3

Meteoroid and Debris Environment Models

CURRENT PROGRAM

NASA has created models of the meteoroid and debris environment to aid in designing the ISS and in evaluating the effectiveness of techniques to mitigate the hazard from meteoroids and debris. The meteoroid environment used in the design of the ISS consists of a flux model (Grün et al., 1985) and a velocity model (Erickson, 1968; Kessler, 1969). The primary model of the debris environment used in the ISS design was created in 1991 (Kessler et al., 1994). Since then, NASA has updated the debris model twice, once in 1994 (Kessler, 1994) and again in 1996 (Zhang, 1996). The 1991 model is now primarily used to assess whether elements of the ISS meet their PNP requirements. The later models are primarily used to assess the effectiveness of shielding and other hazard mitigation approaches.

The 1991 debris model describes the flux of debris on a spacecraft orbiting at any inclination and altitude below 1,000 km. This model consists of a set of equations that describe the existing flux of debris and projected changes in the environment. The part of the model describing the 1991 environment was created by “curve fitting, ” or developing equations that produce results that correspond well with observed data. These data came from a variety of sources, including the analysis of panels returned from the Solar Max satellite (Barrett et al., 1988), the Arecibo (Thompson et al., 1992) and the Goldstone (Goldstein and Randolph, 1990) radars, the U.S. Space Command satellite catalog, the Massachusetts Institute of Technology experimental test site telescope (Taff et al., 1985), and the ground-based electro-optical space surveillance (GEODSS) telescope system (Henize and Stanley, 1990). The 1991 model assumes that objects are in circular



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Protecting the Space Station from Meteoroids and Orbital Debris 3 Meteoroid and Debris Environment Models CURRENT PROGRAM NASA has created models of the meteoroid and debris environment to aid in designing the ISS and in evaluating the effectiveness of techniques to mitigate the hazard from meteoroids and debris. The meteoroid environment used in the design of the ISS consists of a flux model (Grün et al., 1985) and a velocity model (Erickson, 1968; Kessler, 1969). The primary model of the debris environment used in the ISS design was created in 1991 (Kessler et al., 1994). Since then, NASA has updated the debris model twice, once in 1994 (Kessler, 1994) and again in 1996 (Zhang, 1996). The 1991 model is now primarily used to assess whether elements of the ISS meet their PNP requirements. The later models are primarily used to assess the effectiveness of shielding and other hazard mitigation approaches. The 1991 debris model describes the flux of debris on a spacecraft orbiting at any inclination and altitude below 1,000 km. This model consists of a set of equations that describe the existing flux of debris and projected changes in the environment. The part of the model describing the 1991 environment was created by “curve fitting, ” or developing equations that produce results that correspond well with observed data. These data came from a variety of sources, including the analysis of panels returned from the Solar Max satellite (Barrett et al., 1988), the Arecibo (Thompson et al., 1992) and the Goldstone (Goldstein and Randolph, 1990) radars, the U.S. Space Command satellite catalog, the Massachusetts Institute of Technology experimental test site telescope (Taff et al., 1985), and the ground-based electro-optical space surveillance (GEODSS) telescope system (Henize and Stanley, 1990). The 1991 model assumes that objects are in circular

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Protecting the Space Station from Meteoroids and Orbital Debris 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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Protecting the Space Station from Meteoroids and Orbital Debris FIGURE 3-1 Comparison of meteoroid and debris flux in ISS orbit. Source: NASA. flux predicted in the 1991 model. Figure 3-2 compares the flux of debris in the ISS orbit predicted by the 1991, 1994, and 1996 models. Another difference is that the latest model includes objects in elliptical orbits, while the 1991 and 1994 models assume that all objects travel in circular orbits. A third change is that the predicted average impact velocity has been reduced. (The small increase in average collision velocity due to collisions with objects in elliptical orbits is over-shadowed by the reduction in average collision velocities for the much larger population of objects in nearly circular orbits.) Figure 3-3 compares the orbital debris impact velocity distribution predicted by the three models. Table 3-1 compares the 1991, 1994, and 1996 models. ANALYSIS AND FINDINGS NASA has developed a series of increasingly reliable orbital debris environment models using the limited data available. The 1996 model, which

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Protecting the Space Station from Meteoroids and Orbital Debris incorporates new data from a number of sources and divides debris into size, inclination, and eccentricity ranges, will be a useful tool for assessing both the risk posed to the station by debris and the steps that can be taken to manage that risk. The committee believes the model is generally sound, but there is still room for improvement. The ISS program is most concerned about debris ranging from about 0.5 cm to 20 cm in diameter. Debris with diameters in this range may be too large to shield against and too small to track and avoid. Further efforts to more accurately determine the current population of these objects in the ISS orbit, however, may not be the most effective way to help improve the models. At the altitude of the ISS, atmospheric drag steadily removes debris from orbit, and new debris may enter the altitude band as satellites and rocket bodies break up, solid rocket motors eject slag, and the orbits of higher-altitude objects decay. The population of debris at the altitude of the ISS can thus change dramatically in just a few years, FIGURE 3-2 Comparison of model flux predictions. Source: NASA.

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Protecting the Space Station from Meteoroids and Orbital Debris FIGURE 3-3 Comparison of model impact velocity predictions. Source: NASA. depending on the solar cycle (which causes the atmosphere to expand and contract over an 11-year period) and the rate at which new debris is created in LEO. Understanding the sources of objects in this size range and the processes that add and remove these objects from the ISS altitude regime should thus be a priority of further efforts to improve debris models. A major gap exists in the available data about another key size range of meteoroids and debris. Figure 3-4 shows the various data sources used in developing the NASA meteoroid and debris environment models. This figure, which groups together data acquired at a variety of altitude regimes over a multiyear period, shows the extreme paucity of data on meteoroids and debris in the 0.1 mm to 0.5 cm size range. The models deal with this gap by essentially drawing a line connecting the measured flux of objects smaller than 0.1 mm with the measured flux of objects larger than 0.5 cm. A better understanding of the population of objects in this range would be valuable for ISS risk management because most of the potentially damaging impacts with the ISS will come from objects in this size range. The population of these small objects is far more volatile than the population of larger objects; therefore, investigations should focus on the sources and characteristics of the debris.

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Protecting the Space Station from Meteoroids and Orbital Debris TABLE 3-1 Comparison of Orbital Debris Models Characteristics (in ISS orbit) 1991 Model 1994 Model 1996 Model Approximate number of impacts of objects larger than 1 cm in diameter with ISS over 10 years 0.7 0.35 0.35 Average impact velocity 10.8 km/s 9.2 km/s 8.7 km/s Growth in future environment 5% of 1988 population per year (300 new objects per year) for d ≥10 cm 5% of 1991 population per year for altitudes with little atmospheric drag 8% of 1995 population per year for 7 degree incl. band; 4% per year for other bands for altitudes with little atmospheric drag   Growth of 2% per year through 2010; 4% per year after 2010 for d <10 cm Reduced population growth at lower altitudes, decreasing to no growth at around 200 km Reduced population growth at lower altitudes, decreasing to no growth at about 200 km Inclination distribution Based on satellite catalog Based on Haystack data Based on Haystack, catalog, and other sources Object shape Sphere Unchanged Unchanged Density of individual objects 2.8d−0.74 g/cm3 (d in cm) for d ≥0.62 cm 4 g/cm3 for d <0.62 cm Unchanged Unchanged Predominant source of objects larger than 1 cm Breakups Breakups Breakups and solid rocket motor ejecta Includes debris in elliptical orbits No No Yes Approximate number of impacts of objects larger than 0.5 cm in diameter with ISS trailing surfaces oer 10 years 0 0 2 × 10−5 per square meter

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Protecting the Space Station from Meteoroids and Orbital Debris FIGURE 3-4 Data used to create environment models. Source: NASA. Another issue of concern is the assumptions in the models about debris shape and composition. The models represent debris as spheres with a density of 4 g/cm3 for objects smaller than 0.62 cm in diameter. For objects larger than 0.62 cm, the modeled density decreases as the size of the object increases and equals about 2.8 g/cm3 for objects around 1 cm in diameter. The actual composition and shape of debris is not well known. Recent NASA analysis of data from the Haystack radar suggests that aluminum oxide ejected from solid rocket motors may be the most common debris detectable from Earth in the ISS orbit (Reynolds and Zhang, 1996). Fragments from the breakup of spacecraft and rocket bodies may be the second most common debris. Because the density of aluminum oxide is 3.9 g/cm3, the density estimates in the models may be close to the actual values for smaller debris, but may underestimate the density of debris 1 cm in

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Protecting the Space Station from Meteoroids and Orbital Debris diameter and larger. In addition, solid rocket motor ejecta and breakup fragments are unlikely to be spherical. Improved knowledge about the actual shapes and composition of debris would be useful for predicting the damage caused by impact and for developing appropriate shields and damage control strategies. Finally, in any model providing a statistical representation, it is essential to have some accompanying estimate of model uncertainties. Although defining the level of uncertainty in a debris environment model is difficult because of sparse data and the large variability in the environment over time, an assessment of the uncertainty in the debris environment model would be very useful for groups involved in shielding the ISS and in developing damage control hardware and procedures. Finding 3. NASA has done a good job of improving its models of the debris environment in LEO by incorporating new data and making reasonable assumptions about areas where data are sparse. The 1996 model appears to be a good tool for ISS meteoroid and debris risk management. RECOMMENDATIONS Recommendation 4. Wherever possible, the meteoroid and debris analysis integration team should use the 1996 model to assess and mitigate the meteoroid and debris hazard to the International Space Station. Contractors should also be encouraged to use the 1996 model to assess the vulnerability of their International Space Station components. Recommendation 5. NASA should continue to update the 1996 debris environment model by using new data and analyses. Efforts should focus on improving understanding of the processes that add and remove objects in the 0.5 to 20 cm size range from the International Space Station altitude regime, the sources and characteristics of smaller debris down to 1 mm in diameter, and debris composition and shape. NASA should also strive to describe the uncertainty in any new models. Changes to the model should undergo a thorough peer review. To support this improvement, NASA should continue to gather more data to better understand the orbital debris environment. REFERENCES Barrett, R.A., R.P. Bernhard, and D.S. McKay. 1988. Impact Holes and Impact Flux on Returned Solar Max Louver Material. Presented at Lunar and Planetary Science XIX, Houston, Texas, March 14–18, 1988. Erickson, J.E. 1968. Velocity distribution of sporadic photographic meteors. Journal of Geophysical Research 73:3721–3726. Goldstein, R., and L. Randolph. 1990. Rings of Earth Detected by Orbital Debris Radar. JPL Progress Report 42-101. Pasadena, California: NASA Jet Propulsion Laboratory.

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Protecting the Space Station from Meteoroids and Orbital Debris Grün, E., H.A. Zook, H. Fechtig, and R.H. Giese. 1985. Collisional balance of the meteoritic complex. Icarus 62:244–272. Henize, K., and J. Stanley. 1990. Optical Observations of Space Debris. AIAA Paper 90-1230. Presented at AIAA/NASA/DoD Orbital Debris Conference: Technical Issues and Future Directions, Baltimore, Maryland, April 16–19, 1990. Kessler, D.J. 1969. Average relative velocity of sporadic meteoroids in interplanetary space. AIAA Journal 7: 2337–2338. Kessler, D.J. 1994. Update on the Orbital Debris Environment for Space Station. NASA Memorandum SN3-94-164. Houston, Texas: National Aeronautics and Space Administration. November 16, 1994. Kessler, D.J., R.C. Reynolds, and P.D. Anz-Meador. 1994. Space Station Program Natural Environment Definition for Design. NASA SSP 30425, Revision B. Houston, Texas: National Aeronautics and Space Administration Space Station Program Office. Levine, A., ed. 1991. LDEF—69 Months in Space: First Post-Retrieval Symposium. NASA Conference Publication 3134, Part 1. Hampton, Virginia: NASA Langley Research Center. Reynolds, R.C. 1993. Orbital debris environment predictions for space station. In Proceedings of the First European Conference on Space Debris, April 5–7, 1993, Darmstadt, Germany. Darmstadt: European Space Operations Center, pp. 337–339. Reynolds, R.C., and J.C. Zhang. 1996. Orbital Debris Environment Modeling at NASA Johnson Space Center. Briefing presented to the NRC Committee on International Space Station Meteoroid/Debris Risk Management, Houston, Texas, April 1, 1996. Stansbery, E.G., T.E. Tracy, D.J. Kessler, M. Matney, and J.F. Stanley. 1994. Haystack Radar Measurements of the Orbital Debris Environment; 1990 –1994. JSC-26655. Houston: NASA Johnson Space Center. Stansbery, E.G., T.J. Settecerri, M.J. Matney, J. Zhang, and R. Reynolds. 1996. Haystack Radar Measurements of the Orbital Debris Environment; 1990 –1994. JSC-27436. Houston: NASA Johnson Space Center Space and Life Sciences Directorate, Solar System Exploration Division, Space Science Branch. Taff, L.G., D.E. Beatty, A.J. Yakutis, and P.S. Randall. 1985. Low altitude one centimeter space debris search at Lincoln Laboratories (M.I.T.) experimental test system. Advances in Space Research 5(2):35–45. Thompson, T.W., R.M. Goldstein, D.B. Campbell, E.G. Stansbery, and A.E. Potter. 1992. Radar detection of centimeter-sized orbital debris: Preliminary Arecibo observations at 12.5 cm wavelength. Geophysical Research Letters 19(3):257. Zhang, J.C. 1996. The 1996 Orbital Debris Engineering Model. Briefing presented to the NRC Committee on International Space Station Meteoroid/Debris Risk Management, Houston, Texas, April 1, 1996.