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Introduction

Spacecraft in low Earth orbit (LEO) continually collide with meteoroids passing through near-Earth space and with orbital debris created by human activities in space. The vast majority of these meteoroids and debris are much smaller than a millimeter in diameter and cause little damage. A small fraction of the meteoroid and debris populations, however, are larger and can cause severe damage in a collision with a spacecraft. The chance of impact with larger objects relates directly to the size and orbital lifetime of a spacecraft. The larger the spacecraft and the longer it remains in orbit, the more likely it will collide with potentially damaging objects (NRC, 1995a).

The International Space Station (ISS) will be the largest spacecraft ever built. ISS assembly in LEO is due to begin in late 1997, and the station is expected to remain operational for at least 15 years. When assembly is complete, the multibillion-dollar ISS will have a mass of 419,000 kg, a crew of approximately six researchers, and more than 11,000 m2 of surface area exposed to the space environment (NRC, 1995b). Figure 1-1 depicts the ISS at the end of its assembly sequence. Due to its large surface area, long functional lifetime, and the potential for a catastrophic outcome from a collision, protecting the ISS from meteoroids and debris poses a unique challenge.

The National Aeronautics and Space Administration (NASA) has been aware of the potential hazard to the space station from meteoroids and debris since the inception of the program (Portree and Loftus, 1993). The agency has addressed the problem by `king to better understand the meteoroid and debris hazard and by taking steps to protect the space station from the hazard. Both these tasks are formidable.



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Protecting the Space Station from Meteoroids and Orbital Debris 1 Introduction Spacecraft in low Earth orbit (LEO) continually collide with meteoroids passing through near-Earth space and with orbital debris created by human activities in space. The vast majority of these meteoroids and debris are much smaller than a millimeter in diameter and cause little damage. A small fraction of the meteoroid and debris populations, however, are larger and can cause severe damage in a collision with a spacecraft. The chance of impact with larger objects relates directly to the size and orbital lifetime of a spacecraft. The larger the spacecraft and the longer it remains in orbit, the more likely it will collide with potentially damaging objects (NRC, 1995a). The International Space Station (ISS) will be the largest spacecraft ever built. ISS assembly in LEO is due to begin in late 1997, and the station is expected to remain operational for at least 15 years. When assembly is complete, the multibillion-dollar ISS will have a mass of 419,000 kg, a crew of approximately six researchers, and more than 11,000 m2 of surface area exposed to the space environment (NRC, 1995b). Figure 1-1 depicts the ISS at the end of its assembly sequence. Due to its large surface area, long functional lifetime, and the potential for a catastrophic outcome from a collision, protecting the ISS from meteoroids and debris poses a unique challenge. The National Aeronautics and Space Administration (NASA) has been aware of the potential hazard to the space station from meteoroids and debris since the inception of the program (Portree and Loftus, 1993). The agency has addressed the problem by `king to better understand the meteoroid and debris hazard and by taking steps to protect the space station from the hazard. Both these tasks are formidable.

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Protecting the Space Station from Meteoroids and Orbital Debris It is difficult to characterize the hazard posed by meteoroids and debris to the ISS because most of the meteoroids and debris that could harm the space station are small, dark, and fast moving and thus difficult to detect from the Earth. Moreover, the meteoroid and debris environment in the ISS orbit can vary greatly, depending on the state of the solar cycle and the number and severity of recent breakups of orbiting objects. Adequately protecting the ISS from this environment is also challenging because of the uncertainty of the threat and the difficulty of accurately simulating the effects of high-speed meteoroid and debris impacts. The team building the ISS has developed a strategy to manage the hazard posed by meteoroids and debris to the ISS. To support this strategy, the team has developed models that predict the flux of meteoroids and debris in the ISS orbit. The ISS program uses these models to determine the chances that the station will collide with meteoroids and debris of various sizes. The program plans to reduce the hazard by: (1) shielding elements of the ISS to protect them from impacts with the smallest meteoroids and debris, (2) moving the ISS out of the path of the rare pieces of debris large enough to be tracked by ground-based sensors, and (3) implementing design features and operational procedures to minimize the FIGURE 1-1 The International Space Station. Source: NASA.

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Protecting the Space Station from Meteoroids and Orbital Debris hazard to the station or crew if the ISS collides with meteoroids or debris too large to be shielded against but too small to be tracked by ground based-sensors. In this report, the committee examines the ISS program strategy for reducing the hazard of meteoroid and debris impact and recommends alternative strategies where appropriate. Chapter 2 examines the overall ISS meteoroid and debris risk management strategy. Chapter 3 looks at the NASA meteoroid and debris environment models, and Chapter 4 examines the vulnerability of the ISS to impact and the use of protective shields. Chapter 5 addresses methods to reduce the risk to the station and crew in the event of a damaging impact. Finally, Chapter 6 explores the use of collision warning and avoidance systems. REFERENCES NRC (National Research Council). 1995a. Orbital Debris: A Technical Assessment. Committee on Space Debris, Aeronautics and Space Engineering Board . Washington, D.C.: National Academy Press. NRC (National Research Council). 1995b. The Capabilities of Space Stations. Committee on Space Station, Aeronautics and Space Engineering Board . Washington, D.C.: National Academy Press. Portree, D.S.F., and J.P. Loftus, Jr. 1993. Orbital Debris and Near-Earth Environmental Management: A Chronology . NASA Reference Publication 1320. Linthicum Heights, Maryland: NASA Center for Aerospace Information.