MESSENGER’s “brain” is its Integrated Electronics Module (IEM), a space- and weight-saving device that combines the spacecraft’s core avionics into a single box. The spacecraft carries a pair of identical IEMs for backup purposes; both house a 25-megahertz (MHz) main processor and 10-MHz fault protection processor. All four are radiation-hardened RAD6000 processors, based on predecessors of the PowerPC chip found in some models of home computers. The computers, slow by current home-computer standards, are state of the art for the radiation tolerance required on the MESSENGER mission.
Programmed to monitor the condition of MESSENGER’s key systems, both fault protection processors are turned on at all times and protect the spacecraft by turning off components and/or switching to backup components when necessary. The main processor runs the Command and Data Handling software for data transfer and file storage, as well as the Guidance and Control software used to navigate and point the spacecraft. Each IEM also includes a solid-state data recorder, power converters, and the interfaces between the processors and MESSENGER’s instruments and systems.
Intricate flight software guides MESSENGER’s Command and Data Handling system. MESSENGER receives operating commands from Earth and can perform them in real time or store them for later execution. Some of MESSENGER’s frequent, critical operations (such as propulsive maneuvers) are programmed into the flight computer’s memory and timed to run automatically.
For data, MESSENGER carries two solid-state recorders (one backup) able to store up to 1 gigabyte each. Its main processor collects, compresses, and stores images and other data from MESSENGER’s instruments onto the recorder; the software sorts the data into files similar to how files are stored on a PC. The main processor selects the files with highest priority to transmit to Earth, or mission operators can download data files in any order the team chooses.
Antenna signal strength (and downlink rate) varies with spacecraft-Earth distance and ground-station antenna size. While orbiting Mercury MESSENGER will store most of its data when it’s farther from Earth, typically sending only information on its condition and the highest-priority images and measurements during regular eight-hour contacts through NASA’s Deep Space Network. The spacecraft will send most of the recorded data when Mercury’s path around the Sun brings it closer to Earth.