30. End of Mission Considerations 30.1 Disposal Guidelines and Requirements 30.2 LEO End-of-Mission Options (could be divided into several subsections) 30.3 Disposal from Orbits Other than LEO 30.4 Passivation Options 30.4.1 Propulsion Systems 30.4.2 Power Systems 30.4.3 Other Systems 30.4.4 Passivation Challenges 30.5 Disposal Planning 30.6 Examples 30.6.1 FireSat II 30.6.2 SCS Annotated Bibliography 30 End of Mission Considerations Scott M. Hull, NASA Goddard Space Flight Center While a great deal of effort goes into planning and executing successful mission operations, it is also important to consider the End of the Mission during the planning, design, and operations phases of any mission. Spacecraft and launch vehicles must be disposed of properly in order to limit the generation of orbital debris, and better preserve the orbital environment for all future missions. Figure 30-1 shows a 1990’s projected growth of debris with and without the use of responsible disposal techniques. This requires early selection of a responsible disposal scenario, so that the necessary capabilities can be incorporated into the hardware designs. The mission operations must then be conducted in such a way as to preserve, and then actually perform, the planned, appropriate end of mission disposal. Figure 30-1 Debris Growth with Various Mitigation Approaches (reference iii, page 22) Computer simulations have shown that the orbital debris population already present on- orbit is self- propagating; that is, the orbital debris density will continue to increase through random collisions alone, unless reduced by outside effortsi. This may well result in a cascade effect that eventually renders some orbits impractical for space operations. Since it is not yet economically practical to remove a significant amount of existing debris from orbit, it is critical that responsible end of mission disposal be practiced for all current and future missions, in order to help control the rate of increase of mission-lethal debris objects in commonly used orbits. Had such methods been employed throughout the history of space operations, the cascade effect might have been prevented, or at least substantially delayed. End of Mission disposal (also known as End of Life disposal, Decommissioning, or simply Disposal) has been addressed primarily at the international level in discussions by the Inter-Agency Space Debris Coordination Committee (IADC). The IADC is an international forum of national space agencies and the European Space Agency (ESA) for the coordination of activities related to the issues of man-made and natural debris in spaceii. In 2002, the IADC issued a set of guidelines (IADC-02-01) addressing, among other things, prevention of post mission explosions, and acceptable disposal options. These guidelines, described in Section 30.1, were slightly refined in 2007. Spacecraft mission designers need to consider disposal early in the design process , in order to incorporate the necessary hardware and procedures to ensure a safe disposal. The first step is to select a baseline disposal method, as described in Sections 30.2 and 30.3 below. That disposal method will determine the key design factors which will need to be considered throughout the remainder of the design process. It may, for example, be necessary to size the propulsion system and navigation hardware for an orbit change or controlled reentry. Alternatively, it may be necessary to design the power and propulsion systems for postmission passivation, as described in Section 30.4. As the design develops, and the reentry risk is determined, it is occasionally necessary to change the baselined disposal method, but at a cost which increases dramatically as the design matures. In any event, it will be necessary to develop and test spacecraft operations procedures specific to the disposal (Section 30.5). The disposal of the FireSat II and SCS sample missions are discussed in Section 30.6, as examples of the application of the disposal principles. Early consideration of the end of mission disposal is among the most effective ways to minimize the growth of orbital debris, to the benefit of all missions. 30.1 IADC End of Mission Guidelines While they do represent agreements among the leading space agencies of the world, it is important to note that the IADC guidelines are not currently legally binding. They are generally reflected, however, in numerous national space policies, and are followed at least in part for the majority of scientific and military space missions. Despite whether commercial and other missions are legally bound to comply with the IADC guidelines, meeting the standards has been shown to be crucial for limiting the growth of orbital debris, which is in the interests of all space users. The guidelines are summarized here, and the specific text is readily available on the internet. In addition, there is a “Support Document to the IADC Space Debris Mitigation Guidelines” (IADC-04-06), which provides valuable insights and background information on the specific guidelines. The IADC guidelines provide guidance for limiting the generation of orbital debris both during and after space operations. They begin by describing the need for limiting the growth of orbital debris, and defining the relevant terms used throughout the document. The guidelines refer to direct creation of debris through operational debris (lens caps, for example), and potential breakage of tethers. They also consider on-orbit breakups caused by explosions during and after the mission, as well as intentional destruction by internal or external sources. The guidelines also define accepted disposal orbits and other conditions such as the timeline for abandoning commonly used orbits, and controlling the risk to people and property on the Earth. Finally, they address limiting the potential for damage by collisions with other space objects and with small orbital debris that could prevent the ability to successfully execute end of mission disposal. The IADC guidelines are written to apply throughout the mission lifetime, from design through operations and decommissioning. It is worth noting that while the IADC Guidelines do lay the foundation for general agreements on the limitation of orbital debris, with few exceptions they do not provide specific quantitative requirements. In fact, the stated purpose of the guidelines is to “demonstrate the international consensus on space debris mitigation activities and constitute a baseline that can support agencies and organizations when they establish their own mitigation standards”. Only in the case of the definitions of the protected orbit regions, and the GEO disposal conditions, do the guidelines provide specific limits. The remaining limitations are described as qualitative measures, which are left to individual agency requirements documents to define in detail. In addition to the IADC guidelines, various other organizations have adopted similar guidelines and requirements. In 2007, the United Nations General Assembly endorsed the “Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space”, which are very similar to, and cover the same general topics as, the IADC guidelines, while being generally less specific. Most nations look to their national space agency (often an IADC member) for orbital debris policy and control. The United States, however, has issued not only the “US Government Orbital Debris Mitigation Standard Practices”, but also has a number of individual agency documents that address orbital debris limitation to varying degrees. In general, the requirements documents issued by the IADC member agencies themselves are the most specific and restrictive. Examples of national orbital debris limitation documents are shown in Table 30.1-1. Table 30.1-1 International Orbital Debris Limitation Documents (self-generated) Domain Document IADC IADC-02-01, Rev 1 US U.S. Government Orbital Debris Mitigation Standard Government Practices NASA NPR 8715.6A, NASA-STD-8719.14 US DoD DoD Space Policy Directive, 3100.10, AFI 91-217 US FAA Title 14, CFR Part 415.39 Japan JAXA JMR-003 France CNES MPM-50-00-12 Europe (ESA) European Code of Conduct for Space Debris Mitigation Russia Space Technology Items. General Requirements on Mitigation Of Space Debris Population 30.2 LEO Disposal Options Responsible exit from Low Earth Orbit (LEO) is one of the most important steps that can be taken to limit the growth of debris in that region of space. An IADC working group has examined the long-term effects of various guideline optionsiii, and shown that by limiting the amount of time that each vehicle remains in LEO, growth of the orbital debris environment is greatly reduced. Thus, it is desirable and recommended for space objects (both launch vehicle stages and spacecraft) to be removed from the LEO region as soon as practical. The minimum orbital lifetime possible for an individual vehicle might be determined, however, by the remaining maneuvering capability of the vehicle at the end of the mission or by the initial orbit. There are three basic approaches to LEO disposal. The most desirable, if possible, is to perform a controlled (or ‘targeted’) reentry into an unpopulated region of ocean soon after the end of the mission, either using on- board propulsion or by external retrieval. If this is not possible, it may be possible to boost into a storage orbit between LEO and GEO, safely removed from both regions. Finally, a spacecraft can be allowed to reduce its orbit by atmospheric drag, resulting in an uncontrolled reentry and impact on an unpredictable portion of the Earth’s surface. Controlled reentry is the preferred LEO disposal method for several reasons. Not only does it provide positive removal of the vehicle from orbit, but the removal also tends to occur as soon as possible after the mission. By selecting a reentry location over a large unoccupied area of ocean, the reentry risk to the ground population is minimized. This, along with the lack of postmission passivation, can result in greater flexibility for the mission hardware designers. Controlled reentry is not without significant challenges, however. Reentry maneuvers typically require extensive planning, and notification to the relevant air and maritime traffic authorities prior to performing the maneuvers. In practice, controlled reentry is best performed using at least three separate maneuvers in order to better control and refine the orbit, with a final perigee of less than 50 km, to prevent atmospheric skip. In order to accomplish this, the spacecraft design must incorporate sufficiently large thrusters to ensure adequate control authority at low altitude. Controlled reentry also requires that the vehicle reserve sufficient fuel to reliably perform the reentry maneuvers at the end of the mission, which will result in a larger fuel mass at launch. Section 9.6 discusses the ΔV needed to accomplish controlled reentry, and gives examples. With the advent of robotic servicing and retrieval capabilities, it also may become a practical option for missions to further extend mission lifetime, and still perform a controlled reentry, but at the expense of an additional launch. Disposal into a storage, or ‘graveyard’, orbit may be a practical option for some high- altitude LEO missions. The storage region is located between LEO and GEO, and is generally considered to extend from 2000 km to approximately 35,586 km (GEO – 200 km) altitude. Within this region, however, care must be taken to also avoid commonly used orbits, such as the circular 12 hour orbits used by navigation and other satellites. Both the apogee and perigee of the disposal orbit must be within the storage orbit region. As with controlled reentry, the spacecraft design must incorporate sufficiently large maneuvering thrusters, and fuel must be reserved for the orbit raising operation. Figure 30.2-1 shows the typical ΔV required to maneuver from a circular LEO orbit to the 2000 km storage orbit. Any vehicle left in a storage orbit must be passivated at the end of the mission, as described in a later section. In general, only those missions operating above about 1400 km altitude can reach the storage orbit region with less delta V than re- entering within the recommended timeframe. If neither controlled reentry nor storage orbit disposal are practical for a LEO mission, then disposal will eventually occur by uncontrolled reentry. If possible, the final altitude and Area–to-Mass Ratio need to be tailored to ensure that a reentry by atmospheric drag is predicted to occur within 25 years after the end of the mission. The IADC study mentioned above concluded that this orbit duration is a reasonable compromise between unlimited orbital lifetimes and immediate de-orbit at the end of the mission. For a spacecraft with no propulsion system, that limits the maximum orbit altitude to about 600 to 700 km, depending on the Area–to-Mass Ratio and launch year. Figure 30.2-1 shows the typical ΔV required to maneuver from a circular LEO orbit to an orbit that will reenter the atmosphere within 25 years. Note that the solar flux is an important component of this prediction, and varies throughout the solar cycle, complicating the reentry date prediction considerably. Current predictions of future solar activity are available for download from NASA, NOAA, and other sources, and are typically updated frequently. Earlier orbit decay will further reduce the likelihood of collision, and is therefore recommended if possible. Figure 30.2-1 Disposal Delta-V Requirements (self-generated, see Excel spreadsheet “SMAD End of Mission Calculations.xls”) While at first glance uncontrolled reentry may appear to be a preferred disposal method (it is surely the simplest and lowest mass approach), there are considerable challenges to doing so responsibly. It is necessary for any non-operational object left in orbit to be passivated during the orbit-decay period, as discussed in Section 30.4, in order to prevent inadvertent explosion or breakup during the potentially long orbit decay period. The risk to the ground population may also be controlled by requirements of the launching or operating organization. For example, several space agencies require a detailed assessment to show that the spacecraft hardware will burn up sufficiently during atmospheric reentry to pose less than a 1 in 10,000 risk of causing a serious injury to even one human. In some cases, the risk to the public from an uncontrolled reentry is sufficient to dictate that a controlled reentry is the only acceptable method of disposal from LEO. Meeting a requirement to limit reentry risk can be extremely difficult or impossible for some large spacecraft, and usually necessitates specialized design techniques and materials selection. The risk is determined by how much of the spacecraft survives reentry, and by the ground population over which the reentering debris might land. In the case of an uncontrolled reentry, the surviving debris might land anywhere in the latitude band covered by the orbit inclination, so an average population density over this band is used for the risk estimate. Object survivability is largely driven by the thermal properties of the primary construction material for the object in question, expressed as a heat of ablation. Heat of ablation is typically expressed in terms of mass, but it can also be useful to express it in terms of volume (multiplying by the material density), for comparing two material options for the same size part. Heat of Ablation = Specific Heat x Δ Temperature + Latent Heat of Fusion The heats of ablation for several typical spacecraft materials are shown in Table 30.2-1. Notice that materials such as aluminum and graphite/epoxy composite are readily demiseable, whereas titanium, glass, and beryllium all have high heats of ablation, and should therefore be avoided when possible, if reentry risk is a concern. In general, objects made from materials with melting temperature greater than 1000 K, or heat of ablation greater than 1000 kJ/kg or 2500 kJ/m3 are more likely to survive atmospheric reentry. Oxidation heating (essentially burning) on reentry also contributes the demisability of aluminum and graphite/ epoxy. Table 30.2-1 Heats of Ablation for Several Common Spacecraft Materials (self-generated) Melting/ Heat of Softening Heat of Ablation Material Temperature (K) Ablation (kJ/kg) (kJ/m3) Graphite/Epoxy 700 350 550 Aluminum 850 900 2400 Stainless Steel 1700 900 7250 Titanium 1940 1600 7050 Zerodur Glass 2000 1400 3550 Beryllium 1557 4100 7550 Object survivability is also influenced by the object’s ballistic coefficient, which is a function of the shape, mass, and dimensions of the object, and determines its velocity. In general, faster moving objects accumulate more heat, and are more likely to demise. Reentry risk has been successfully reduced on some flight missions by modifying a component’s shape, size, or material, when possible. Another approach that can be used to reduce the reentry risk is to ensure that several high survivability objects are bound together, since multiple objects are more likely to cause injury than a single object. For example, if several surviving battery cells are contained within a robust battery box, then the single surviving box presents less risk than the multiple cells would. Because the survivability of spacecraft components depends on so many factors, it is necessary to combine very specialized assessment software with detailed knowledge of the spacecraft construction in order to determine and limit the reentry risk. Early avoidance of high survivability materials in component designs can help to prevent difficult and expensive redesigns late in the design phase. For all LEO mission disposal options, it is desirable to minimize the total time that a vehicle spends in orbit. This is done to reduce the likelihood that the vehicle will experience collisions with resident objects, which usually result in the creation of additional orbital debris. Debris generation potential increases rapidly with projectile size, and even existing objects as small as 1 cm can create additional debris on impact. Simulations predict that on-orbit collisions will be the primary source of new orbital debris, so minimizing orbital lifetime is the most important step toward limiting future debris generation. Figure 30.2-2 shows that even with no new launches, collisions among existing on-orbit objects will eventually cause the debris population to rise. Any fuel remaining at the end of the mission should be used to lower perigee as much as possible during the passivation process, resulting in the earliest possible reentry. One caveat to this general rule, though, is that the last burn before an uncontrolled reentry should leave the spacecraft high enough so that its orbit remains stable long enough for ground tracking to get an accurate fix. This will allow monitoring of the vehicle for conjunction assessment and collision avoidance purposes as its apogee descends past other operating spacecraft. Figure 30.2-2 Projected Debris Generation by Mechanism (with no new launches after 2009) Originally from the 13th Annual FAA Commercial Space Transportation Conference, Orbital Debris & Space Traffic Control presentation by Gene Stansbery (JSC/ODPO) Also at http://www.orbitaldebris.jsc.nasa.gov/newsletter/pdfs/ODQNv14i1.pdf, page 8. Complicated by the 2006 data. ODPO has offered to print this for us without the confusing solid line data. 30.3 Non-LEO Disposal Options The IADC guidelines define two protected regions of space: LEO and GEO. No space vehicles should be left in either of these regions on a long- term basis. The LEO region is simply defined as the “spherical region that extends from the Earth’s surface up to an altitude (Z) of 2000 km”. Removal from the LEO region within 25 years is described in Section 30.2. The GEO protected region is defined as 35,786 km +/- 200 km, with latitude of 0° +/- 15°. Any mission that passes through that protected region must be maneuvered at the end of the mission to remain clear. Removal from GEO should occur as soon as practical after the end of the mission. Figure 30.3-1 The IADC-defined Protected Regions of Space (from reference iii, page 5) Figure 30.3-2 Artist’s rendering of the IADC-defined Protected Regions (from reference iii, page 6) Disposal from GEO is performed by increasing the orbital radius sufficiently to remain well in excess of 200 km above the GEO altitude (35,786 km + 200 km = 35, 986 km minimum altitude) for a minimum of 100 years. Due to the effects of solar radiation, as well as luni-solar and geopotential perturbations, the recommended minimum increase above GEO is defined as: (equation adapted from reference iii, page 18) 200 km + 35 km + (1000 km x C x A/m) R Where: 35 km represents the effects of luni-solar and geopotential perturbations C is the solar radiation pressure coefficient (typically 1 – 2 kg/m2) R A/m is the spacecraft cross-sectional area to dry mass ratio (m2/kg) The altitude is generally increased, as opposed to decreasing, to prevent an accumulation of debris that future GEO missions would need to pass through to get to GEO, and to prevent potential signal interference. In addition to increasing the orbital radius, the final orbit should be circularized to an eccentricity of no more than 0.003, and the spacecraft needs to be passivated as described in section 30.4. It is estimated that the fuel required for responsible disposal of GEO missions is equivalent to that used for about 3 months of station-keeping for most spacecraftiv. The “Support to the IADC Space Debris Mitigation Guidelines” document contains further details on disposal from GEO. Disposal from Earth orbits other than LEO and GEO (high eccentricity science orbits, for example) should minimize the orbit lifetime and avoid any highly used regions. While the IADC guidelines are not specific, some national requirements documents include a protected region for 12 hour orbits, commonly used for navigation satellites. Due to the wide variety of unusual orbits, it would be impossible to cover all possibilities in detail here. The first priority of the end of mission planning, however, is minimization of the potential for on-orbit collisions. As with LEO and GEO missions, any vehicle left in orbit must be passivated (see section 30.4) after the final maneuvers. There are, as yet, few guidelines available for disposal of lunar, planetary, or Lagrange orbit missions. The primary consideration, as it is for Earth orbits, is to minimize the potential for collisions with other current and future spacecraft, either directly or through generated debris. Therefore, it is best to avoid leaving spacecraft in long-term orbits (>10 years) at the end of the mission, except in those cases where the spacecraft continues to serve as a communications relay after its primary mission. Likewise, it is best to passivate any orbital hardware at the end of the mission, to prevent explosions and other breakups, which would generate additional debris that might interfere with future missions. If disposal is to include lunar or planetary impact, care should be taken to avoid sites of scientific or historic value, as well as preventing organic contamination whenever possible. 30.4 Passivation At the end of the mission, the IADC guidelines call for all on-board sources of stored energy to be “depleted or safed when they are no longer required for mission operations or post-mission disposal”, also known as passivation. The main concern is that stored residual energy has in the past resulted in explosions, which have been a major source of orbital debris. Propulsion systems, batteries, and reaction wheels all contain stored energy, and are the most common components identified for passivation. Since a spacecraft or launch vehicle could be in orbit and unattended for many years (even centuries) after the mission, it is important to passivate to prevent generation of debris that would increase the likelihood of collisions for other missions.