THEMIS

NASA satellite of the Explorer program
NASA satellites

Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission began in February 2007 as a constellation of five NASA satellites (THEMIS A through THEMIS E) to study energy releases from Earth's magnetosphere known as substorms, magnetic phenomena that intensify auroras near Earth's poles. The name of the mission is an acronym alluding to the Titan Themis. Three of the satellites orbit the Earth within the magnetosphere, while two have been moved into orbit around the Moon. Those two were renamed ARTEMIS for Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun. THEMIS B became ARTEMIS P1 and THEMIS C became ARTEMIS P2. ARTEMIS P1 and P2 together comprise the THEMIS ARTEMIS mission. The THEMIS satellites were launched 17 February 2007 from SLC 17B aboard a Delta II launch vehicle. Each satellite carries five identical instruments, including a fluxgate magnetometer (FGM), an electrostatic analyzer (ESA), a solid state telescope (SST), a search coil magnetometer (SCM) and an electric field instrument (EFI). Each probe has a mass of , including of hydrazine fuel. THEMIS data can be accessed using the SPEDAS software. Canada, Austria, Germany, and France also contributed to the mission.

Swales Aerospace, now part of Orbital ATK which is now part of Northrop Grumman, Beltsville, Maryland, manufactured all five probes for this mission. Each was built up and tested at the Beltsville facility, before being delivered to University of California, Berkeley for instrument integration. Swales was responsible for integrating the BAU, IRU, Solar arrays, antenna, battery, and other components necessary for functionality. This was the second major satellite built by Swales, the first being the Earth Observing 1 (EO 1) spacecraft, which continues to orbit Earth. Swales was also responsible for designing and building the Electrical Ground Support Equipment (EGSE) used for monitoring the probes during all phases of pre launch activities, including use at the launch site.

After the installation of instruments at SSL, Berkeley, pre launch testing including thermal vacuum, vibration and acoustic tests, was conducted at NASA's Jet Propulsion Laboratory in Pasadena, California.

THEMIS was originally scheduled to launch on 19 October 2006. Owing to delays caused by workmanship problems with Delta II second stages — an issue that also affected the previous mission, STEREO — the THEMIS launch was delayed to 15 February 2007. Due to weather conditions occurring on 13 February 2007, fueling of the second stage was delayed, and the launch pushed back 24 hours. On 16 February 2007, the launch was scrubbed in a hold at the T 4 minute point in the countdown due to the final weather balloon reporting a red, or no go condition for upper level winds. A 24 hour turnaround procedure was initiated, targeting a new launch window between 23:01 and 23:17 UTC on 17 February 2007. Favorable weather conditions were observed on 17 February 2007, and the countdown proceeded smoothly. THEMIS successfully launched at 23:01:00 UTC. The spacecraft separated from the launch vehicle approximately 73 minutes after liftoff. By 03:07 UTC, on 18 February, mission operators at the Space Sciences Laboratory (SSL) of the University of California, Berkeley, commanded and received signals from all five spacecraft, confirming nominal separation status. The launch service was provided by the United Launch Alliance through the NASA Launch Services Program (LSP).

The Fast Auroral SnapshoT Explorer (FAST) mission supported THEMIS in 2008 and 2009 before being retired. FAST was a Small Explorer program (SMEX) mission launched in 1996. In 2007, THEMIS "found evidence of magnetic ropes connecting Earth's upper atmosphere directly to the Sun", reconfirming the theory of solar terrestrial electrical interaction (via "Birkeland currents" or "field aligned currents") proposed by Kristian Birkeland circa 1908. NASA also likened the interaction to a "30 kiloVolt battery in space", noting the "flux rope pumps 650,000 Ampere current into the Arctic!" On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms. Two of the five probes, positioned approximately one third the distance to the Moon, measured events suggesting a magnetic reconnection event 96 seconds prior to Auroral intensification. Vassilis Angelopoulos of the University of California, Los Angeles, who is the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger".