This chapter contains all of the findings and recommendations from the various chapters of this report.
Finding: NASA’s meteoroid and orbital debris programs have used their resources responsibly and have played an increasingly essential role in protecting the safety of both crewed and uncrewed space operations.
Finding: The increasing responsibilities given to NASA’s meteoroid and orbital debris programs have put pressure on the programs’ allotted resources. The increasing scope of work, and the complexity and severity of the debris and meteoroid environment, are outpacing in real dollars the decreasing funding levels of NASA’s MMOD programs.
Finding: The current lack of radar cross-section calibrations using fragments from a larger range of materials used in modern satellites and rocket bodies, as well as non-fragmentation debris, represents a significant source of uncertainty in interpreting key measurements of the orbital debris environment.
Finding: NASA’s orbital debris programs do not include the capability to monitor with in situ instrumentation the penetrating flux of objects smaller than a few millimeters. Data collected by in situ monitoring could be used to resolve uncertainties in measurements made remotely, to help identify new sources of debris, and to provide clues to the causes of spacecraft anomalies.
Finding: Correctly characterizing the shape and material properties of orbital debris is critical to correlating the results of ground-based satellite impact tests with radar cross-section data and thus to predicting 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 accurately 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.
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.
Finding: The models used to relate measurements of plasma to fundamental parameters of a meteoroid contain large uncertainties and errors. These models include, but are not limited to, electromagnetic scattering models, luminous emission models, and meteoroid fragmentation models.
Finding: Because the scientific community infers the properties of a meteoroid indirectly from its effects on the atmosphere (a meteor) or the effects of its impact on a spacecraft, it is imperative to understand observational biases inherent in each instrument that affect the detection of these secondary effects.
Finding: The Meteoroid Environment Model incorporates in its predictions the latest available data on the meteoroid environment, including the directionality and full velocity distribution of the meteoroids. It is currently the NASA model that is most consistent with the known meteoroid environment, although some major uncertainties still remain.
Recommendation: The NASA meteoroid and orbital debris programs should establish a baseline effort to evaluate major uncertainties in the Meteoroid Environment Model regarding the meteoroid environment in the following areas: (1) meteoroid velocity distributions as a function of mass; (2) flux of meteoroids of larger sizes (>100 microns); (3) effects of plasma during impacts, including impacts of very small but high-velocity particles; and (4) variations in meteoroid bulk density with impact velocity.
Finding: The earlier SSP 30425 meteoroid model does not reproduce existing observational meteoroid data with a fidelity equal to that of the Meteoroid Environment Model. Numerous disparate sources of data have been fused to produce the current meteoroid flux model used by NASA, sometimes incorporating differing underlying assumptions.
Finding: The Meteoroid Environment Model currently does not extend to prediction of the meteoroid environment in the outer solar system, and the measurements it incorporates are poorly constrained in the cis-martian region.
Recommendation: An effort should be made to re-examine earlier data used in the Grün Interplanetary Flux Model and to reconcile the data with more recent measurements in the literature on meteoroid flux, and a technical evaluation should be undertaken to synthesize and document such data as it is incorporated into the Meteoroid Environment Model (MEM). Updates of the MEM and technical development should follow a technical pathway as rigorous as that being taken for updates of the Orbital Debris Environment Model.
Recommendation: NASA should adopt the Meteoroid Environment Model for agency-wide official use and extend its capabilities to the outer solar system.
Finding: NASA’s MMOD risk assessment processes have evolved beyond focusing primarily on the damage to spacecraft from collisions with debris that are too small to track, to incorporating a more complete range of risks. More remains to be accomplished, however, including the need in some cases for more measurements as parameters for risk analyses. As gaps are filled, NASA’s MMOD efforts can progress toward ever more integrative risk assessment in which all sources and types of risk are modeled and assessed.
Recommendation: Although NASA should continue to allocate priority attention and resources to collision risks and conjunction analysis, it should also work toward a broad integrative risk analysis to obtain a probabilistic risk assessment of the overall risks present in the MMOD domain in which all sources of risk can be put in context.
Finding: The calculation and communication of information about uncertainty are critical to properly assessing operational alternatives based on calculated risks posed by orbital debris.
Recommendation: NASA’s meteoroid and orbital debris programs should increase their efforts to reduce the uncertainty and variability in models through acquisition of measurements (and where necessary, to do testing and analysis) for continually improving assessment of risk and characterization of uncertainty. Together with its MMOD efforts, NASA should continue to advance the agency’s efforts to present information on uncertainty in risk analyses. Special attention should be given to maximizing public understanding of uncertainty analysis through peer-reviewed papers and other publications.
Finding: The BUMPER program was not designed to fully address the probability of spacecraft failure following penetration by a meteoroid(s) or pieces of orbital debris.
Recommendation: NASA’s own MSCSurv code might offer insights for development of an expanded, improved MMOD risk analysis code that fully addresses the risk to a valuable spacecraft following an MMOD impact and, as such, should be coupled with results from BUMPER for use as needed.
Finding: It is not possible to obtain uncertainty bounds and/or confidence intervals as part of the current procedures being used to derive damage predictor equations in BUMPER.
Recommendation: Considering the critical need to develop overall uncertainty bounds for predictions of MMOD impacts (which in turn could be used in a probabilistic risk assessment), NASA should refine its damage prediction models so that they include uncertainty bounds and/or confidence intervals.
Finding: Using aluminum spheres to develop ballistic limit equations for risk assessments for spacecraft may not accurately portray the range of damage likely from impact with an orbital debris particle of any given characteristic size and thus may result in a non-optimum design of the spacecraft’s MMOD protection systems.
Recommendation: A priority in the next release of the Orbital Debris Environment Model and Standard Breakup Model should be the inclusion of shape characteristics in the particle distributions to more accurately portray the range of potential damage from an impact with orbital debris.
Finding: NASA’s current orbital debris programs are recognized both nationally and internationally as leaders in providing support for defining the environment and related impact hazards associated with orbital debris, and mitigation techniques to effectively minimize the hazards associated with the current and future orbital debris environment.
Finding: Most relevant federal agencies accept all or some of the components of NASA’s orbital debris mitigation and prevention guidelines.
Finding: Enhanced mitigation standards or removal of orbital debris are likely to be necessary to limit the growth in the orbital debris population. Although NASA’s orbital debris programs have identified the need for orbital debris removal, the necessary economic, technology, testing, political, or legal considerations have not been fully examined, nor has analysis been done to determine when such technology will be required.
Finding: NASA’s Object Reentry Survival Analysis Tool provides results as point estimates without confidence bounds or uncertainty estimates.
Recommendation: In regard to debris reentry risk, NASA should provide confidence bounds on and uncertainty estimates of the resulting risk levels for use in both the Debris Assessment Software and Object Reentry Survival Analysis Tool.
Finding: The reentry hazard programs used by NASA and the European Space Agency to determine the risk to people on the ground from reentering debris differ in how those thresholds are defined. NASA’s Object Reentry Survival Analysis Tool defines a “casualty” as personal injury, whereas ESA models equate a “casualty” with death.
Recommendation: NASA should update the Object Reentry Survival Analysis Tool so that it provides the probabilities of both injury and death as standard outputs.
Finding: The computation of the probability of collision for use in an assessment of risk requires the uncertainty parameters in the orbits of the two objects at conjunction, and assumes that these uncertainties are represented by a Gaussian distribution. Research has shown that the uncertainty distribution typically is Gaussian for several days, but when propagating for more than 2 to 3 days it may no longer be Gaussian. In addition, the uncertainties provided by the JSpOC are known to be usually too small, and the probability of collision can be very sensitive to errors in the size of the uncertainty.
Recommendation: NASA should develop a research plan for (1) assessing the impact of inaccuracy in the uncertainty on computations of the probability of collision and on the ensuing risk assessment, and (2) improving the accuracy of the computation of the probability of collision, given the presence of these uncertainty errors.
Finding: The large uncertainties in the launch dispersions (deviations from a planned trajectory) that yield a probability of collision of less than 10–5 translate to a very low return on investment in launch collision avoidance (COLA), and funds could probably be used more effectively in some other area of debris mitigation. However, in the event of a collision during launch, the political realities of potentially having done nothing probably mean that the use of COLA needs to continue, especially for crewed launches.
Finding: Spacecraft anomalies are a direct measurement of both the state of the particulate environment in space and the adequacy of a spacecraft design. However, no formal recording, analyzing, sharing, and reporting procedures exist to take advantage of data on spacecraft anomalies despite that data’s potential as valuable information about particulates in a critical size range that is typically not sampled continuously.
Recommendation: NASA should initiate a new effort to record, analyze, report, and share data on spacecraft anomalies in order to better quantify the risk posed by particulates too small to be cataloged yet large enough to disrupt spacecraft operations. The results of this effort would provide general insights into the effects of meteoroids and orbital debris on operational space systems. Eventually, this effort could provide data to upgrade current MMOD models—the Meteoroid Environment Model, Orbital Debris Environment Model, and BUMPER.
Finding: NASA’s Orbital Debris Mitigation Standard Practices, including the “25-year rule,” and NASA’s Procedural Requirements for Limiting Orbital Debris do not uniformly apply to non-NASA missions, launches, and payloads.
Recommendation: NASA should continue to engage relevant federal agencies as to the desirability and appropriateness of formalizing NASA’s Orbital Debris Mitigation Standard Practices, including the “25-year rule,” and NASA Procedural Requirements for Limiting Orbital Debris as legal rules that could be applicable to U.S. non-NASA missions and private activities.
Finding: The institutions and agreements that have been used to address issues related to orbital debris are primarily political, not legal, in nature. The success of those agreements will thus depend on a complex interplay of good faith; political will; and political, economic, and, sometimes, legal forces.
Recommendation: NASA should continue to engage the international community in efforts to develop cooperation and political will regarding activities concerning orbital debris.
Recommendation: NASA should assess the value of alternative data sets, such as by participating in the not-for-profit Space Data Association, to determine how sharing operator ephemerides might improve the accuracy and efficiency of NASA’s Conjunction Assessment Risk Analysis (CARA) by incorporating the best data possible in its CARA process.
Finding: Debris removal activity that involves selecting and removing any given object—debris or otherwise—from space, crosses crucial national and international legal thresholds.
Recommendation: NASA’s meteoroid and orbital debris programs should engage the NASA General Counsel’s Office and, through that office, the U.S. State Department regarding the legal requirements and diplomatic aspects of active debris removal.
Finding: NASA’s management structure has not kept pace with the expanding responsibilities of its MMOD programs. Consequently, the MMOD programs do not have a single management and budget point that can efficiently coordinate all of the current and planned activities and establish clear priorities.
Recommendation: NASA should review the current management structure of its MMOD programs in order to achieve better coordination, provide improved central decision making, and establish a framework for setting priorities. This framework should include a major interface with Congress, other federal and state agencies, and the public.
Finding: NASA’s MMOD researchers do not consistently communicate the results of their work to the scientific community, with the result that users of NASA’s codes and models have less understanding regarding the underlying assumptions and intricacies in each code and model.
Recommendation: NASA should encourage its MMOD researchers to more fully communicate the results of their work and their development activities, such as in appropriate peer-reviewed publications when possible, so that users of NASA’s codes and models gain a greater appreciation for and more clearly understand the underlying assumptions and intricacies in each code and model.
Finding: Nearly all of NASA’s MMOD programs are only one person deep in staffing. This shortage of staffing makes the programs highly vulnerable to budget reductions or changes in personnel. Further reductions in real budgetary support over the coming years could threaten the viability and scope of ongoing MMOD programs.
Recommendation: NASA should develop a formal strategic plan that provides the basis for prioritizing the allocation of funds and effort over various MMOD program needs. Among the potential research needs and management issues to be considered is the selection listed in Box 12.1. The strategic plan should consider short- and long-term objectives, a schedule of benchmark achievements to be accomplished, and priorities among them. Stakeholders should be engaged to help develop and review this plan. Finally, the MMOD strategic plan should be revised and updated at regular intervals.
Finding: The long-lived problem of growth in the orbital debris population as a result of debris self-collision and propagation requires that NASA take a long-term perspective to safeguard the space environment for future generations.
Finding: Although the meteoroid and orbital debris environment may be manageable at present, debris avoidance, mitigation, surveillance, tracking, and response all require money. At present, these costs usually come in the form of additional spacecraft mass and fuel and in the maintenance of debris surveillance systems. Such costs are usually absorbed in the budgets for space mission design, operations, and, in the case of commercial activities, insurance premiums. In the absence of appropriate meteoroid and orbital debris management to deal with the issue, these costs may grow over time. Although they can serve to highlight the importance of NASA’s debris measurement and monitoring activities, at present these costs are not routinely measured and reported.
Finding: The cost of replacing spacecraft has been used as a measure of the economic harm of a catastrophic debris impact but may underestimate the full cost of harm for two reasons: (1) actual replacement may be difficult because of funding, launch window limitations, or other constraints; and
(2) replacement cost, insurance premiums, and other measures of the cost incurred to protect a spacecraft understate the full cost to society as a whole if that spacecraft, damaged by a meteoroid or orbital debris, itself generates debris that then creates potential harm to other spacecraft.
Recommendation: NASA should lead public discussion of the space debris problem to emphasize debris as a long-term concern for society that must continue to be addressed today. Necessary steps include improvements in long-term modeling, better measurements, more regular updates of the debris environment models, and other actions to better characterize the long-term evolution of the debris environment.
Recommendation: NASA should join with other agencies to develop and provide more explicit information about the costs of debris avoidance, mitigation, surveillance, and response. These costs should be inventoried and monitored over time to provide critical information for measuring and monitoring the economic impact of the meteoroid and orbital debris problem, signaling when mitigation guidelines may need revision, and helping to evaluate investments in technology for active debris removal.