The MMS is comprised of four primary modules and two sub-modules, each designed to provide the spacecraft with a specific functionality. The Propulsion Module provides maneuvering and orbit maintenance, the Attitude Control Module provides attitude determination and pointing control, the Command and Data Handling Module provides computer control and communications, and the Power Module provides electrical power. The Earth Sensor Assembly Module supplies the satellite's attitude relative to the earth, and the Signal Conditioning and Control Unit performs miscellaneous control functions.
The MMS is considered "off-the-shelf" because the design has been an integral part of three previous spaceflight missions: the Solar Maximum Mission and Landsat 4 and 5. Using off-the-shelf hardware was determined to be safer and more cost effective than developing a custom spacecraft for the mission. A separate Instrument Module is attached to the MMS to accommodate the mission-specific sensors and viewing geometry.