MMS (Magnetospheric Multiscale)

Magnetic reconnection is a physical process in which magnetic field lines from different magnetic domains collide and snap into new configurations, explosively releasing vast amounts of stored magnetic energy into plasma. This process drives solar flares, auroras, geomagnetic storms that can disrupt power grids, and is fundamental to how energy and mass are transferred throughout the universe. The MMS mission studies this phenomenon in Earth's magnetosphere because it is one of the few places where reconnection can be directly measured with spacecraft.

The MMS mission consists of four identical spacecraft that fly in a tightly controlled tetrahedral formation, sometimes as close as 7 kilometers apart. Flying in formation allows the constellation to make three-dimensional measurements of magnetic fields and plasma at multiple points simultaneously, which is necessary to observe the structure and dynamics of reconnection events. The four spacecraft were launched together on a single Atlas V rocket on March 12, 2015.

MMS flies in a highly elliptical orbit that takes the four spacecraft from as close as approximately 2,500 km to as far as about 152,000 km from Earth, allowing it to sample both the magnetopause (the boundary where solar wind meets Earth's magnetic field) and the magnetotail. In Phase 2 of the mission, NASA doubled the apogee to reach the more distant magnetotail, where reconnection also occurs. This orbit allows MMS to observe reconnection in both the day-side and night-side regions of the magnetosphere.

In 2016, the MMS team announced the first direct detection of the electron dissipation region, the microscopic heart of magnetic reconnection where magnetic field lines actually break and reform. This was a decades-long goal in plasma physics, achieved by the four spacecraft flying through the reconnection region at precisely the right moment at unprecedentedly high data rates. MMS has since provided detailed measurements of how energy is converted to heat and particle acceleration during reconnection, fundamentally advancing our understanding of this universal process.