFD but it was built in Japan. Dr. Mozer was also active in spacecraft operations, involving multiple trips to Japan. The science obtained from the spacecraft resulted in several published papers by Dr. Mozer.

More about the Geotail Mission