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 microphysics of magnetic reconnection, energetic particle acceleration, and turbulence⁠ — processes that occur in many astrophysical plasmas. As of March 2020, the MMS spacecraft has enough fuel to remain operational until 2040.

The mission builds upon the premise of the ESA Cluster mission, but MMS instrumentation surpasses 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.

In order to resolve the three-dimensional structure of magnetic reconnection at varying spatial scales, the four identical MMS spacecraft orbit the Earth in a tetrahedral formation with adjustable separation distances. This enables simultaneous sampling of the plasma and fields at multiple points in space to measure spatial gradients and temporal variations.  Such measurements are essential for quantifying terms in Maxwell's equations that describe the evolution of the electromagnetic fields, and makes it possible to distinguish between spatial and temporal structures. The capability for multi-point measurements is crucial for studying magnetic reconnection and cannot be achieved with a single-spacecraft mission.

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 Sun sensors.

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 Earth and 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.