NASA - National Aeronautics and Space Administration

THEMIS Overview


Image above: Artist's concept of THEMIS in orbit. Credit: NASA

NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) aims to resolve one of the oldest mysteries in space physics, namely to determine what physical process in near-Earth space initiates the violent eruptions of the aurora that occur during substorms in the Earth's magnetosphere.

THEMIS is a 2-year mission consisting of 5 identical probes that will study the violent colorful eruptions of Auroras.

Understanding and predicting space weather is important to describe the environment in which spacecraft and astronauts operate and ensure their safety. Just as hail and tornadoes accompany the most severe thunderstorms, substorms accompany the most intense space storms – those that disrupt communications, cause power line transmission failures, and produce the most penetrating radiation. THEMIS will study substorms to gain insight into the most severe space storms.

Substorms occur when the magnetosphere suddenly releases vast amounts of stored solar wind energy. Substorms start from a small region in space but within minutes cover an immense region of the magnetosphere. Different possible triggers have different locations, so the key to solving this mystery is placing spacecraft in various locations in Earth's magnetic field to figure to help find the elusive substorm point of origin.

For the first time NASA will launch a constellation of five satellites to study substorms. The THEMIS probes will line up over North America once every four days. Over the mission’s two-year lifetime, the probes should be able to observe some 30 substorms.

THEMIS is the fifth medium-class mission under NASA's Explorer Program, which was conceived to provide frequent flight opportunities for world-class scientific investigations from space within the Heliophysics and Astrophysics science areas. The Explorers Program Office at Goddard Space Flight Center, Greenbelt, Md., manages this NASA-funded mission. The University of California, Berkeley's Space Sciences Laboratory and Swales Aerospace, Beltsville, Md., built the THEMIS probes.

Why "THEMIS?"


Image above: Image of substorm models. Credit: UC/Berkeley

Themis is the goddess of justice, wisdom and good counsel, the guardian of oaths, and the interpreter of the gods' will in Greek mythology. The daughter of Uranus (Sky) and Gaia (Earth), she was Zeus' second consort and the mother of the Horae (the Seasons) and the Moirae (the Fates). Legend also holds that Themis was the mother of Prometheus.

Themis is typically depicted with a sword and scales, symbolizing both her power and her impartiality. Her blindfolding dates from the 16th Century and signifies Themis' famed neutrality. This accounts for the commonly used term, "blind justice." The modern depiction is of a young woman, often blindfolded, holding her scales and sword. This image of Themis is today prominently displayed in the hall of justice and flown on flags worldwide.

The THEMIS mission will impartially distinguish, as implied by the goddess' name, between two disparate phenomenological and plasma-physical models of substorm onset in order to solve a tantalizing mystery: Where and when do substorms start in the Earth's magnetosphere? This question has been the subject of scientific contention for over thirty years.