IMPROVING KNOWLEDGE OF THE DEBRIS ENVIRONMENT
Understanding the orbital debris environment (including debris size ranges, compositions, and distribution by orbital altitude, eccentricity, and inclination) is necessary to assess the debris hazard to spacecraft in various orbits, to understand the future evolution of the debris population, and to enable wise decisions to be made on methods to reduce the future hazard. However, data are lacking on many debris sources, size ranges, and orbital regions; current understanding of the debris environment is based on incomplete measurements and models that are not yet mature.
Increasing our knowledge of the orbital debris environment and applying that knowledge to debris mitigation practices may be the most cost-effective means of reducing the future impact of the debris hazard. First, better understanding of the environment would help spacecraft designers to protect spacecraft more effectively against debris. Although some meaningful measurements have been made at lower altitudes, current understanding of the debris environment is not sufficient for most spacecraft designers to predict accurately the level of debris protection that spacecraft may require; this may result in costly over- or under-protection. Second, a better understanding of the environment could be applied to determine which debris prevention measures will most effectively reduce the future hazard. Currently, there is much uncertainty about the cost-effectiveness of some methods of reducing the future debris hazard; models of the future debris population incorporating new
data (e.g., data on previously uncataloged large debris) could help to identify the best methods with which to deal with the problem and the orbital regions at which these methods should be targeted.
This is not to suggest an effort to characterize all debris in all orbits; rather, characterization efforts should focus on gathering the information needed to fill critical data gaps. Previously, most measurements of the debris environment were made when opportunities arose. Although these measurements added greatly to our knowledge of the debris environment, and further ad hoc measurements will doubtless continue to add to our knowledge, future debris characterization efforts should focus on either (1) providing information that will be directly useful to spacecraft designers and operators, or (2) answering questions about the debris environment that will increase understanding of the population's long-term evolution.
Currently, the only national or international guidance on either the most important areas in the debris field to be investigated or potential methods to investigate these areas comes from the Inter-Agency Space Debris Coordination Committee (IADC), which is made up of representatives from ESA, the Russian Space Agency, space agencies from Japan, and NASA. To provide future guidance for debris research, the committee recommends the following:
Recommendation 1: An expanded international group should be formed to advise the space community about areas in the orbital debris field needing further investigation and to suggest potential investigation methods. This group, which could include representatives from industry and academia, as well as from governments, could build on the work of the IADC. The group could identify the highest-priority areas of interest to orbital debris researchers and spacecraft operators, the data required to understand each area, and potential methods to acquire the data.
The committee recommends the following as an interim set of debris characterization research priorities:
Recommendation 2a: Models of the future debris environment should be further improved by refining theoretical models, acquiring and incorporating new data to lessen uncertainties, and testing the models against new data. Ensuring that these models incorporate all major sources of debris and increasing the accuracy of breakup models (for both collisions and explosions) should be major components of this effort. Improving these models is crucial because potentially very expensive decisions on the adoption of debris mitigation measures depend on their conclusions. These decisions must be based on the best information possible.
Recommendation 2b: Uncataloged medium-sized and large debris in LEO should be carefully studied. This should include a long-term measurement campaign to understand more fully the fluctuations in the uncataloged population due to perturbation forces and various generation mechanisms, thorough processing of the data, etc. Although the composition and dynamics of cataloged debris have been studied fairly well, knowledge of uncataloged large and medium-sized debris is limited. Although uncataloged large and medium-sized debris will not contribute significantly to collisional population growth, this population of debris is more likely to cause significant damage to typical spacecraft than the populations of either cataloged debris or small debris.
Recommendation 2c: Further studies (including both measurements and modeling) should be conducted to better understand the GEO debris environment. These should include efforts to determine the current debris population in GEO as well as to model its future evolution . Data on the debris environment in GEO are extremely sparse. Although the chance of a damaging impact in GEO is likely to be much lower than the chance in LEO, it is important to better understand the GEO debris environment because (1) the geostationary orbit is a limited and valuable resource that should be preserved for the future, (2) the orbital lifetime of space objects in GEO is extremely long (on the order of tens of thousands to millions of years), and (3) there are currently many highly valuable spacecraft in GEO.
Recommendation 2d: A strategy should be developed to gain a better understanding of the sources and evolution of the small debris population . Because the population of small debris is so time dependent, this strategy should focus on answering questions about the long-term nature of this population. The orbital debris community (including experts in modeling, detection and tracking, impact damage, and damage mitigation) should develop a strategy of observing requirements to effectively provide information about the sources and evolution of the small debris population.
Recommendation 2e: The data acquired from continuing studies of the debris environment should be compiled into a standard population characterization reference model. Methods should be adopted to validate or indicate the state of validation of this model. Such a model would aid experimenters in properly interpreting their data and spacecraft designers in properly assessing the hazard to their spacecraft.
In addition, the committee recommends two measures to improve the efficiency and accuracy of research on orbital debris:
Recommendation 3: The creation of an international system for collecting, storing, and distributing data on orbital debris should be explored. This would
include the creation of a unified database and catalog of debris that would receive measurements from all sensors gathering data on debris (including those in the SSS and the SSN). The information from this database would be accessible to interested parties under certain conditions. Currently, there is no formal mechanism allowing the nations of the world that engage in space monitoring to share data. Sharing of data from the SSS and the SSN could increase confidence in the catalogs of large debris and would also be useful in determining the desirability of future collaborative space debris monitoring efforts. The distribution of data from other sensors would enable an expanded group of researchers worldwide to analyze orbital debris data.
Recommendation 4: The orbital debris community should exercise more peer review over its research. Orbital debris is sometimes studied with an eclectic and often not fully developed set of observational, experimental, and modeled data and methods. The field needs a more rigorous scientific structure to give it a better theoretical underpinning and to logically link its elements. The practices of using external technical peer review panels, publishing in peer-reviewed journals, and establishing a close working relationship with related scientific fields should be expanded to provide some of this rigor.
IMPROVING SPACECRAFT PROTECTION AGAINST DEBRIS
Even if fairly drastic steps are taken to reduce the generation of new debris, a hazard will likely continue to exist, and probably grow, in some important orbital regions for a great many years. Without remedial steps, the debris hazard will grow more rapidly. In either case, orbital debris is now a part of the space environment and should be considered during the design of spacecraft and the planning of space operations. As described in Chapter 6, the growing availability of (1) analytic and experimental tools to quantify the debris threat to spacecraft and (2) techniques to protect against debris impacts make it feasible for designers to assess the debris hazard and protect spacecraft appropriately. However, not all spacecraft designers have knowledge of these tools and techniques. For this reason, the committee recommends the following:
Recommendation 5: A guide to aid spacecraft designers in dealing with the debris environment should be developed and distributed widely. This design guide should include information on environmental prediction, damage assessment, and passive and operational protection techniques . Such a guide would enable spacecraft designers (1) to assess the need to incorporate protective measures in spacecraft design or operations and (2) to choose and
implement appropriate measures of protection, if necessary. It could also serve as a useful reference text for advanced students in space engineering. Chapter 6 provides a top-level description of the processes and methods that should be discussed in this guide.
Considerable effort has already been invested in studying the effects of debris impact on spacecraft and the ability of shielding to reduce impact damage. However, as discussed in Chapter 5, knowledge gaps remain in (1) the effects of impact by the variety of debris shapes and compositions likely to exist in orbit, (2) the vulnerability of different spacecraft components to debris impact, and (3) the effects of impact at typical LEO collision velocities. To better predict impact damage and design debris shields, the committee recommends the following:
Recommendation 6: Research should be continued to characterize the effects of hypervelocity impacts on spacecraft systems in the following areas:
further development of techniques to launch projectiles to the velocities typical of collisions in LEO;
improved models of the properties of newer spacecraft materials.
studies of damage effects on critical components;
development of analytical tools consistent over a range of debris impact velocities, shapes, and compositions; and
improved models of catastrophic spacecraft breakup from debris impact.
The first four of these research areas aim at improving spacecraft and shield design; the final research area aims at improving models of the future debris population.
These research goals could be achieved more easily if data from hypervelocity facilities worldwide were made more readily available. Unfortunately, as discussed in Chapter 5, the capabilities of many hypervelocity facilities are not well known, and the impact data generated at these facilities are often inaccessible. This has resulted in duplication of effort both within and between nations, slowing the development of good models of debris impact damage. Thus, the committee recommends the following:
Recommendation 7: A handbook describing the capabilities of the international hypervelocity impact facilities generally available for debris research should be developed. Such a handbook would facilitate the sharing of impact results generated at different facilities, perhaps leading to the establishment of a debris-related database of impact results accessible via the Internet.
REDUCING THE FUTURE DEBRIS HAZARD
Unless the production of new debris is reduced, it will become necessary for increasing numbers of spacecraft to adopt measures to avoid debris impact damage, and the chance of losing functional spacecraft to debris will increase. As discussed in Chapter 7, cleaning up debris via active removal will be uneconomical for the foreseeable future, so efforts must focus on reducing the creation of new debris. There are many possible means of accomplishing this goal, but the decision on which should be implemented cannot be made solely on technical grounds. As with other environmental issues, decisions on the adoption of debris reduction methods, and on the means to implement these methods, must balance political and economic as well as technical factors and thus must be made in forums that are capable of balancing all of these factors.
Current international law does not specifically address the orbital debris issue, so there is a fairly clean slate upon which to draft future regulations to reduce the generation of new debris. (Existing international agreements pertaining to orbital debris, as well as some of the efforts under way that may affect future rule making on orbital debris-related issues, are discussed briefly in Appendix A.) Possible future regulatory schemes may be voluntary or mandatory; they may provide incentives to spacecraft operators who reduce debris creation, or they may specify particular debris-mitigating measures all manufacturers must incorporate. It is clear, however, that debris reduction measures enacted by any single nation will not be sufficient to prevent a growing future hazard. For this reason, and because unilaterally adopted debris reduction measures may reduce economic competitiveness, the committee recommends the following:
Recommendation 8: The spacefaring nations should develop and implement debris reduction methods on a multilateral basis. Given the long development cycle for new space vehicles with debris-minimizing features, the technical development, cost-benefit assessments, and international discussion required to implement countermeasures should start as soon as possible.
Although these multilateral discussions cannot be conducted on a purely technical basis, it is crucial that they be based on sound technical advice. The committee's consensus technical assessment of the actions that should be implemented to reduce future growth in the debris hazard, based on its current understanding of the debris environment and of the costs and benefits of various mitigation measures, is represented in the following recommendations (Chapters 7 and 8 discuss each of these actions in greater detail):
Recommendation 9: Space system developers should adopt design requirements ensuring that spacecraft or rocket bodies do not explode after their functional lifetimes. Ensuring that all potential sources of stored energy on a spacecraft or rocket body are depleted at the end of their functional lifetime is the primary means of accomplishing this goal. Explosions of spacecraft and rocket bodies have been major contributors to the debris hazard, so preventing such explosions will significantly reduce the growth in the short-term debris hazard. Implementing design features to passivate spacecraft and rocket bodies after their functional lifetimes will generally not be very costly.
Recommendation 10: The release of mission-related objects during spacecraft deployment and operations should be avoided whenever possible. Release of mission-related objects in long-lifetime orbits should be particularly avoided. Mission-related debris is a significant fraction of the population of large debris in orbit. Reducing the release of mission-related debris during spacecraft deployment and operations can typically be accomplished without significant expenditure and, in general, without new technology, although some hardware development may be required.
Recommendation 11: Developers should incorporate requirements that spacecraft and rocket bodies be designed to minimize the unintentional release of surface materials, including paint and other thermal control materials, both during and after their functional lifetimes. To aid in meeting these requirements, surface materials that minimize the release of small particles should be developed and used. The deterioration of spacecraft surfaces (paint, etc.) is believed to be a major contributor to the population of small debris, so ending its release would prove beneficial to the space environment.
Recommendation 12: Intentional breakups in orbit (especially those expected to produce a large amount of debris) should be avoided if at all possible. No intentional breakups expected to produce numerous debris with orbital lifetimes longer than a few years should be conducted in Earth orbit. Occasionally, an organization may want to explode a space object in orbit for defense, scientific, or calibration purposes. If it is absolutely necessary that the breakup take place in Earth orbit, it should be at a low altitude to limit the maximum orbital lifetime of fragments.
All of these actions will help to reduce the short-term debris hazard, but (as described in Chapter 8), models of the future debris population show that EOL reorbiting of large objects (generally rocket bodies and spacecraft) in LEO or in orbits that pass through LEO may be necessary to reduce collisional growth in the LEO debris population. However, removing these objects from orbit (particularly from the higher orbits) can be costly. Ensuring that spacecraft and rocket bodies passing
through LEO are (after their functional lifetime) placed into orbits that will decay in a reasonable amount of time appears to be the most costeffective reorbiting measure. Determining exactly how long this time should be will be as much a political and economic decision as a technical one, due to the relatively large costs that such a maneuver may impose on some missions. Because of the long lead time required to develop and qualify new space hardware, however, it is necessary to begin setting standards now. For this reason, the committee recommends the following:
Recommendation 13: Spacecraft and rocket bodies in LEO and in highly elliptical orbits passing through LEO should be reorbited after their functional lifetime. This reorbiting maneuver should either remove them from LEO or reduce their orbital lifetime. Effort should be made to achieve an international consensus on the magnitude of such reorbiting maneuvers. A draft NASA guideline suggested that spacecraft in orbits that pass through LEO should be limited to orbital lifetimes in LEO of no longer than 25 years after mission completion; this standard does not seem unreasonable. However, any orbital lifetime limitation guideline that is adopted should be based on thorough scientific analysis.
Although the geosynchronous region may not be subject to collisional cascading and current GEO hazard levels from orbital debris appear to be very low, the hazard from debris left in GEO can persist for millennia. Currently, the long-term evolution of the debris environment is not well enough understood to determine the best long-term strategy for managing the debris hazard in GEO. Experts have not yet reached a consensus on the best locations for disposal orbits, or even on whether the use of disposal orbits is the optimal strategy for containing the GEO debris hazard. However, it may not be wise to let the GEO debris population grow until a permanent solution is divined. For these reasons, the committee recommends the following:
Recommendation 14: The use of GEO disposal orbits should be further studied. Until such studies produce a verifiably superior long-term strategy for dealing with the GEO hazard, operators of GEO spacecraft and rocket bodies should be encouraged to reorbit their spacecraft at EOL if they are capable of safely performing a reorbiting maneuver to a disposal orbit at least 300 km from GEO. Studies on the use of GEO disposal orbits should be focused on the development of a long-term strategy for maintaining a low debris hazard in GEO. Such studies should include the development of accurate models capable of predicting the effects of various debris reduction measures on the future hazard in GEO.