Geotail provides information about the way the magnetic envelope surrounding Earth, called the magnetosphere, responds to incoming material and energy from the Sun.
Surviving more than six times longer than planned, Geotail continues to send back crucial information about how aurora form, how energy from the Sun funnels through near-Earth space and the ways in which magnetic field lines move and rebound, creating explosive bursts that rearrange the very shape of our magnetic environment.
|Nation(s)||Japan and United States|
|Objective(s)||High Elliptical Earth Orbit|
|Spacecraft Mass||2,225 pounds (1,009 kilograms)|
|Mission Design and Management||Institute of Space and Astronautical Science / NASA|
|Launch Vehicle||Delta 6925 (no. D212)|
|Launch Date and Time||July 24, 1992 / 14:26 UT|
|Launch Site||Cape Canaveral Air Force Station / Launch Complex 17A|
|Scientific Instruments||1. Magnetic Fields Measurement Monitor (MGF)
2. Low Energy Particles Experiment (LEP)
3. Electric field monitor (EFD)
4. Energetic Particles and Ion Composition Experiment (EPIC)
5. High-Energy Particle Monitors (HEP)
6. Plasma Wave Instrument (PWI)
7. Comprehensive Plasma Instrument (CPI)
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.
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 farside 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). Tbe 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”).
Geotail’s 14th and last flyby of the Moon occurred on Oct. 25, 1994 at a range of 14,000 miles (22,445 kilometers) and as a result, placed the spacecraft in orbits with progressively lower apogees. In November 1994, Geotail’s apogee was 50 Re and by February 1995, it was down to 30 Re.
The lower orbit allowed the spacecraft to begin the second part of its mission, to study magnetotail substorms near Earth. During these orbits, perigee was about 10 Re, while the orbital inclination to the ecliptic plane was about –7 degrees in order that the apogee would be located in the magnetotail’s neutral plane during the winter solstice.
Later in the decade, Geotail’s orbit was adjusted so that it passed just inside Earth’s magnetosphere’s boundary plane on the dayside.
In 2012, the spacecraft celebrated 20 years of continuous operation and, in 2019, despite having an original lifetime of only four years, was still sending back data on the formation of auroras, the nature of energy funneled from the Sun into near-Earth space, and the ways in which Earth’s magnetic field lines move and rebound, thus producing explosive bursts that affect our magnetic environment.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.