Predicting the Future Space Debris Environment
First published Fall 2015, Crosslink® magazine.
The Aerospace Corporation’s ADEPT simulation is being used to assess the effectiveness of mitigation practices on reducing the future orbital debris population.
In a landmark 1978 publication, NASA scientists Donald Kessler and Burton Cour-Palais concluded that collisions of satellites and spent rocket bodies would eventually form the dominant source of orbital debris in low Earth orbit (LEO). They predicted that debris from such collisions would collide with other satellites and rocket bodies and create even more debris. As a result of this chain reaction, the risk to satellites in certain regions of space would increase exponentially with time, even without further launches into those regions. In a 1991 paper, Kessler used the term “collisional cascading” to describe this process. Since then, the term “Kessler syndrome” has become widely used in the popular literature.
In February 2009, the first of the predicted catastrophic collisions occurred between the Iridium 33 satellite and the Russian Cosmos 2251 satellite. This single event generated more than 2200 trackable fragments and significantly more that were too small to track. An antisatellite test performed by China in 2007 had already produced more than 3400 trackable fragments. Between the two, the number of tracked objects had increased by about 65 percent.
Following these events, the U.S. Air Force initiated a study in 2009 to assess the effects of an increasing debris population on the performance of future U.S. military space systems. To support this effort, the Air Force asked Aerospace to generate discrete future LEO debris populations for input to its simulations. This resulted in a new capability at Aerospace to model the future debris environment in LEO. This initial capability was largely independent of models developed by other organizations, but still used a database of object masses supplied by NASA.
During the course of subsequent studies, Aerospace significantly enhanced its ability to model the future LEO debris environment. Portions of the process were reconfigured to run on distributed high-performance computing clusters, and the system was made fully independent of other debris models by establishing an Aerospace-developed database of object masses, sizes, and ballistic areas. The capability became sufficiently mature to receive a name: the Aerospace Debris Environment Projection Tool (ADEPT).
In 2012, the Air Force Space and Missile Systems Center (SMC) requested a study to determine the effect of potential changes to National Space Policy on the future debris environment in medium Earth orbit (MEO), with the goal of assessing the risk to the Global Positioning System (GPS). This was known as the MEO Debris Environment Projection Study. For this effort, ADEPT was extended to model not just LEO but all orbital regimes. This was necessary to account for possible cross-coupling between the LEO, MEO, and geosynchronous (GEO) populations via collisions involving objects on highly eccentric orbits. A number of other improvements have been made to ADEPT through internal research and development. These include faster generation of future collisions, extension of Monte Carlo processing, generation of future random solar cycles, greater fidelity of the original population, better modeling of active debris removal, assessment of modeling accuracy via comparison with actual data, and improved fragmentation modeling.
The discrete populations generated by ADEPT can be used to derive a variety of products. For example, plots of the on-orbit population vs. time can measure the growth rate for the debris population—overall, or in specific orbital regions.
Plots of object spatial density vs. altitude and time indicate which regions of space will see higher debris growth. This information can influence where a satellite might be flown to minimize risk; it can also help show how different disposal practices might affect different regions of space.
Plots of probability vs. severity enable the user to rank orbital objects by the amount of debris they might create from collisions in various scenarios. This is useful in identifying objects for active removal that would achieve the greatest reduction in future debris growth.
The ADEPT discrete populations can also be used to predict the frequency of collision avoidance maneuvers on orbit, which could affect the amount of propellant needed on board and help forecast mission outages that might be induced by the maneuvers.
ADEPT is currently used at Aerospace to perform collision probability analyses for space debris assessment reports, which are required by Air Force Instruction 91-217 (Space Safety). ADEPT has also been used by Aerospace to support the NASA delegation at the Inter-Agency Space Debris Coordination Committee (IADC).
The 2012 MEO Debris Environment Projection Study used ADEPT to simulate two scenarios. In the first (compliance), all worldwide future launches comply with internationally recommended disposal guidelines. In the second (business as usual), all worldwide future launches move to disposal orbits near their mission orbits and do nothing else to comply with any guidelines. Results showed that the rate of growth of the future collisional debris population in the business as usual scenario increases with time. The rate of growth also increases in the compliance scenario, but much more slowly.
ADEPT scenarios have also shown the effect of conservation of mass. In essence, as collisions occur, the amount of mass in orbit is redistributed from large objects (for example, satellites) to smaller debris pieces. Smaller objects are less likely to collide, and when they do, they have less momentum and kinetic energy to impart to other objects. ADEPT runs start with an initial population and create “first-generation” debris, caused by collisions between objects in the initial population, and “second-generation” debris, caused by the collision of first-generation debris objects with both initial population objects and other first-generation objects. ADEPT simulations over 200 years have shown that second-generation debris grows much more slowly than first-generation debris. So, although the future collisional debris population increases with time, it does not increase exponentially, at least for simulated time periods up to 200 years in the future.
ADEPT Future Studies
Studies to date using ADEPT assume that historical launch patterns will continue into the future. While this has been the standard practice in the debris modeling community, the future launch pattern will almost certainly be different. Russian launch patterns and orbits have changed significantly, and China is emerging as a dominant spacefaring nation. The French Space Operations Act of 2010 imposes more- stringent debris mitigation requirements than previous laws, and could significantly change the future distribution of Ariane upper stages.
The introduction of CubeSats has also brought a significant change in launch patterns. Typically, a relatively large number of CubeSats will hitch a ride on a launch of a standard satellite. Ultimately, CubeSats may form a large population occupying a wide range of orbits—but that will not necessarily result in a larger future debris population, because the effect of conservation of mass is present. Their small size reduces their probability of collision, while their small mass reduces their potential for creating large amounts of debris when they do collide with other objects. So, as with second-generation debris, the effect of their small size and mass on the creation of future debris may offset the effect of their greater numbers. Also, the ballistic coefficients of CubeSats are different from those of standard large satellites. This means they will lose altitude (if their orbits are low enough to be affected by drag) at different rates than larger satellites. ADEPT can be used to help quantify how these opposing attributes of the CubeSat population will affect the future orbital debris population.
Future development plans for ADEPT include reconstructing the current debris population down to 1 centimeter and smaller from all previous space activity. This will enable independent assessment of the debris risk posed to spacecraft by the existing small, untracked debris population. It will then be possible to improve current estimates of the cost of shielding (typically feasible only for debris up to 1 centimeter) or constellation replenishment to compensate for failures caused by debris impacts.
The ADEPT simulation process enables projections of the future orbital debris environment resulting from various scenarios. It can model the impact of changes in launch traffic patterns and identify effective debris mitigation approaches. The future debris environment representations generated by ADEPT can be used to determine satellite collision avoidance frequency and associated maneuver requirements and to support other types of mission utility analysis. Used effectively, ADEPT studies can help identify debris mitigation approaches that maximize the long-term sustainability of space for future generations at reasonable cost to the current generation.
About the Authors
Alan B. Jenkin, Senior Engineering Specialist, System Analysis and Simulation Subdivision, joined Aerospace in 1985. He has worked for 24 years in the field of astrodynamics, specializing in collision risk analysis, orbital debris and meteoroid risk assessment, and postmission disposal of satellites and upper stages. He has a B.S. in aeronautical engineering and an M.S. in computer and systems engineering from Rensselaer Polytechnic Institute. He has also completed graduate course work at the University of California, Los Angeles. He has written 39 conference papers, 10 journal articles, and one book chapter. He is an associate fellow of the American Institute of Aeronautics and Astronautics.
Marlon E. Sorge, Senior Project Engineer, Space Innovation Directorate, joined Aerospace in 1989. He has worked on space debris issues for more than 25 years, encompassing fragmentation modeling, risk assessments, debris environment projection, mitigation techniques, and policy development. He also coordinates Aerospace’s debris research program. He has a B.S. in physics and an M.S. in aeronautical and astronautical engineering from Purdue University.
Glenn E. Peterson, Senior Engineering Specialist, System Analysis and Simulation Subdivision, joined Aerospace in 1997. He works on a variety of topics including space debris, collision analysis, meteor showers, and satellite disposal. He has a B.S. and an M.S. in aerospace engineering from San Diego State University, and a Ph.D. from the University of Texas at Austin.
John P. McVey, Engineering Specialist, Astrodynamics Department, joined Aerospace in 2003. He has performed research on a variety of orbital mechanics topics including orbit dynamics and propagation, orbital debris studies, collision analysis, and orbit analysis of small satellites. He has a B.S. in physics from the University of Florida and an M.S. in aerospace engineering from the University of Colorado, Boulder. He is currently a Ph.D. candidate in the Astronautical Engineering Department at the University of Southern California.
Bernard B. Yoo, Engineering Specialist, Astrodynamics Department, joined Aerospace in 1993. He specializes in optimal resource allocation and scheduling of space and communications systems. Current projects include model-based systems engineering, architecture performance analysis, and integrated atmospheric modeling. He has experience and general expertise in visibility analysis, constellation design, launch and on-orbit operations support, and space-based weather monitoring. He has a B.S. in mechanical engineering from Rice University and an M.S. in aerospace engineering (astronautics) from the University of Southern California.
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