CHAPTER 17 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION 286 Micrometeoroid and Orbital Debris Spacecraft in orbit around the Earth encounter small fragments of rock called micrometeoroids (as opposed to meteorites, which are larger pieces of rock that survive reentry into Earth’s atmosphere). Orbiting spacecraft also encounter a growing amount of debris such as rocket bodies and pieces of spacecraft that have exploded. Collectively, these particles are called MMOD. The entire ISS must be maneuvered to dodge larger pieces that could slam into the station and create a large hole in the structure. However, the ISS is constantly exposed to tiny particles that are too small to track from the ground (Figure 5). This issue is discussed, in greater detail, in Chapter 8. Partway through the ISS assembly, serious cuts in the EMU gloves were seen, which caused alarm within the EVA community. ISS EVAs were already rough on an astronaut’s gloves due to connector manipulation and moving long distances, but crews also started pointing out and photographing more and more MMOD strikes on the ISS that could be contributing factors. When a piece of debris strikes the ISS, it can leave a small pit or hole with sharp points that could tear an EMU glove (Figure 6). Initially, the crew was told to mark an MMOD strike with a wire tie (i.e., a few inches of wire similar to a coat hanger) by wrapping it around the handrail as a visual indication to the next astronaut. However, the strikes became too numerous. Crew members are now given a briefing where they view several photos that show the many known MMOD strikes. The message to the crew: Look before touching. Each time an astronaut performs an EVA, a new strike might have happened since the last EVA. The EMU glove materials were upgraded to be more durable, but the gloves are still carefully photographed and inspected after each EVA. Tethers—a crew member’s lifeline to the ISS—are also inspected for MMOD strikes before reuse. Depending on the EVA, the calculated odds are approximately 1 in 8000 that an EVA crew member’s spacesuit will be struck by MMOD during that EVA, although the strike would have to create a hole approximately 4 mm (0.2 in.) or larger to be fatal. The odds that any crew member, inside the ISS or performing an EVA, will not survive because of an MMOD strike to the ISS are around 1 in 120 over a 6-month period, since a hurtling piece of debris could puncture a pressurized module and cause rapid depressurization that is too fast to allow astronauts or equipment to react. Figure 5. Two views of an MMOD strike on a tool that had been on the ISS exterior, exposed to the elements. The crew found this crater during Space Shuttle mission STS-123/ISS-1J/A. Although a lot of the outer-most material is missing, the hole in the fabric is the area of greatest concern. Figure 6. Glove tears that were noted during an EVA on STS-118/ISS 13A.1, causing the flight control team to terminate the EVA (i.e., bring the crew member back into the airlock early) as a preventative measure. Fortunately, the bladder that holds in the oxygen and keeps pressure on the astronaut was not damaged. Only the outer layers and fabric were affected.