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3 Orbital Debris Modeling and Simulation As indicated in Chapter 1, “Introduction and Historical Background,” NASA’s meteoroid and orbital debris (MMOD) models create the foundation for the technical services provided by the Orbital Debris Program Office (ODPO), Meteoroid Environment Office (MEO), Hypervelocity Impact Technology Facility (HITF), and other NASA offices, in addition to being tangible artifacts used by the aerospace community to support analyses regarding space system performance related to debris and meteoroids. Figure 3.1 shows that relationships among the various models used. Meteoroid modeling and simulation, including the Meteoroid Environment Model (MEM), are discussed separately in Chapter 4, “The Meteoroid Environment and its Effects on Spacecraft.” In addition, the committee notes that the collision avoidance operations are not discussed since they represent an operational capacity that is not directly supported by the NASA MMOD offices. Although there is significant related dialogue between NASA centers and personnel, collision avoidance operations are based solely on specific mission needs, as opposed to the foundational research and development that are the focus of the ODPO, the MEO, and the HITF. GEOPROP and PROP3D (Table 3.1) are general-purpose orbital propagator models, and Solar Flux is a model for the solar flux component (i.e., 10.7-cm wavelength) that affects atmospheric density calculations. Figure 3.1 and Table 3.1 highlight the wide suite of MMOD models that NASA has developed (and continues to develop) in support of national policy development, mission operations, and international technical discussions. The following models, which (with one exception) are used only by NASA, provide valuable insights and reflect the results of decades of testing, measurements, studies, and analysis. • SBM produces distributions of fragments from a variety of breakup events for use by other models such as LEGEND and SBRAM. The model leverages empirical data from laboratory and on-orbit testing to provide an accurate depiction of explosions and collisions in space. 1 For further information on SBM see D. McKnight, Determination of breakup initial conditions, Journal of Spacecraft and Rockets 1 28(4):470-477, 1991; D. McKnight, Determining the effects of space debris impacts on spacecraft structures, Acta Astronautica 26(7):501- 512, 1992; D. McKnight, “Key Aspects of Satellite Breakup Modeling,” ESA SP-1, First European Conference on Space Debris, Darmstadt, Germany, April 5-7, 1993; D. McKnight, L. Nagl, and R. Maher, “Refined Algorithms for Structural Breakup Due to Hypervelocity Impact,” Hypervelocity Impact Symposium, Paper 82, Santa Fe, N.M., October 1994 (also International Journal of Impact Engineering Proceedings 17:547-558, 1994); D. McKnight, L. Nagl, C. Dobosz, and R. Maher, “Explosion Modeling and Simulation,” DNA-TR-94-11, August 1994; D. McKnight, L. Nagl, and R. Maher, “Fragmentation Algorithms for Strategic and Theater Targets (FASTT) Empirical Breakup Model,” DNA-TR-94-104, December 1994; D. McKnight, M. Fudge, and T. Maclay, Satellite Orbital Debris Characterization Impact Test (SOCIT) Series Data Collection Report, prepared under NASA contract NAS 9-19215, April 1995; D. McKnight, N. Johnson, M. Fudge, and T. Maclay, Analysis of SOCIT Debris Data and Correlation to NASA’s Breakup Models, prepared under NASA contract NAS 9-19215, July 1995. 23

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24 LIMITING FUTURE COLLISION RISK TO SPACECRAFT TABLE 3.1 Summary of NASA MMOD Models and Their Interdependencies Model Function/Usage Development Latest Version Models Used Used By ORDEM User-friendly, semi-empirical Since 1970s, ORDEM2000; SBM DAS/ (Orbital environment characterization, NASA has ORDEM3.0 Solar Flux BUMPER Debris for current and short-term evolved through due out in late GEOPROP Engineering future (~30 years), of debris several codes; 2011/early 2012 PROP3D Model) impact flux down to 10 µm in ORDEM96 was Earth orbit (LEO and GEO) released in 1996. based on returned samples, remote observations, modeling, historical changes, and trends. Available to the public. MEM Semi-empirical meteoroid Based on 2000- Version 2.0 Grün IFM BUMPER (Meteoroid velocity and direction 2004 work at Lunar MEM2/ Environment distribution for near-Earth and University of MEM CXP in Model) interplanetary (Mercury to Western Ontario. 2008 asteroid belt) natural particulates down to 1 µg to predict flux on spacecraft surfaces. Available to the public. DAS Suite of tools (ORDEM, orbit NASA developed DAS 2.0.1 ORDEM None (Debris propagators, and ballistic limit to accompany 2008 “BLEs”/SBM Assessment equations [BLE]) to assist NASA standard Software) NASA offices in verifying practices starting compliance with NASA STD in 1995. 8719.14. Used widely inside and outside NASA. Available to the public. BUMPER Semi-empirical model to NASA developed BUMPER II ORDEM/MEM None determine the potential for and started to 2005 debris and meteoroids to apply in mid- strike and penetrate spacecraft 1990s. surfaces. Not available for public use. ORSAT (Object Simulates reentry of hardware to NASA evolved ORSAT 6.0 None None Reentry Survival determine debris casualty area since 1993 but 2006 Analysis Tool) to calculate the reentry risk to also used to people on Earth. Not available analyze many for public use. non-NASA objects. LEGEND Statistical, three-dimensional, NASA EVOLVE 2005, but SBM None (LEO-to-GEO debris evolutionary model for leveraged to undergoing GEOPROP Environment the study of the long-term create LEGEND continual PROP3D Debris) debris environment for LEO, in 2003. upgrades Solar Flux HEO, and GEO. Provides debris Launch Traffic characteristics as functions of time, altitude, longitude, and latitude. Not available for public use. SBM Semi-empirical model that NASA has been 2001 None ORDEM (Standard determines the number, mass, developing SBM DAS Breakup velocity, and ballistic coefficient since 1970s; used LEGEND Model)a distributions of fragments by IADC since SBRAM produced down to 1 mm from 2008. a breakup event—collision and explosion. Not available for public use.

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25 ORBITAL DEBRIS MODELING AND SIMULATION TABLE 3.1 (continued) Model Function/Usage Development Latest Version Models Used Used By SBRAM Used to determine the short- NASA developed 2006 SBM None (Satellite term hazard (hours to days) its own version GEOPROP Breakup from a single breakup event and in 2006 and one PROP3D Assessment separate from “background risk.” for the Missile Model) Provides probability of collision Defense Agency over time for a given asset from in 2007. a simulated breakup cloud. Not available for public use. NOTE: The latest version is given if appropriate and/or known. a The SBM is not a standalone software application available from NASA, although it is embedded in ORDEM, DAS, LEGEND, and SBRAM. In addition, all of the equations used in the SBM are available in N.L. Johnson, P.H. Krisko, J.-C. Liou, and P.D. Anz-Meador, NASA’s new breakup model of Evolve 4.0, Advances in Space Research 28(9)1377-1384, 2001. • LEGEND depicts the long-term evolution of the debris flux as a function of latitude, longitude, altitude, size, and time. Considering longitude permits analysis of constellations or any orbits that may include a stable argument of perigee or right ascension of ascending node. LEGEND contains a number of submodels: the traffic model describes the frequency of launches, the initial orbits of objects placed into orbit, and the physical charac - teristics of each object placed into orbit; the atmospheric decay model includes the Jacchia atmospheric density model plus a solar activity model; and various debris source models describe sources, such as SBM, exhaust from solid rocket motors, and coolant leaks. Values for the probability of explosion are assigned to certain objects, and probabilities of collision are calculated for all objects. Probabilities are combined with a random number genera - NASA MMOD Model Development and Use Protect Spacecraft Protect Orbital Environment Protect People on Earth BUMPER DAS Read Read Particle If reentry NASA NPR Compliance Vulnerability; criteria Shielding Design not met Embedded LEGEND MEM ORDEM ORSAT OD Environment Micrometeoroid Orbital Debris Reentry Hazard Evolution Environment Environment SBRAM MODEL LEAD KEY Event-Specific ODPO Hazard HITF MEO Embedded Embedded SBM Breakup Debris FIGURE 3.1 NASA MMOD model development and use. The models developed, used, and delivered by NASA cover a wide 3.1 from NASA MMOD Model Figures 31MAY11.eps spectrum of applications and needs for both NASA and non-NASA space operators. When a model is “embedded” in another, it is subsumed by that top-level model and does not run independently of it. When a model is “read” by another, the top-level model is using the output of the first model as an input to its analysis.

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26 LIMITING FUTURE COLLISION RISK TO SPACECRAFT tor to predict if an explosion or collision will occur. Consequently, every run of the program results in a different sequence of events. Typically after 100 runs the resulting fluxes are averaged. Predicted averages are compared with observational data to determine if updates to the model are required; after adjustments in the model, the results become part of a historical database that sets the initial conditions for any future projections. 2 • ORSAT provides a high-fidelity determination of the risk to people on the ground from reentering space systems. ORSAT is discussed more fully in Chapter 8. • BUMPER determines spacecraft vulnerability from particulates and facilitates shield-design processes for spacecraft based on input from the MEM and ORDEM particulate environment models. BUMPER is discussed more fully in Chapter 6. NASA used the BUMPER code to calculate the probability of MMOD critical impact damage for space shuttle missions, and uses it to calculate the risk of MMOD penetration for the International Space Station (ISS), extravehicular activity (EVA) suits, and other spacecraft. The major limitations of BUMPER are that (1) it calculates only a portion of the MMOD risk to a spacecraft (the probability of a penetration, however that is defined for the particular spacecraft under consideration) and is not able to calculate the total MMOD risk (which would include the probability of spacecraft loss or kill), (2) it provides only a point estimate of MMOD risk with no assessment of the associated uncertainty, and (3) it does not take into consideration the possibility of non-spherical particle impacts in its risk-calculating modules and algorithms. 3 • SBRAM provides calculations of short-term hazards to other spacecraft from fragmentation events on orbit. A basic combination of the SBM and general purpose propagators, SBRAM permits rapid examination of the effect that a breakup event may have on space assets hours to days after an event. As such, it is uniquely tailored to permit the creation of specific scenarios and has been developed for use by NASA and for other government partners. Because they model complex physical phenomena and require operation by skilled users, LEGEND, ORSAT, and BUMPER are not made available to the public. Although the NASA SBM model has not been updated since 2001,4 it is used by the LEGEND, SBRAM, and DAS programs. As such, any deficiencies in the model will affect a variety of studies, analysis, and support to customers. In addition, the data from the major on-orbit collisional breakup events in 2007 (the Chinese ASAT event; see Box 1.2 in Chapter 1) and 2009 (the Iridium–Cosmos col - lision; see Box 9.1 in Chapter 9) have not been considered in the current model. 5 NASA is planning to update the SBM with full-scale tests involving “new-construction” satellites—satellites constructed using processes and materials designed to prevent or at least minimize the creation of addition orbital debris. However, it is not at all clear that the new-construction satellites will be generating significant orbital 2 For further information on LEGEND see J.-C. Liou, D.T. Hall, P.H. Krisko, and J.N. Opiela, LEGEND—A three dimensional Leo-to-Geo debris evolutionary model, Advances in Space Research 34:981-986, 2004; D.J. Kessler, M.J. Matney, R.C. Reynolds, R.P. Bernhard, E.G. Stansbery, N.L. Johnson, A.E. Potter, and P.D. Anz-Meador, The Search for a Previously Unknown Source of Orbital Debris: The Possibility of a Coolant Leak in Radar Ocean Reconnaissance Satellites, JSC-27737 and LMSMSS32426, NASA Johnson Space Center, Houston, Tex., February 21, 1997; E.L. Christiansen, Handbook for Designing MMOD Protection, NASA TM-2009-214785, NASA Johnson Space Center, Houston, Tex., 2009; W.H. Jolly and J. Williamsen, “Ballistic Limit Curve Regression for Freedom Station Orbital Debris Shields,” AIAA Paper No. 92-1463, AIAA Space Programs and Technologies Conference, Huntsville, Ala., American Institute of Aeronautics and Astronautics, Reston, Va., 1992; W.P. Schonberg, H.J. Evans, J.E. Williamsen, R.L. Boyer, and G.S. Nakayama, Uncertainty considerations for ballistic limit equations for aerospace structural systems, Paper No. IMECE2005-79709, Proceedings of the 2005 ASME International Mechanical Engineer- ing Congress and Exposition, Orlando, Fla., November 5-11, 2005, American Society of Mechanical Engineers, New York, N.Y., 2005; J.E. Williamsen, W.P. Schonberg, and A.B. Jenkin, On the effect of considering more realistic particle shape and mass parameters in MMOD risk assessments, Advances in Space Research 47:1006-1019, 2011; D. Kessler, N. Johnson, E. Stansbery, R. Reynolds, K. Siebold , M. Matney and A. Jackson, The importance of non-fragmentation sources of debris to the environment, Advances in Space Research 23(1):149-159, 1999. 3 National Research Council, Protecting the Space Shuttle from Meteoroids and Orbital Debris, National Academy Press, Washington, D.C., 1997, available at http://www.nap.edu/catalog.php?record_id=5958, accessed July 7, 2011. 4 N.L. Johnson, P.H. Krisko, J.-C. Liou, and P.D. Anz-Meador, NASA’s new breakup model of Evolve 4.0, Advances in Space Research 28(9):1377-1384, 2001. 5 See J.-C. Liou and N.L. Johnson, Physical properties of the large Fengyun-1C breakup fragments, Orbital Debris Quarterly News 12(2):4- 5, 2008; M. Matney, Small debris observations from the Iridium33/Cosmos 2251 collision, Orbital Debris Quarterly News 14(2):6-8, 2010, available at http://orbitaldebris.jsc.nasa.gov/newsletter/pdfs/ODQNv14i2.pdf; J. Oberg, “U.S. Satellite Shootdown: The Inside Story,” IEEE Spectrum, August 2008, available at http://spectrum.ieee.org/aerospace/satellites/us-satellite-shootdown-the-inside-story.

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27 ORBITAL DEBRIS MODELING AND SIMULATION debris in the future. Since there are far more “old-construction” satellites in orbit, it would appear that the old- construction satellites would contribute more debris to the environment. It would be instructive to know what will be the relative mass of old-construction to new-construction satellites in orbit over time; that information would help NASA determine how test targets should be constructed and with which materials. In addition, the shape of a debris fragment is known to have an influence on the penetrability of impacts from debris and on the damage levels sustained by spacecraft,6,7 and the fragmentation physics varies significantly between (1) thin-walled, hollow cylinders like rocket bodies and human habitation modules and (2) compact, robotic satellites, a consideration that should also be factored into the development of a new SBM. Finding: Correctly characterizing the shape and material properties of orbital debris is critical to cor- relating the results of ground-based satellite impact tests with radar cross-section data and thus to pre- dicting the damage caused by debris particles, yet there has been little effort to include realistic effects of shape in the standard breakup model. These enhancements would also serve to improve BUMPER’s accuracy in predicting risks. Recommendation: The NASA Orbital Debris Program Office should expand its efforts to more accu- rately incorporate data on sources of debris into the standard breakup model, especially (1) empirical results from recent major on-orbit collisions, (2) data from laboratory rocket body collision tests (which need to be planned and conducted), (3) results from hypervelocity impact tests with payloads using newer construction methods and materials, and (4) enhanced data on fragment shape characteristics. The following two models developed by NASA are provided to the public: 8 • DAS, a suite of tools originally provided to assist NASA offices in verifying compliance with NASA procedural requirements for NASA STD 8719.14, has been used by many non-NASA missions to ascertain their systems’ debris-related performance.9 DAS contains ORDEM, SBM, ballistic limit equations (BLEs) predicting the penetrability of debris, and a simplified reentry demise model integrated within a user-friendly graphical user interface that provides threshold evaluations of adherence to the NASA standard for limiting debris. DAS puts significant analytic capability in the hands of the operator, but it does not provide the flexibility to analyze these issues quantitatively. For example, to examine reentry disintegration of hardware more precisely, one would apply ORSAT. Similarly, to determine a shield design (versus a simple ability to survive the particulate environment), one would apply BUMPER. • ORDEM is the empirical space debris environment model that estimates as a function of size and altitude the number of debris objects that are likely to impact a spacecraft. This engineering model is based almost entirely on measurements, with some interpolations and extrapolations required when no measurements are available (e.g., for debris smaller than 1 mm at altitudes above 600 km where no measurements have been taken). Since the mea - surements of the debris environment include measurements of flux, the uncertainties associated with the ORDEM engineering model are limited almost entirely to uncertainties in translating any measured “size” into some level of damage. Although NASA has made several important improvements in ORDEM since its latest formal release in 2000, a new version of that model that incorporates those changes has not been released. Given that the environment is 6 L. Anselmo, A. Cordelli, P. Farinella, C. Pardini, and A. Rossi, Modelling the evolution of the space debris population: Recent research work in Pisa, pp. 339-344 in Proceedings of the Second European Conference on Space Debris, Darmstadt, Germany, March 17-19, 1997, ESA SP-393, European Space Operations Centre, European Space Agency, Paris, France, 1997, available at http://articles.adsabs.harvard.edu// full/1997ESASP.393..339A/0000339.000.html, accessed July 8, 2011. 7 J. Opiela and N. Johnson, “Improvements to NASA’s Debris Assessment Software,” 2007. 8 The Meteoroid Environment Model (MEM), which is also provided free to the public, is discussed in Chapter 4. 9 J. Opiela and N. Johnson, “Improvements to NASA’s Debris Assessment Software,” 58th International Astronautical Congress, IAA-01- IAA.6.6076, Hyderabad, India, International Academy of Astronautics, Paris, France, September 2007, available at http://ntrs.nasa.gov/archive/ nasa/casi.ntrs.nasa.gov/20070013703_2007011171.pdf, accessed July 8, 2011.

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28 LIMITING FUTURE COLLISION RISK TO SPACECRAFT constantly changing, updates should be made consistent with major changes in the environment and with enhanced understanding of environmental features. Debris-shape characterization is ongoing within NASA, and some rudi - mentary information is to be included in the forthcoming ORDEM update. This information is important because there is significant shape variation among larger particles, where criticality for human-rated spacecraft is defined relative to mission failure or crew loss, and not just in terms of whether there is impact damage. The update to ORDEM2000, known as ORDEM 3.0 (formerly referred to as ORDEM 2010), is slated to include a definition of the environment past LEO, an explicit characterization of orbital debris flux uncertainties, and the introduction of material densities into the model. Recommendation: NASA’s Orbital Debris Program Office should release the next version of the Orbital Debris Environment Model as soon as possible and provide updates on a regular basis or as often as required as a result of major changes to the orbital debris environment or improved characterization of that environment, including characterization of debris shape, as applicable.