Five years ago today — February 17, 2007 — a Delta-II rocket launched from Cape Canaveral, carrying five nearly identical satellites on a mission to study magnetic field “substorms.”
(Artist’s concept of THEMIS in orbit. NASA image.)
The Time History of Events and Macroscale Interactions during Substorms, or THEMIS, spacecraft — in practical NASA fashion, designated THEMIS-1 through THEMIS-5 (or sometimes -A through -E) — were designed to “track the origins of substorms within the Earth’s magnetic field.” Energetic particles from such substorms cause the famed Northern and Southern Lights: the Aurorae Borealis and Australis, respectively.
The National Space Science Data Center page about THEMIS-1 describes the different mission phases and the unique orbits of the five spacecraft:
The mission consists of several phases. In the first phase, the spacecraft will all orbit as a tight cluster in the same orbital plane with apogee at 15.4 Earth radii (RE). In the second phase, also called the Dawn Phase, the satellites will be placed in their orbits and during this time their apogees will be on the dawn side of the magnetosphere. During the third phase (also known as the Tail Science Phase) the apogees will be in the magnetotail. The fourth phase is called the Dusk Phase or Radiation Belt Science Phase, with all apogees on the dusk side. In the fifth and final phase, the apogees will shift to the sunward side (Dayside Science Phase).
All five satellites will have similar perigee altitudes (1.16-1.5 Re) but varying apogee altitudes (P1: ~30 RE, P2: ~20 RE, P3 & P4: ~12 RE, P5: ~10RE) with corresponding orbital periods of ~4, 2, and 1 days, respectively. This results in multi-point magnetic conjunctions. Every four days the satellites will line up along the Earth’s magnetic tail with magnetic foot points in the North American sector, allowing the tracking of disturbances through different geospace regions from tail to ground.
The whole “magnetic storm” thing sounds science fiction-y, doesn’t it? But that makes it cool.
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