The Magnetospheric Multiscale, or MMS, mission relies on four spacecraft with an identical set of 11 instruments made of 25 sensors. The four spacecraft fly in an adjustable, pyramid formation that enables them to observe the 3-dimensional structure of magnetic reconnection. Four spacecraft give MMS the necessary observational perspectives to determine whether reconnection events occur in an isolated locale, everywhere within a larger region at once, or traveling across space.
Each MMS observatory is in the shape of an octagon, roughly 11 feet across and 4 feet high, built around a central cylindrical thrust tube. The majority of the science instruments and associated electronics are mounted on the underside of the top deck. The flight control hardware is installed on the upper side of the bottom deck. Each observatory is also equipped with six long electric antennas with science sensors on the end as part of the science experiments.
Primary power is provided by eight solar array panels, with a secondary battery for energy storage and use during eclipses. The propulsion system consists of 12 thrusters and four hydrazine propellant tanks located within the central thrust tube. The MMS spacecraft are spin-stabilized, with a spin rate of three revolutions per minute. Attitude information is provided by four star cameras, two three-axis accelerometers, and two sun sensors. The thrusters are used for attitude and orbit adjustment maneuvers.
MMS is also equipped with a new navigator based on extremely sensitive GPS equipment to provide absolute position information. The observatories require such sensitive sensors because the satellites fly in an orbit higher than that of the GPS satellites, so they must rely on the weaker signals from GPS satellites on the far side of Earth.
The MMS spacecraft are being developed at NASA's Goddard Space Flight Center in Greenbelt, Md. The four observatories are being built, tested and integrated nearly simultaneously.