The Magnetospheric Multiscale (MMS) Mission is a
NASA robotic space mission to study the
Earth's magnetosphere, using four identical spacecraft flying in a
tetrahedral formation. The spacecraft were launched on 13 March 2015 at 02:44
UTC. The mission is designed to gather information about the
magnetic reconnection, energetic particle acceleration, and
turbulence — processes that occur in many astrophysical
plasmas. As of March 2020, the MMS spacecraft have enough fuel to remain operational until 2040.
The mission builds upon the successes of the
ESACluster mission, but will surpass it in spatial resolution and in temporal resolution, allowing for the first time measurements of the critical electron diffusion region, the site where magnetic reconnection occurs. Its orbit is optimized to spend extended periods in locations where reconnection is known to occur: at the dayside
magnetopause, the place where the pressure from the
solar wind and the planets' magnetic field are equal; and in the
magnetotail, which is formed by pressure from the solar wind on a planet's magnetosphere and which can extend great distances away from its originating planet.
Magnetic reconnection in
Earth's magnetosphere is one of the mechanisms responsible for the
aurora, and it is important to the science of controlled
nuclear fusion because it is one mechanism preventing
magnetic confinement of the fusion fuel. These mechanisms are studied in outer space by the measurement of motions of matter in stellar atmospheres, like that of the Sun. Magnetic reconnection is a phenomenon in which energy may be efficiently transferred from a magnetic field to the motion of charged particles.
The MMS mission consists of four spacecraft. Each has a launch mass of 1,360 kg (3,000 lb). In their stowed launch configuration, each are approximately 3.5 by 1.2 m (11.5 by 3.9 ft), and when stacked together they have a total height of 4.9 m (16 ft). After being deployed in orbit, a total of eight axial and wire booms are deployed, including four Spin-Plane Double Probe (SDP) wire booms each 60 m (200 ft) long. 
The MMS spacecraft are spin stabilized, turning at a rate of three revolutions per minute to maintain orientation. Each spacecraft contains 12 thrusters connected to four hydrazine fuel tanks. Position data is provided by highly sensitive GPS equipment, while attitude is maintained by four
star trackers, two
accelerometers, and two
The mission is broken into three phases. The commissioning phase will last approximately five and a half months after launch, while the science phases will last two years. The first science phase will focus on the magnetic boundary between the
Sun (day side operations) for one and a half years, with the spacecraft formation orbiting the Earth at 2,550 by 70,080 km (1,580 by 43,550 mi). The second science phase will study reconnection in Earth's magnetic tail (night side operations) for half a year, increasing the orbit to 2,550 by 152,900 km (1,580 by 95,010 mi).
Each spacecraft carries several experiments, divided into three suites: the Hot Plasma Suite, the Energetic Particles Detector Suite, and the Fields Suite.
Hot Plasma Suite
The Hot Plasma Suite measures
plasma particle counts, directions, and energies during reconnection. It consists of two instruments:
Fast Plasma Investigation (FPI), a set of four dual electron
spectrometers (DES) and four dual ion spectrometers (DIS).
Hot Plasma Composition Analyzer (HPCA), detects particle speed in order to determine its mass and type.
Energetic Particles Detector
The Energetic Particles Detector Suite detects particles at energies far exceeding those detected by the Hot Plasma Suite. It consists of two instruments:
Fly's Eye Energetic Particle Sensor (FEEPS), a set of silicon solid state detectors to measure electron energy. Between two FEEPS per spacecraft, the individual detectors are arranged to provide 18 different view angles simultaneously; hence the term "fly's eye".
Energetic Ion Spectrometer (EIS), measures energy and total velocity of detected ions in order to determine their mass. The EIS can detect helium and oxygen ions at energies higher than that of the HPCA.
The Fields Suite measures magnetic and electric field characteristics. It consists of six instruments:
magnetometer (AFG), determines the strength of magnetic fields.
Digital Fluxgate magnetometer (DFG), determines the strength of magnetic fields.
Electron Drift Instrument (EDI), measures electric and magnetic field strength by sending a beam of electrons into space and measuring how long it takes the electrons to circle back in the presence of these fields.
Spin-plane Double Probe (SDP), consists of
electrodes on the end of four 60 m (200 ft) wire booms that extend from the spacecraft to measure electric fields.
Axial Double Probe (ADP), a set of electrodes on two 15 m (49 ft) antennas mounted axially on the spacecraft.
Search Coil Magnetometer (SCM), an induction magnetometer used to measure magnetic fields.
In order to collect the desired science data, the four satellite MMS constellation must maintain a tetrahedral formation through a defined region of interest in a highly elliptical orbit. The formation is maintained through the use of a high altitude rated
GPS receiver, Navigator, to provide orbit knowledge, and regular
formation maintenance maneuvers. Through Navigator, the MMS mission broke the
Guinness World Record twice for highest altitude fix of a GPS signal (at 70,000 km (43,000 mi) and 187,200 km (116,300 mi) above the surface in 2016 and 2019 respectively).
MMS has since detected magnetic reconnection occurring in unexpected places. In 2018, MMS made the first-ever detection of magnetic reconnection in the
magnetosheath, a region of space previously thought to be too chaotic and unstable to sustain reconnection.Magnetic flux ropes and
Kelvin–Helmholtz vortices are other phenomena where MMS has detected reconnection events against expectations.
Sharma, A. Surjalal; Curtis, Steven A. (2005). "Magnetospheric Multiscale Mission". Nonequilibrium Phenomena in Plasmas. Astrophysics and Space Science Library. Vol. 321. Springer Netherlands. pp. 179–195.
National Research Council (2003). The Sun to the Earth - And Beyond. National Academies Press.
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ).
Cubesats are smaller. Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).