orbits and bases the distribution of orbital inclinations on the inclinations of objects tracked and cataloged by the U.S. Space Command. Terms of the equations predicting future changes in the debris flux were based on assumptions about future spacecraft launches and the number and nature of future spacecraft and rocket body breakups.

The 1994 debris model updated the 1991 model for altitudes between 350 km and 600 km and inclinations of approximately 51.6 degrees. The updated model incorporates new data primarily from the Haystack radar (Stansbery et al., 1994), but also from an analysis of data from the Goldstone radar, the analysis of the Long Duration Exposure Facility (LDEF) surfaces (Levine, 1991), and a recalibration of U.S. Space Command radars. The 1994 model uses many of the same assumptions as the 1991 model, including estimates for object density and shape and the assumption that objects travel in circular orbits.

The ISS program replaced the 1994 debris model with the 1996 model for ISS safety evaluations conducted after May 1996. The new debris model, which was peer-reviewed by NASA and outside reviewers, incorporates additional data from the Haystack radar (Stansbery et al., 1996), LDEF, space shuttle impacts, and from an analysis of the perturbing force of solar radiation. The 1996 model provides debris flux information for spacecraft in all orbital inclinations for altitudes up to 2,000 km. Unlike earlier models, this model begins by defining a population of debris divided into six inclination bands, two eccentricity families, and six size ranges. These populations are based on the existing data, but, where data are lacking, estimates derived from the complex NASA EVOLVE model (Reynolds, 1993) and other support models are used. The debris model then calculates the flux of this population on a spacecraft in a given orbit. The 1996 model is thus better than previous models at accurately representing changes in the size distribution of debris with altitude and inclination. This is also the first debris model that incorporates the large amounts of debris that travel in elliptical orbits.

In the meteoroid model, the impact velocity of meteoroids with orbiting spacecraft velocities can range up to about 70 km/s, with an average velocity of about 19 km/s. The mean density of meteoroids is modeled as 2 g/cm3 for meteoroids smaller than 10−6 g, as 1 g/cm3 for meteoroids between 10−6 and 0.01 g, and as 0.5 g/cm3 for masses above 0.01 g. The meteoroid model includes the effects of the normal annual meteor showers, but it does not account for rare meteor storms that occur when the Earth passes through a particularly dense portion of a comet dust trail. The ISS program, however, is aware of the potential hazard from such storms and is evaluating potential actions (e.g., restricting extravehicular activity during meteor storms) to reduce the hazard. Figure 3-1 compares the modeled flux of meteoroids and debris in the ISS orbit.

The debris environment predicted by the 1996 model differs in a number of ways from the environment predicted by the 1991 model. For example, the predicted flux of objects larger than 1.0 cm in diameter in the 1996 model is half the



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