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Short description: NASA mission for studying substorms
Five Themis satellites seen from above.jpg
THEMIS in an Astrotech cleanroom
Mission typeMagnetospheric research
COSPAR ID2007-004 (A, B, C, D, E)
SATCAT no.30580, 30581, 30582, 30583, 30584
Mission durationelapsed: 14 years, 9 months and 4 days
Spacecraft properties
ManufacturerSwales Aerospace
Launch mass630 kilograms (1,390 lb)
Dry mass77 kilograms (170 lb) each
Power37.0 W each
Start of mission
Launch dateFebruary 17, 2007, 23:01:00 (2007-02-17UTC23:01Z) UTC
RocketDelta II 7925-10C
Launch siteCape Canaveral Air Force Station SLC-17B
Orbital parameters
Reference systemGeocentric
RegimeHighly Elliptical
Perigee altitude470 kilometres (290 mi)
Apogee altitude87,330 kilometres (54,260 mi)
Period1,870 minutes
THEMIS logo.png  

The 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.[1]

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.[2] ARTEMIS P1 and P2 together comprise the THEMIS-ARTEMIS mission.[3]

The THEMIS satellites were launched February 17, 2007 from Cape Canaveral Air Force Station Space Launch Complex 17 aboard a Delta II rocket.[1][4] Each satellite carries identical instrumentation, 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 126 kg, including 49 kg of hydrazine fuel.[5]

THEMIS data can be accessed using the SPEDAS software.


The launch of THEMIS atop the Delta II 7925 rocket, at SLC-17B, Cape Canaveral

THEMIS was originally scheduled to launch on October 19, 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 Thursday, February 15, 2007. Due to weather conditions occurring on Tuesday, February 13, fueling of the second stage was delayed, and the launch pushed back 24 hours. On February 16, 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 February 17.

Favorable weather conditions were observed on February 17, and the countdown proceeded smoothly. THEMIS successfully launched at 6:01 p.m. EST. The spacecraft separated from the launch vehicle approximately 73 minutes after liftoff. By 8:07 p.m. EST, 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).

Mission status

From February 15, 2007 until September 15, 2007 the five THEMIS satellites coasted in a string-of-pearls orbital configuration. From September 15, 2007 until December 4, 2007 the satellites were moved to more distant orbits in preparation for data collection in the magnetotail. This phase of the mission was called the "Dawn Phase" because the satellites' orbits were in apogee on the dawn side of the magnetosphere. On December 4, 2007 the first tail science phase of the mission began. In this segment of the mission scientists will collect data from the magnetotail of the Earth's magnetosphere. During this phase the satellites' orbits are in apogee inside the magnetotail. The scientists hope to observe substorms and magnetic reconnection events. During these events charged particles stored in the Earth's magnetosphere are discharged to form the aurora borealis. Tail science is performed in the winter of the northern hemisphere because the ground magnetometers that Themis scientists correlate the satellite data with have relatively longer periods of night. During the night, observations are not interrupted by charged particles from the Sun.[6]

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.[7][8] NASA also likened the interaction to a "30 kiloVolt battery in space," noting the "flux rope pumps 650,000 Amp current into the Arctic!"[9]

On February 26, 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms.[10] 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.[11] Dr. 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."[12]

Extended mission

On May 19, 2008 the Space Sciences Laboratory (SSL) at Berkeley announced NASA had extended the THEMIS mission to the year 2012. NASA officially approved the movement of THEMIS B and THEMIS C into lunar orbit under the mission name "ARTEMIS" (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun), which was revised to "THEMIS-ARTEMIS" in 2019.[3] In February 2017, THEMIS celebrated ten years of science operations. As of August 2017, the three THEMIS inner probes continue to collect valuable data on the Sun's interaction with the Earth's magnetosphere.


ARTEMIS probes in lunar orbit

In early 2010, ARTEMIS P1 (THEMIS B) performed two lunar flybys and one Earth flyby, and approached insertion into a Lissajous orbit around a lunar Lagrange point. Lunar orbit insertion was targeted for April 2011. ARTEMIS P2 (THEMIS C) completed a lunar flyby and was on the inbound leg of the first of three deep space excursions on its way to a Lissajous orbit and was targeted for lunar orbit in April 2011.[13]

On June 22, 2011, ARTEMIS P1 began firing its thrusters to move out of its kidney-shaped libration orbit on one side of the Moon, where it had been since January.[14] On July 2, 2011 12:30 p.m. EDT, ARTEMIS P1 entered lunar orbit. The second spacecraft, ARTEMIS P2, moved into lunar orbit on July 17, 2011.[15] Along the way, the two spacecraft were the first to ever achieve orbit around the Moon's Lagrangian points.[14][16]

As of October 2019, both lunar probes are in stable orbits, and are expected to remain operational for a long time.[3]

Animation of THEMIS-B
Geocentric orbit phase
Trans-lunar injection phase
Lissajous orbit phase
Selenocentric orbit phase
  THEMIS-B ·   Earth ·   Moon


Aboard the spacecraft

  • Instrument Data Processing Unit (IDPU): The IDPU houses most of the electronics for the instruments on the THEMIS spacecraft.
  • Electric Field Instruments (EFI): The EFI is designed and built to sense the electric field in Earth's ever-changing magnetosphere
  • Fluxgate magnetometer (FGM): The FGM measures the background magnetic field to identify and time the abrupt reconfigurations of the magnetosphere during substorm onset.
  • Search-coil magnetometer (SCM): The SCM measures low frequency magnetic field fluctuations and waves in three directions in Earth's magnetosphere.
  • Electrostatic Analyzer (ESA): The ESA measures thermal electrons and ions to identify and track high-speed flows through the magnetotail and identify pressure pulses.
  • Solid State Telescope (SST): The SST measures energetic particle distribution functions.
  • Digital Fields Board (DFB): The DFB uses an FPGA to perform configurable on-board band pass processing and fast Fourier transformations(FFT) on instrument data.

Ground based

As the satellites monitor the magnetosphere from orbit, twenty ground stations in North America simultaneously monitor auroras. Ground station mission and science operations are being managed by the University of Calgary's Auroral Imaging Group and the University of California Space Sciences Laboratory.

  • Ground-Based All-Sky Imager (ASI) Array: The ground-based All-Sky Imager (ASI) array observes the aurora over the Northern American continent from Canada to Alaska in order to determine where and when the auroral substorm onset occurs.
  • Ground-Based Magnetometer (GMAG) Array: The GMAG Measure changes in Earth's magnetic field near Earth's surface due to substorm onset to help determine the timing of substorm events.

Research results

The THEMIS instruments have been used to track whistler-mode chorus waves during can persist in periods of low geomagnetic activity.[17][18]

Swales Aerospace

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 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-vac, vibration and acoustic tests, was conducted at NASA's Jet Propulsion Laboratory in Pasadena, California.


The Fast Auroral Snapshot Explorer (FAST) mission supported THEMIS in 2008 and 2009 before being retired.[19] FAST was a Small Explorer program (SMEX) mission launched in 1996.[19]

See also

Lists of relevant topics

Other relevant spacecraft


  1. 1.0 1.1 Lynn Jenner (May 28, 2009). "THEMIS Mission Pages". NASA. 
  2. Phillips, Tony. "Dead Spacecraft Walking". ARTEMIS mission site. NASA. 
  3. 3.0 3.1 3.2 Johnson-Groh, Mara (October 7, 2019). "Artemis, meet ARTEMIS: Pursuing Sun Science at the Moon". NASA. 
  4. Justin Ray (February 18, 2007). "Mission Status Center: THEMIS". SpaceFlight Now. 
  5. World Data Center for Satellite Information (March 1, 2007). "SPACEWARN Bulletin, No. 640". National Space Science Data Center. 
  6. Space Sciences Laboratory (2009). "Themis Orbits". UC Berkeley. 
  7. Cynthia O'Carroll (December 11, 2007). "NASA Spacecraft Make New Discoveries About Northern Lights". Goddard Space Flight Center. 
  8. Tony Phillips (March 20, 2008). "Spring is Aurora Season". Science@NASA. 
  9. Cynthia O'Carroll (December 7, 2007). "Multimedia for the Press Event for THEMIS". Goddard Space Flight Center. 
  10. Laura Layton (July 24, 2008). "THEMIS Satellites Discover What Triggers Eruptions of the Northern Lights". NASA Goddard Space Flight Center. 
  11. Vassilis Angelopoulos (August 15, 2008). "Tail Reconnection Triggering Substorm Onset". Science 321 (5891): pp. 931–935. doi:10.1126/science.1160495. Bibcode2008Sci...321..931A. 
  12. Tariq Malik (July 24, 2008). "Secret of Colorful Auroras Revealed". 
  13. Space Sciences Laboratory. "THEMIS News and Events". UC Berkeley. 
  14. 14.0 14.1 Fox, Karen C.. "First ARTEMIS Spacecraft Successfully Enters Lunar Orbit". The Sun-Earth Connection: Heliophysics. NASA. 
  15. Hendrix, Susan. "Second ARTEMIS Spacecraft Successfully Enters Lunar Orbit". The Sun-Earth Connection: Heliophysics. NASA. 
  16. Broschart, S.B. (2009). "Preliminary Trajectory Design for the ARTEMIS Lunar Mission". AAS/AIAA Astrodynamics Specialists Meeting. Pittsburg. 
  17. Li, W.; Thorne, R. M.; Angelopoulos, V.; Bortnik, J.; Cully, C. M.; Ni, B.; LeContel, O.; Roux, A. et al. (7 May 2009). "Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft". Geophysical Research Letters 36 (9): L09104. doi:10.1029/2009GL037595. 
  18. Li, W.; Bortnik, J.; Thorne, R. M.; Angelopoulos, V. (2011). "Global distribution of wave amplitudes and wave normal angles of chorus waves using THEMIS wave observations" (in en). Journal of Geophysical Research: Space Physics 116 (A12). doi:10.1029/2011JA017035. ISSN 2156-2202. 
  19. 19.0 19.1 FAST EPO News

External links

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