143 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT CHAPTER 8 Space junk that is marginally above the 1-cm (0.4-in.) capability of the shielding but less than the 10-cm (3.9-in.) threshold that can be ground tracked may cause a penetration of the ISS hull, which can result in an overboard leak and depressurization. The crew has the tools, procedures, and training to arrest such a leak by locating and placing a patch over the interior hull if the hole can be found before the ISS stack pressure drops too low. In the most extreme case, the crew can close off a leaking module by closing the connecting hatches and isolating it, which may cause the loss of the module but would leave the crew members and their escape vehicles safe and intact. Protection against larger pieces is done by altering the orbit to actively avoiding the debris, as described below. Active Orbital Debris Tracking from the Ground and from Space The first step in this protection is the tracking of debris by the US DoD Space Surveillance Network (SSN). The SSN uses radar and optical sensors, both on the ground and in space, that detect and track debris in orbit and build a catalog of objects in space along with their orbital characteristics. The SSN tracks and updates the orbital location of the debris as the debris changes orbit, due to atmospheric drag, or breaks apart. Figure 4 shows one space- scanning radar complex that is part of the SSN. Figure 4. Millstone/Haystack radar complex used to track orbital debris. The installation is located in Tyngsboro, Massachusetts. Currently, the SSN uses 29 optical and radar sensors to characterize space debris, and makes approximately 400,000 measurements per day. The sensors are divided into dedicated sensors (used exclusively for space surveillance) and contributing/ collateral sensors (sometimes used for other purposes). These sensors are spread around the Earth to better cover possible orbital