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7 Mitigation of Orbital Debris President Reagan’s 1988 National Space Policy directed that “all space sectors will seek to minimize the creation of space debris . . . consistent with mission requirements and cost effectiveness.” At that time, the NASA orbital debris program had already established procedures to minimize the possibility of growth in the debris population from future explosions of the U.S. Delta 2nd stage, and had approached the European Space Agency (ESA) concerning a similar problem with ESA’s Ariane upper stage. These were the events that began NASA’s involvement in establishing standards for mitigation of orbital debris on an international basis. Political, legal, technical, and economic considerations had to contribute to any standards that were established. The political and legal issues are discussed in Chapter 11. The economic issues were considered by consulting with the manufacturers and operators of space hardware. For example, most upper-stage rockets either have the ability to deplete the excess fuel remaining after delivering their payload, or could have that ability with a simple modification. Consequently, implementation of the mitigation standard for a spacecraft and upper stages to deplete their on-board stored energy after mission operations began in the early 1980s, well before there were any writ - ten requirements. This single mitigation action greatly reduced the risk of accidental explosions for those who exercised it and lowered the growth rate of cataloged fragments resulting from rocket body explosions from 150 fragments per year between 1964 and 1984 to only 50 fragments per year for the next 20 years (see Figure 1.2). Other mitigation standards, such as minimizing any debris intentionally released, also had minimal impact on mission costs and were quickly accepted by the community. However, by the early 1990s it was becoming increasingly obvious that for as long as any object was in orbit it would always be subject to the external energy source of kinetic energy. The kinetic energy involved in collisions between cataloged objects is much greater than most remaining internal energy and is therefore capable of producing even more fragments than explosions. Con - sequently, it was concluded that the increasing accumulation of orbital mass in the form of intact spacecraft and upper stages would inevitably lead to collisions between these objects and become the dominant source of future debris.1 While there was uncertainty in when collisions would become the dominant source of debris, there was concern that at some point in the near future the rate of growth in low Earth orbit (LEO) would become irrevers - ible. This led to a mitigation standard for upper stages and payloads at the end of their mission to either maneuver Interagency Group (Space) for the National Security Council, Report on Orbital Debris, Washington D.C., February 1989; D.J. Kessler, 1 Orbital debris environment for spacecraft in low Earth orbit, Journal of Spacecraft and Rockets 28(3):347-251, 1991; J.P. Loftus, Jr., D.J. Kessler, and P.D. Anz-Meador, Management of the orbital debris environment, Acta Astronautica 26(7):477-486, 1992; D.J. Kessler and J.P. Loftus, Jr., Orbital debris as an energy management problem, Advances in Space Research 16(11):139-144, 1995. 57

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58 LIMITING FUTURE COLLISION RISK TO SPACECRAFT to one of a set of defined disposal regions or maneuver to an orbit where atmospheric drag would remove the object within 25 years. The rationale for 25 years rather than any other time period was that it was an acceptable compromise between the amount of fuel required to maneuver to a lower orbit, and the effectiveness of such a maneuver to the long-term environment, as a result of predictions by various orbital debris models. Models such as NASA’s LEGEND and the earlier EVOLVE have consistently predicted that there is only a small difference in the long-term environment between an object being removed immediately and 25 years later. Before ESA accepted the 25-year rule, ESA considered everything from zero years to 100 years for a post-mission lifetime, using the ESA MASTER’99 model. ESA concluded that “a 25-year post-mission lifetime is the shortest possible before propellant requirements start to become disproportionately high.”2 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. There are two problems with the current post-mission disposal standards: (1) As described in Chapter 11, “Issues External to NASA,” not all of the spacecraft community are required to follow NASA’s standards, with at least one U.S. agency not even encouraging compliance with the 25-year rule. 3 (2) Current model predictions conclude that even 90 percent compliance is insufficient to prevent future debris growth in LEO. These same models predicted the collision rates that are observed from the past four collisions between cataloged objects, as well as the amount of debris generated as a consequences of China’s anti-satellite test (Box 1.2 in Chapter 1) and the accidental Iridium–Cosmos collision (Box 9.1 in Chapter 9), providing additional evidence that the models are correct, and that mitigation alone is not sufficient. The possibility that current mitigation standards may not be adequate requires either more aggressive mitigation or the introduction of removal operations; however, the agency is not prepared for either. The only study to determine what actions would result in a stable orbital debris environment concluded that the retrieval of pre-selected objects could do so, and would be significantly helped by compliance with the 25-year rule.4 A study by both NASA and ESA has identified some alternative techniques to remove objects. 5 However, the largest activity was the “International Conference on Orbital Debris Removal” in Chantilly, Virginia, on December 8-10, 2009, sponsored by NASA and the Defense Advanced Research Projects Agency (DARPA). The conference identified many possibilities, but all required further technology development, most raised legal issues, and some introduced policy conflicts. A few might be more accurately described as enhanced or active mitigation. A report to DARPA after the conference included the observation that “any future debris removal strategy must be tested to ensure that it will work in the operating environment.”6 None of the removal or “enhanced mitigation” concepts have been fully tested or tried in the operating environment. R. Walker, C. Martin, H. Stokes, J. Wilkinson, H. Sdunnus, S. Hauptmann, P.Beltrami, and H. Klinkrad, Executive summary, in Update of 2 the ESA Space Debris Mitigation Handbook, Ref. QINETIQ/KI/SPACE/CR021539, European Space Agency, Paris, France, July 2002, avail - able at 3 National Research Council, Summary of the Workshop to Identify Gaps and Possible Directions for NASA’s Micrometeoroid and Orbital Debris Programs, The National Academies Press, Washington, D.C., 2011. 4 J.-C. Liou, N.L. Johnson, and N.M. Hill, Controlling the growth of future LEO debris populations with active debris removal, Acta As- tronautica 66(5-6):648-653, 2010. 5 H. Klinkrad and N.L. Johnson, “Sustainable Use of Space through Orbital Debris Control,” Paper AAS 10-016 presented at the 33rd Annual AAS Guidance and Control Conference, Breckenridge, Colo., February 6-10, 2010; also in Advances in the Astronautical Sciences 137:63-74, 2010. 6 D. Baiocchi and W. Welser IV, Confronting Space Debris, Strategies and Warnings from Comparable Examples including Deepwater Horizon, prepared for DARPA by RAND Corporation, Defense Advanced Research Projects Agency, Arlington, Va., 2010.

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59 MITIGATION OF ORBITAL DEBRIS FIGURE 7.1 LDEF was returned to Earth using the space shuttle. The space shuttle also returned the Satellites Palapa and Weststar; however, these satellites were “cooperative” in that they were stable and designed to be handled by the space shuttle. Returning a possibly spinning satellite that was not designed to be handled is a more difficult problem, even more so without the capabilities of a crewed space shuttle. SOURCE: Courtesy of NASA-JSC. Objects have been removed from orbit, such as the LDEF satellite shown in Figure 7.1, but these were planned and designed for easy removal using the space shuttle. Various concepts for removing debris are discussed in DARPA’s recently released “Catcher’s Mitt” Final Report,7 and include the use of nets and harpoons to capture large objects, and tethers, drag augmentation devices, and solar sails to remove the objects. Other potential removal solutions that have flown in orbit or been tested on the ground are electrodynamic and momentum tethers; drag augmentation devices; solar sails; ground-based and space-based lasers; and soft-catch collection media. Many show some promise; however, necessary safeguards must also be addressed and tested to ensure that any operation system does not contribute to the production of orbital debris through unintentional consequences. 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 con- siderations have not been fully examined, nor has analysis been done to determine when such technology will be required. W. Pulliam, Catcher’s Mitt Final Report, Tactical Technology Office, Defense Advanced Research Projects Agency, Arlington, Va. 7