CHAPTER 3 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON 38 transfer between the inside and the outside (Figure 4). The feedthroughs are designed to ensure the holes in the module have at least two seals to vacuum. Additionally, the internal pieces of each feedthrough are specially sealed and tested to ensure no leaks occur from the cabin out to space through the connector. Every effort was made to minimize the number of items that must cross the pressure shell of the modules to minimize the risk of air leaks. Cabin Vent Valve Feedthroughs Group of Electrical Feedthroughs Figure 4. The interior of the Permanent Multipurpose Module pressure shell endcone with a number of power, data, and gas feedthroughs. Each electrical feedthrough in this photo is approximately 4 cm (1.5 in.) in diameter. Hatchways are also feedthroughs that provide a means to get into and out of each module (reference the “Hatches” section of this chapter). Windows provide a means for science experiments to study the Earth, for crew members to view their home planet, and for the operation of a number of educational programs. Windows are essentially large feedthroughs with optical panes installed instead of power or data connectors (reference the “Windows on the World” section of this chapter). Each of the holes in the primary structure of the modules for these feedthroughs was designed and reviewed for its ability to keep the atmosphere in and keep the crew safe. Each feedthrough is required to have at least two seals to the vacuum of space. Even with these precautions, the ISS crews are trained extensively on how to handle unexpected depressurizations that are either due to a failure in a seal of a module feedthrough or from an impact by orbital debris. Crew members are equipped with emergency response procedures and equipment that they can use to try to pinpoint the leak location and attempt to repair it. Pressurized Module Assembly Figure 5. The probe docking mechanism of an incoming Progress cargo vehicle. This funnel-shaped probe interfaces with a receiving cone on the docking mechanisms of the Russian segment of the space station. The hatchway that the crew will translate through after docking, which is the space inside the two orange rubber o-rings, is 80 cm (31.5 in.) in diameter. Although the Russian modules of the ISS are all connected using an automated “probe-and-cone” docking system (Figure 5), this system does not allow for large hatchways that can accommodate transferring large objects, including various payload racks (reference the “Racks” section of this chapter), between modules.