Not including items attached on the outside, the equipment module measures approximately 80 centimeters (2.6 feet) tall. Most of the electronics that run the spacecraft lie inside the module, while the science instruments are bolted outside on the end opposite the propulsion module.
Inside, two identical computers will orchestrate almost all of the spacecraft's flight activities. Although only one of the two units will control Surveyor at any one time, identical software will run concurrently in the backup unit in case of an emergency. Each computer control unit consists of a Marconi 1750A microprocessor, 128 Kbytes of RAM for storing programs to control the spacecraft, and 20 Kbytes of ROM that contain code to run basic survival routines in the event that the computers experience a reset.
Additional storage space for science and spacecraft health data will be provided by two solid-state recorders with a combined capacity of 375 megabytes. Surveyor will be America's first spacecraft sent to another planet to exclusively use computer RAM instead of a tape recorder for mass data storage. This technological improvement will dramatically reduce operational complexity, thereby reducing mission planning costs during flight.
The equipment module also houses three "reaction wheels" mounted in directions at right angles to each other. Surveyor's flight computers can control the spin of each one of these disks. By spinning different sets of disks at fast speeds, the spacecraft will be able to rotate its body and point its rocket engine or science instruments in any direction. Engineers refer to this concept of spinning disks to change pointing directions as "attitude control."