53 SYSTEMS: STRUCTURE AND MECHANISMS—THE INTERNATIONAL SPACE STATION’S SKELETON CHAPTER 3 The design concept of these windows is that any high-velocity debris would hit the outermost debris pane, followed by the outer pressure pane. If the debris were going fast enough to break through both panes, the particles would be slowed enough such that they would not penetrate the inner pressure pane. As shown in the diagram in Figure 21, this design is similar to the Whipple Shield design of the module debris shields described earlier. The smaller windows on the ISS, such as the US hatch windows, have the same dual- pressure pane design concept as well as inner and outer protective covers. Window Assembly Components Window Panes Figure 21. Overview of the US Laboratory window. Note that the flex hose was the source of the leak discussed in Chapter 16. The windows of the US hatches, US Laboratory, JEM, and Cupola are designed to be replaceable in the event a window pane breaks. An astronaut would install an external pressure cover over the window, via an EVA, to replace the windows exposed to space. The window would then be removed from the inside of the ISS. The windows themselves cannot be removed by a spacewalking astronaut the removal must be done from the pressurized environment of the ISS cabin. Thus, if debris were to damage both panes of a window and cause a module to depressurize, it would not be possible to replace that damaged window. Instead, the crew would install the external pressure cover, via EVA, and repressurize the evacuated module. The crew could then go into that module and remove the window. With the window removed, the crew would install an internal pressure cover over the window’s hole until the new window was ready to be installed. The pressure covers are on orbit to provide a means to respond to a broken window pane however, no spare windows are kept on board the ISS. A spare would need to be manufactured and flown after the failure.