CHAPTER 5 SYSTEMS: COMMAND AND DATA HANDLING—THE BRAINS OF THE INTERNATIONAL SPACE STATION 92 Astronaut Susan Helms does a little “light reading” on the International Space Station. Brains. That is essentially what the Command and Data Handling (C&DH) system is for a spacecraft. Part of what is termed avionics, the C&DH system is responsible for the control of the primary systems of a spacecraft. Early spacecraft used electrical and mechanical switches to operate the vehicle. From the beginning of the Space Age, up through the Space Shuttle era, the astronaut was a primary component of the C&DH system—adjusting dials as needed, throwing a switch to configure a system, and responding when a component malfunctioned. Over time, computers played an ever-more-crucial role. Computers performed critical calculations that required accuracy and speed such as calculating the trajectory of a spacecraft as it descended to the lunar surface. The lunar module navigation computer possessed less than 40 kilobytes of memory and ran with a processing speed of 2.048 megahertz (MHz). By comparison, a basic Apple iPhone in 2014 contained 200 times more memory, ran 1,000 times faster, and produced pictures typically 2 megabytes (MB) in size. For the International Space Station (ISS), the crew would no longer be a primary component of the C&DH system. Computers took over virtually every aspect of the vehicle’s operations. Whereas the crew on the Space Shuttle interacted with the flight systems primarily through the use of switches or dials, nearly all aspects of the spacecraft operating system on the ISS are operated by computer interface and are therefore readily operable by the ground control team (i.e., the ground). This frees the crew to focus on research. The C&DH system directs the operations of other systems via