Orbital Debris Cascades: Population Stability, Growth, and the Usability of Space
The orbital debris population from collisions will grow over the next 200 years. How soon the usability of space will be affected depends on future launch rates, the level of compliance with disposal guidelines, and evolving spacecraft design practices.
The term collisional cascading was used by NASA scientist Donald Kessler in a 1991 paper describing the growth of the future orbital debris population from a chain reaction of collisions. This was predicted to occur if the production rate of debris from collisions became greater than the rate of removal from atmospheric drag. Since then, the term “Kessler syndrome” has become widely used in popular literature.
Computerized tools to model future growth of the orbital debris environment have been developed over the last 20 years by several international organizations. Within the United States, the Orbital Debris Program Office (ODPO) at NASA Johnson Space Center developed the EVOLVE model. This tool was used to help establish criteria in the U.S. Government Orbital Debris Mitigation Standard Practices (ODMSP), including the 25-year threshold on orbital lifetime for disposal by uncontrolled reentry. ODPO later developed a model called LEGEND (LEO-to-GEO Environment Debris Model), which is now the baseline debris environment projection tool used by NASA. ODPO has used LEGEND to study the effect of various levels of compliance with post-mission disposal (PMD) rules, such as those in the ODMSP, as well as the effect of various rates of active debris removal (ADR). NASA uses these findings to identify effective mitigation practices and support activities of the Inter-Agency Space Debris Coordination Committee (IADC), which coordinates international debris activities and identifies mitigation options.
The Aerospace Corporation has been developing its own long-term debris modeling capability since 2009. The Aerospace Debris Environment Projection Tool (ADEPT) uses component models developed over several decades and employs algorithms and data sets that are independent from the models of other international organizations. ADEPT has been used to quantitatively assess the issues of environmental stability and the effectiveness of PMD vs. ADR.
Most published studies of future debris population growth assume a steady launch rate of new satellites. The studies typically project 100 to 200 years out and the historical launch pattern is almost always repeated because it is difficult to predict actual launch patterns over long timeframes. Launch patterns are a function of technological, political, and economic conditions, which are far more uncertain than physics-based phenomena.
Because there is random variation involved in predicting future events and phenomena (e.g., future collision events), debris environment projection models are run many times (Monte Carlo variations) to understand variability in scenarios. Thus, single debris growth trend lines typically represent the average of many possible futures.
Studies typically consider variations on two hypothetical scenarios: In the business-as-usual scenario, worldwide missions remain near their mission orbits at end of life and make no effort to comply with international disposal guidelines. In the compliance scenario, all worldwide missions comply with current or proposed international disposal guidelines. Often, various levels of compliance are considered. The goal is to show the effects of different mitigation and disposal options on the debris environment relative to levels of action or inaction.
Is the Orbital Debris Environment Unstable?
A debris population is considered unstable if there is a growth trend by the end of a simulated time frame. Studies conducted by NASA using LEGEND and by Aerospace using ADEPT show that the orbital debris population in low Earth orbit (LEO) will grow in the business-as-usual scenario. These studies show a quadratic-type growth of the debris population over 200 years. In compliance scenarios, the average growth rate of the LEO debris population is reduced, but not eliminated. No matter the compliance scenario, some form of ADR will likely be needed to stabilize the future LEO debris population.
Moreover, presenting population counts for LEO to determine stability is an oversimplification of the problem. Stability depends on orbital altitude and fragment size. The atmosphere is generally effective at clearing out debris of 1 centimeter and larger, but only up to an altitude of approximately 1000 kilometers. The highest density regions of LEO are in the 800–1000 kilometer altitude range. There is some clearing of smaller debris in this range, but the clearing is less effective for larger, trackable fragments. The population growth is significantly reduced when there is worldwide compliance with international disposal guidelines, which includes moving objects to lower orbits that are more strongly affected by atmospheric drag. However, stability of the population in lower LEO (1000 kilometers and below) remains marginal, even within compliance scenarios.
Since there is no effective atmosphere between 1000 and 2000 kilometers, the population in the upper region of LEO must be unstable. The growth rate of the collisional debris population will initially be slow, but will eventually increase. The growth in total collision rates depends on the future launch rate into upper LEO and associated disposal practices. The amount of debris released will depend on the amount of mass that is launched into this region and left there.
Orbital regions above LEO do not have any significant atmosphere. Debris populations in medium Earth orbit (between LEO and GEO) (MEO) are typically unstable, but the current population density is low because of the large volume of space in MEO. It will take longer for collisions to start and for their rates to increase in this region. Debris populations in GEO and the graveyard region above GEO are also unstable. The population density is higher here than in MEO because of the altitude and latitude bounds associated with GEO (GEO is torus-shaped, MEO shell-like).
Orbits at high altitude and inclination can undergo large eccentricity variations from the effects of solar and lunar gravity. In some cases, perigee altitude can drop into the atmosphere, resulting in reentry. Some disposal orbits near the MEO navigation constellations, including for the Global Positioning System (GPS), could decay after 100 to 200 years; some high inclination GEO orbits could decay after 25 years. Even without the occurrence of reentry, the variations will cause these orbits to spread out over a larger volume of space over time, resulting in a reduction of population density and hence, collision rates. This dilution of collision risk could be used by MEO constellation satellites to significantly delay the onset of collisions between disposed satellites.
Can Short-Term Cascades Occur After a Collision?
It is well established that single breakup events do not result in collisional cascading over a period of hours, days, weeks, or even months. These findings are based on many breakup event analyses performed by Aerospace over two decades as well as long-term examinations conducted by NASA. Two real-world collision events that produced large amounts of debris are the 2007 Chinese antisatellite (ASAT) test and the 2009 accidental collision between Iridium 33 and Cosmos 2251. Although each collision generated thousands of now-cataloged debris fragments and hundreds of thousands of smaller debris, and even though the Iridium-Cosmos collision occurred within the large Iridium constellation, neither event was followed by a short-term cascade. The actual consequence of the collisions was that the number of close approaches between operational spacecraft and debris objects increased significantly.
There are several reasons why cascades do not occur in the short-term. One is that the high level of energy that drives the severe fragmentation during a collision also ejects the fragments at high velocity, causing them to be spread over a wide range of orbits. This reduces the likelihood that a satellite passing through a given debris cloud will be hit by a fragment. Another reason is that the size of both fragments and satellites is small compared to the volume of space they traverse. Many orbital revolutions must pass before a collision between a fragment and a satellite becomes likely. A time period of hours to months is not sufficient, even for breakups in LEO. Added to this is the spreading effect of orbital perturbations, which tend to disperse debris over months. The presence of pinch points, the regions of space where the collision occurred and through which all of the debris passes as it orbits in orbital debris clouds, may result in some interfragment collisions, but the amount of mass in those fragments is small, and any subsequent collisions involving them would primarily result in the generation of debris too small to be a significant threat.
Will the Orbital Debris Environment Prevent Use of Space?
Studies conducted with ADEPT have predicted the future debris population down to 1 centimeter in size. A rule of thumb is that shielding can be designed to protect spacecraft in LEO against debris up to a maximum of 1 centimeter in size. Results from ADEPT indicate that the orbital debris environment down to 1 centimeter will pose a relatively low risk of collision to satellites for at least the next few decades.
In current spacecraft design practices, weight considerations usually limit the practicalities of shielding to particles between 1 millimeter and 1 centimeter, with variations in a spacecraft depending upon how its individual components are constructed. Susceptibility to these smaller particles is a function of each spacecraft’s design and orbital location.
Regions with unstable debris populations due to lack of atmosphere, such as upper LEO, could see the debris population grow to an unacceptable level. Whether this occurs will depend on the total mass available for creating debris and the volume of the region over which the debris is spread. A constant replenishment of satellite mass in a given region without removal at end of life will accelerate the growth of the collision rate and the resulting debris population.
The ADEPT simulations of the noncompliance (business-as-usual) scenario show that the yearly 1-centimeter debris collision risk posed to a generic satellite in upper LEO could become unacceptable sometime after 200 years. This scenario includes a reference constellation of satellites in the upper LEO region that is continually replenished over 200 years. An unacceptable debris population level could be reached sooner in scenarios where satellites are vulnerable to debris smaller than 1 centimeter. The collision risk posed to the same generic satellite was significantly reduced in the compliance scenario. This illustrates the effect of compliance with the ODMSP and international disposal guidelines on reducing debris risk to future operational satellites.
In general, orbital debris poses a higher risk to manned spacecraft than to robotic spacecraft, which is particularly true for the International Space Station (ISS) because of its large size. Normal ISS operations are periodically interrupted by the need to perform collision avoidance maneuvers to avoid close approaches with cataloged objects, which are usually debris. The erosion of ISS surfaces from impacts by debris smaller than 1 centimeter results in jagged edges that can tear astronaut spacesuits during extravehicular activities. Penetration of a spacesuit from small debris could lead to loss of human life. Crew transfer vehicles have thermal shielding for reentry that is vulnerable to small debris impacts. Even a small hole in a thermal tile created by a 1-millimeter debris object can lead to a burn-through during reentry. The Columbia space shuttle accident is emblematic of this vulnerability. Therefore, future debris population growth may well pose limits to human spaceflight. Conducting human activity at lower altitudes in LEO (below 500 kilometers) can help reduce this risk, because the higher level of atmospheric drag cleans out the debris population more rapidly, resulting in lower overall debris population densities.
Studies by a number of organizations show that the orbital debris population will grow because of collisions, but the rate of growth is strongly dependent on future launch rates and end of life disposal practices. Published studies and ADEPT simulation results show that significant reduction in the growth of collisional debris in LEO and the associated risk posed to spacecraft can be achieved by consistent worldwide compliance with international disposal guidelines.
Jenkin, M. Sorge, J. McVey, G. Peterson, and B. Yoo, “MEO Debris Environment Projection Study,” Proceedings of the Sixth European Conference on Space Debris (Darmstadt, Germany, Apr. 22–25, 2013).
Kessler, “Collisional Cascading: The Limits of Population Growth in Low Earth Orbit,” Advances in Space Research, Vol. 11, No. 12, pp. 63–66 (1991).
Liou, “LEGEND—A Three-Dimensional LEO-to-GEO Debris Evolutionary Model,” Advances in Space Research, Vol. 34, pp. 981–986 (2004).
Liou et al., “Stability of the Future LEO Environment—An IADC Comparison Study,” Proceedings of the Sixth European Conference on Space Debris (Darmstadt, Germany, Apr. 22–25, 2013).
Related publication: Crosslink, Fall 2015, Understanding Space Debris