Space activities in Earth orbit are increasingly indispensable to our civilization. Orbiting spacecraft serve vital roles as communications links, navigation beacons, scientific investigation platforms, and providers of remote sensing data for weather, climate, land use, and national security purposes. The spacecraft that perform these tasks are concentrated in a few orbital regions, including low Earth orbit (LEO), semisynchronous orbit, and geosynchronous Earth orbit (GEO). These orbital regions represent valuable resources because they have characteristics that enable spacecraft operating within them to execute their missions more effectively.
Functional spacecraft share the near-Earth environment with natural meteoroids and the orbital debris that has been generated by past space activities. Meteoroids orbit the Sun and rapidly pass through and leave the near-Earth region (or burn up in the Earth's atmosphere), resulting in a fairly continual flux of meteoroids on spacecraft in Earth orbit. In contrast, artificial debris objects (including nonfunctional spacecraft, spent rocket bodies, mission-related objects, the products of spacecraft surface deterioration, and fragments from spacecraft and rocket body breakups) orbit the Earth and will remain in orbit until atmospheric drag and other perturbing forces eventually cause their orbits to decay into the atmosphere. Since atmospheric drag decreases as altitude increases, large debris in orbits above about 600 km can remain in orbit for tens, thousands, or even millions of years.
Although the uncontrolled reentry of some orbital debris could potentially pose a hazard to activities on the Earth's surface, the major
hazard posed by debris is to space operations. Although the current hazard to most space activities from debris is low, growth in the amount of debris threatens to make some valuable orbital regions increasingly inhospitable to space operations over the next few decades. A responsible approach to orbital debris will require continuing efforts to increase our knowledge of the current and future debris population, the development of tools to aid spacecraft designers in protecting spacecraft against the debris hazard, and international implementation of appropriate measures to minimize the creation of additional debris.
CHARACTERISING THE DEBRIS ENVIRONMENT
The debris environment is difficult to characterize accurately. First, the debris population changes continually as new debris is created and existing debris reenters the Earth's atmosphere. Detection of such changes requires that measurements of the debris environment be updated periodically. Second, only the largest objects can be repeatedly tracked by ground-based sensors; tracking of the numerous smaller pieces of debris is much more difficult. The U.S. and Russian space surveillance systems are able to track and catalog virtually all objects larger than 20 cm diameter in LEO. However, as altitude increases, the minimum-sized object that these systems are capable of tracking increases, until at GEO only objects larger than about 1 meter in diameter are presently cataloged.
Characterization of the debris population that cannot be cataloged must thus be accomplished by sampling the orbital debris flux at particular locations and times and using these data as a basis for estimating the characteristics of the general population. The flux can be sampled either directly (with spacecraft surfaces that are struck by debris) or remotely (by using ground- or space-based radars or optical telescopes that record debris passing through their fields of view). Presently, ground-based remote sensing is the most effective method for sampling the medium-sized (approximate diameter 1 mm–10 cm) debris population, and in situ impact sampling is the most effective method for measuring the small (approximate diameter <1 mm) debris population.
Current measurements of the debris environment contain gaps, such as a lack of information on objects smaller than 1 meter in diameter in GEO, on the small debris population at altitudes above 600 km, and on the medium-sized debris population above LEO. There are, however, several promising means for better characterizing the debris population. For example, large-aperture optical telescopes or telescopes equipped with charge-coupled devices could be employed to improve cataloging of large (approximate diameter >10 cm) debris in orbits above LEO,
shorter-wavelength radars situated at low latitudes could be used to improve our knowledge of the medium-sized debris population in LEO, and active impact detectors deployed at altitudes above 600 km could extend our knowledge of the distribution of the small debris population.
Using such means to better characterize the orbital debris environment and applying the knowledge thus acquired can increase the cost-effectiveness of efforts both to reduce the future debris population and to protect spacecraft from debris. This is not to suggest an effort to characterize all debris in all orbits; rather, characterization efforts should aim at providing information needed to fill critical gaps in the data. To focus this effort the committee recommends that an international group be formed (1) to advise the space community about areas in the orbital debris field needing further investigation and (2) to suggest potential investigation methods.
As an interim set of debris characterization research priorities, the committee recommends the following:
models of the future debris environment should be further improved,
uncataloged debris in LEO should be carefully studied,
further studies should be conducted to better understand the GEO debris environment,
a strategy should be developed to gain an understanding of the sources and evolution of the small debris population, and
the data acquired from this research should be compiled into a standard population characterization reference model.
To improve the efficiency of orbital debris research, the committee recommends exploring the creation of an international system for collecting, storing, and distributing data on orbital debris. Finally, to ensure the accuracy of the data produced by these efforts, the committee recommends that the orbital debris community exercise more peer review over its research.
HAZARD TO SPACE OPERATIONS FROM DEBRIS
The natural meteoroid environment does not pose a serious hazard to well-designed spacecraft in Earth orbit. However, there are now orders of magnitude more large orbital debris objects than large meteoroids in the near-Earth area at any given time. Although measurements of the medium-sized debris environment are sparse, the population of medium-sized orbital debris also appears to be greater than the population of medium-sized micrometeoroids in the regions of LEO where measurements have been made.
Spacecraft are much more likely to collide with smaller debris than
with larger objects. In LEO, the probability of collision with debris in each size range is believed typically to increase by more than a factor of 100 for every factor of 10 decrease in size over most of the medium to small debris size ranges. (For example, LEO spacecraft are probably at least 100 times more likely to be struck by 1-mm-diameter objects than by 1-cm-diameter objects.) In the orbital altitude most densely populated with debris (between 900 and 1,000 km), models suggest that a typical spacecraft (10-square-meter cross-sectional area) has only about one chance in 1,000 of colliding with a large debris object over the spacecraft's 10-year functional lifetime. The chance of colliding with 1 to 10 cm debris over the same period, however, is estimated to be about 1 in 100, a collision with 1 mm to 1 cm diameter debris is believed to be likely, and frequent collisions with debris smaller than 1 mm will occur.
The chance of colliding with debris varies greatly with orbital altitude and, to a lesser extent, with orbital inclination. Based on the best available data, the probability of colliding with large or medium-sized debris in LEO is at least 100 times greater than the average probability in GEO and is likely to be 1,000 times greater than the probability in less used orbital regions. Even within LEO, the collision probability varies greatly with altitude; for example, the chance of collision with medium-sized or large debris is probably higher by a factor of 50 at 900 km altitude than at 250 km. Measurements of small debris are so limited that it is unclear whether this population follows a similar altitude distribution.
The damage that a collision with debris can cause to a spacecraft depends on the kinetic energy released in the collision, the design of the spacecraft, and the geometry of the collision. Due to the typically high relative velocities of the objects involved, collisions in orbit can be highly energetic. For example, a 1-kg object involved in a (typical for LEO) 10 km/s collision will impact with the same relative kinetic energy (about 100 MJ) as a fully loaded 35,000-kg truck moving at 190 km/h. If the kinetic energy released in a collision is large enough compared to the mass of the objects involved, a catastrophic breakup will occur. In such a breakup, numerous fragments capable of causing further catastrophic breakups could be produced. A 1-kg object impacting at 10 km/s, for example, is probably capable of catastrophically breaking up a 1,000-kg spacecraft if it strikes a high-density element in the spacecraft. In such a breakup, numerous fragments larger than 1 kg would be created.
Even if a collision does not fragment a spacecraft, the impact may generate a variety of other damage modes (e.g., spallation, rupturing, leakage, and deformation) possibly degrading spacecraft performance or causing spacecraft failure. In LEO, debris as small as a few millimeters in diameter can puncture unprotected fuel lines and damage other sensitive components, and debris smaller than 1 mm in diameter can erode ther-
mal surfaces and optics. The effect of debris impact on a particular spacecraft is strongly dependent on the spacecraft's design; debris is far more likely to damage unprotected spacecraft than those that were designed with due consideration of the meteoroid and orbital debris environment. Components that are difficult to protect from debris (including photovoltaic arrays, suites of communications antennas, and sensors) may, however, be at risk even in a well-designed spacecraft.
Assessments of the damage caused by debris impact are needed to (1) design spacecraft components and shielding capable of surviving debris impact, and (2) better understand the effect of collisions on the evolution of the future debris population. Since it is very difficult to gather data from the rare impacts of medium-sized or large debris in space, assessment of the potential damage such impacts can cause is accomplished primarily through ground-based experimental testing and analytic/numeric methods. Experimental hypervelocity impact testing generally provides the majority of information for such assessments; analytic and numeric tools currently mainly supplement and extend experimental results.
Current hypervelocity impact facilities cannot, however, simulate all relevant debris compositions, shapes, and velocities, and data on the vulnerability of different spacecraft components to debris impact are limited. Although analytical and numerical techniques can be used to predict impact damage for regimes that hypervelocity testing cannot simulate, these predictions can be inaccurate unless they are based on experimental data. Unfortunately, many of the experimental data are not available due to the general inaccessibility of hypervelocity facility capabilities and the impact data generated at these facilities. As a result of these limitations, current spacecraft protection systems may not provide their desired level of protection, and current models of the effects of collisions on the future debris population may be inaccurate.
To facilitate the development of improved models of debris impact damage and enable the development of improved debris shielding, the committee recommends the continuation of research to characterize the effects of hypervelocity impacts 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 new spacecraft materials;
studies of impact damage effects on critical spacecraft components;
development of analytical tools consistent over a range of debris impact velocities, shapes, and compositions; and
improved models of catastrophic space object breakup due to debris impact.
To reduce duplication of effort and speed this research, a handbook describing the capabilities of international hypervelocity testing facilities should be developed. This handbook would serve to increase opportunities for sharing data generated at different facilities.
DESIGNING FOR THE DEBRIS ENVIRONMENT
Although large uncertainties remain, an improved understanding of the debris environment, combined with (1) the growing availability of analytic and experimental tools to quantify the threat to a spacecraft from debris and (2) the development of techniques to protect against debris impacts, has made it feasible for designers to assess the debris hazard and attempt to protect their spacecraft appropriately.
Most spacecraft designers are, however, unaware of these tools and techniques, and very few understand all of the assumptions on which they are based. Such understanding is important because these tools and techniques may incorporate models that have not yet been clearly validated. For this reason, the committee recommends that a guide for spacecraft designers—including information on environmental prediction, damage assessment, and passive and operational protection schemes—should be developed and distributed widely.
A spacecraft's basic structure should be the first line of defense against the debris hazard, but if necessary, active, passive, and operational techniques can be employed to provide additional protection. Passive protection (shielding) of critical components is one viable means of protection. Shield development continues to decrease the mass of shielding required to protect against debris, though it is uncertain how well these shields will protect against the full range of debris sizes, shapes, and compositions. Operational protection schemes, such as the use of redundant components, may also be appropriate for some spacecraft. Such schemes can add weight and cost but can also protect the spacecraft against non-debris-related failures. Active protection measures, such as on-orbit collision avoidance, are typically expensive and difficult to implement effectively.
REDUCING THE FUTURE DEBRIS HAZARD
If the only additions to the future debris population were rocket bodies, nonfunctional spacecraft, mission-related debris, and the products of explosions and surface deterioration, the space object population would probably continue its roughly linear growth. However, several models that use different methodologies and different assumptions predict that
collisions between space objects will add a potentially large and exponentially growing number of new debris objects to this population. Because of the numerous uncertainties involved in existing models of the debris environment, it is premature to suggest exactly when such collisional growth will begin to occur; it may already be under way, or it may not begin for several decades.
Collisional growth is most likely to occur in regions that (1) have a high debris flux, (2) do not experience a high level of atmospheric drag, and (3) have high typical collision velocities. Experts believe that two LEO regions that meet these criteria, at around 900 to 1,000 km and 1,500 km altitude, have already exceeded their ''critical density," the point at which more fragments will be generated by collisions than will be removed by atmospheric drag, even if no further objects are added. Potential exponential growth in the debris population of these regions could force spacecraft designers and operators to take countermeasures against the threat posed by debris or face a heightened risk of losing spacecraft capability due to impacts. Because fragments from collisions in regions experiencing collisional growth may become widely distributed, the collision hazard may be raised even in regions not now threatened by collisional growth.
There are many possible ways to reduce the hazards posed by debris to future space operations. These include actions taken as a spacecraft enters orbit, during its operations, and after its functional lifetime. The active removal of space debris (e.g., the use of debris collection robots or "sweepers"), however, will not be an economical means of reducing the debris hazard in the foreseeable future. Design of future spacecraft and launch vehicles for end-of-life passivation, autonomous deorbiting, orbital lifetime reduction, and reorbiting are generally far more economical ways of reducing the collision hazard.
Growth in the debris hazard can be abated significantly without exorbitant costs by reducing the number of breakups of spacecraft and rocket bodies and, to a lesser degree, by ending or sharply reducing the amount of mission-related debris released during spacecraft deployment and operations. Such measures, however, will not reduce the total mass of objects in orbit. Since the total mass of objects in orbit is a key determinant of the rate of future collisional population growth, it will be necessary to take measures to remove spacecraft and rocket bodies from some crowded orbital regions at the end of their functional lifetimes in order to reduce the potential for exponential growth of the debris population.
Various methods have been proposed to remove spacecraft and rocket bodies from crowded orbital regions at the end of their functional lifetimes. In lower-altitude orbits, it is often possible to deorbit or reduce the orbital lifetime of spacecraft or rocket bodies, typically through a
final propulsive maneuver. Although direct deorbiting into the Earth's atmosphere eliminates debris from orbit rapidly, it requires more fuel than maneuvering to reduce orbital lifetime. In orbital regions that are too high above the atmosphere for economical deorbiting or orbital lifetime reduction maneuvers, spacecraft or rocket bodies can be moved to "disposal orbits" a safe distance away from valuable orbital regions at the end of their functional lifetimes. Disposal orbits are not a viable alternative within LEO because perturbing forces make all such orbits unstable; objects in LEO disposal orbits will eventually cross heavily trafficked orbital regions. Neither the committee nor the wider debris community can agree on whether disposal orbits should be used by all spacecraft and rocket bodies in GEO.
As with other environmental issues, decisions on the adoption and implementation of particular debris reduction 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. Since implementation of debris mitigation measures can impose additional costs on space operations, international rules are needed to ensure that those engaging in debris mitigation activities are not penalized. For this reason, the committee recommends that the spacefaring nations 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 discussions 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 taken to reduce future growth in the debris hazard, based on our current understanding of the debris environment and of the costs and benefits of various mitigation measures, is represented in the following recommendations:
Space system developers should adopt design requirements to dissipate on-board energy sources to ensure that spacecraft or rocket bodies do not explode after their functional lifetimes.
The release of mission-related objects during spacecraft deployment and operations should be avoided whenever possible.
Spacecraft and rocket bodies should be designed to minimize the unintentional release of surface materials, including paint and other thermal control materials, both during and after their functional lifetimes.
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.
Spacecraft and rocket bodies in LEO and in highly elliptical orbits passing through LEO should either be removed from LEO or have their orbital lifetime reduced at the end of their functional lifetime.
The use of GEO disposal orbits should be studied further. Until such studies produce a verifiably superior long-term strategy for dealing with the GEO debris hazard, operators of GEO spacecraft and rocket bodies should be encouraged to reorbit their spacecraft at the end of their functional lifetimes if they are capable of safely performing a reorbiting maneuver to a disposal orbit at least 300 km from GEO.
The threat that orbital debris poses to international space activities is presently not large, but it may be on the verge of becoming significant. If and when it does, the consequences could be very costly—and extremely difficult to reverse. By contrast, the cost of reducing the growth of the hazard can be relatively low, involving specialized data collection and research along with cooperation and information sharing among the developers and users of space hardware. The committee believes that spacefaring nations should take judicious, timely steps now to understand the risk and agree on ways to reduce it.