When Deciding When To Deorbit A Sattelite

illustration NASA’s EO-1 is an example of a mission that will not comply with guidelines for deorbiting spacecraft within 25 years of the end of their lives. (credit: NASA) Weighing overall societal benefit: Case studies on deciding when to deorbit satellites (part 1) by Marissa Herron Monday, October 21, 2024 Bookmark and Share This paper was inspired by the 2019 Orbital Debris Mitigation Standard Practices (ODMSP) update and, in particular, the enthusiastic debate surrounding the disposal rule. The programmatic and societal perspectives that NASA frequently encounters were not included in the discussions. Programmatic impacts are most simply described as the technical, cost, and schedule impacts, but do not immediately convey the unintended consequences. For example, mandating propulsive capabilities could significantly increase the costs of small satellites and severely challenge the continuation of science programs, or broader societal benefits from a satellite's data could justify an extended lifetime. This paper does not state a recommendation on the disposal rule, but instead seeks to encourage additional perspectives from which to design future disposal rules. What is ODMSP? The Orbital Debris Mitigation Standard Practices (ODMSP) were first established in 2001 and updated in 2019. These standard practices are intended to apply “...guidelines for USG activities…” and serve as a “reference.... for other domestic and international operators .” What is the “25-year rule”? The “25-year rule” refers simply to the removal of an object from orbit within 25 years of mission completion. The 25-year rule was introduced in the 1990s in an effort to limit the growth of debris in Low Earth Orbit (LEO). The 2010 United Nations Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space (UN COPUOS), in a similar spirit, calls for the limitation of a long-term presence of defunct spacecraft and rocket bodies in LEO, but does not explicitly identify the number of years. The June 2021 Inter-Agency Space Debris Coordination Committee (IADC) also specifies disposal within 25 years. A slightly more detailed look at the ODMSP for postmission disposal, or the disposal of a spacecraft following the completion of the mission, is shown in the graphic below. The original 2001 version and the updated 2019 version is shown graphically for comparison. Note that the figure is only referring to the postmission disposal aspects of the ODMSP and not the passivation, large constellation, or other objectives within the ODMSP. illustration Comparison of the 2001 and 2019 ODMSP rules. When considering postmission disposal, there are three major aspects: Dispose of the spacecraft within 25 years of mission completion, If atmospheric reentry is the disposal method, then ensure the human casualty risk (or risk to the public) is less than 1 in 10,000, and Implement a disposal method with a probability of success greater than 90%. Item one inherently refers to passivating the spacecraft such that batteries and energy sources are drained to minimize explosion potential. Ideally, a spacecraft will maintain the ability to do collision avoidance maneuvers and get as low as possible before passivation. The lower a spacecraft is in LEO, the more drag the spacecraft will experience which helps speed up an atmospheric decay disposal. Once the spacecraft runs out of fuel, and thus, the ability to perform collision avoidance maneuvers, the spacecraft is passivated and no longer operational. At this point, the spacecraft should be in an orbit such that the atmospheric disposal option takes no more than 25 years. (This discussion refers to LEO spacecraft, not GEO. The latter maneuver to a disposal orbit above GEO,) Mandating propulsive capabilities could significantly increase the costs of small satellites and severely challenge the continuation of science programs, or broader societal benefits from a satellite's data could justify an extended lifetime. Item number two involves a survivability analysis of the spacecraft components and design prior to launch. This can be accomplished using the ODPO’s Debris Assessment Software or other means to demonstrate the spacecraft components will adequately break up and disintegrate during reentry. The thermal ablative properties of materials used to build the spacecraft are a primary driver for survivability. Those items that do survive reentry are considered in the calculation of the human casualty risk. Item three refers to the use of a proven and reliable method of disposal to ensure the disposal process does not experience problems in the process of disposing of the spacecraft. In other words, an operator does not want to accidentally abandon a spacecraft in a populated orbit due to the failure of the disposal system. Who was involved in the creation of these standards and guidelines? The UN COPUOS consists of membership from several spacefaring and non-spacefaring nations. The IADC membership includes international governmental space agencies . The 2019 update of the ODMSP was accomplished in response to Space Policy Directive-3. The update was led by NASA and supported by relevant government agencies, including regulatory agencies. The higher-level UN COPUOS guidelines, Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space, were informed by the more technical IADC guidelines with the intent of achieving global consensus. The IADC guidelines (UN A/AC.105/C.1/L.260. Inter-Agency Space Debris Coordination Committee space debris mitigation guidelines) were informed by the ODMSP and a number of other resources. What is the effectiveness of the 25-year rule? Why 25-years? The effectiveness of the 25-year rule is assessed (see figure below) for objects in LEO greater than 10 centimeters, a 90% postmission disposal (PMD) success rate, and simulated future explosions. The results presented below are based on a Monte Carlo analysis using the NASA LEGEND tool. Each curve presented below represents the average of 100 Monte Carlo runs. LEGEND models debris as small as one millimeter and is capable of both historical analysis and future projections. The historical simulations are used to validate the model and produce accurate future projections. Averages are used due to uncertainties in parameters such as future launch traffic, solar activity, explosions, collisions, etc. Launch traffic can change dramatically due to large constellations. Solar activity can influence the drag experienced by an object in LEO. Explosions can occur to improperly passivated spacecraft and manufacturing defects. Defunct satellites and/or debris further create collision opportunities. illustration Effects of deorbit lifetimes and success rates. The above figure provides varying levels of mitigation. As expected, no mitigation produces a significant projected increase to the number of objects in LEO over time. With increasing levels of mitigation (or shorter rules), the projected increase in the number of objects in LEO decreases. The relationship between the different simulated rules does not appear to be linear with the number of objects projected. A 2014 IADC study, using the DAMAGE model, concurred with the ODMSP 25-year rule. The IADC study cited that “very short times would involve a substantial increase in the de-orbit propellant requirement.” Both the IADC and ODMSP agreed that 25 years is a maximum timeframe and encouraged proactive steps by the satellite operator to pursue a PMD timeframe that is better than the 25-year rule. They both believed that 25 years was an adequate compromise between high mission cost for a short lifetime disposal (and, thus, effective PMD policy) and a low mission cost for a long lifetime (which increases collision risks). Alternative opinions on the 25-year rule The 2019 ODMSP update was not without controversy or disappointment. There were some opinions that the 25-year rule should be reduced to a 5-year rule. Keep in mind that the ODMSP are intended for government entities but have been globally recognized (albeit in a high level, qualitative form) and have also informed the development of regulations for commercial spacecraft licenses. The latter is a key piece to consider and reflects the intent of SPD-3’s direction to update the ODMSP to “promote efficient and effective space safety practices with U.S. industry and internationally. ” Whether the disposal time frame is best set at 5 or 25 years remains a topic of debate and perspectives. As an alternative and proactive effort, a group of non-governmental entities, called the Space Safety Coalition, voluntarily stated their intent to strive for a five-year disposal timeframe for those spacecraft with chemical or electric propulsion. The group also raised the probability of successful disposal from 90% to 95%. Whether the disposal time frame is best set at 5 or 25 years remains a topic of debate and perspectives. Intuitively, five years seems like the logical choice if your only concern is the sustainability of the space environment. However, if you are considering mission requirements and the cost to build to stricter requirements, then operators may reconsider whether or not to build a satellite. If your concern is the growing popularity of large constellations (as opposed to one or two satellites), then separate requirements can be applied to the large constellations. This is the approach the updated ODMSP pursued in response to SPD-3’s direct requirement to address large constellations. (Although NASA led the ODMSP update, NASA does not have experience building or operating large constellations. The large constellation focus was included in the ODMSP update, because the ODMSP is historically used by the FCC to regulate non-governmental satellites. Thus, although the ODMSP only applies to governmental satellites, the update was created with the understanding that the regulatory agencies would likely implement the updated ODMSP as license requirements for non-governmental satellites.) Can we do better than 25 years? A shorter disposal timeframe will likely encourage future technologies. Under present-day technology, satellites will have shorter operational lifetimes if they use their fuel for a faster disposal. This will result in more maneuvering (versus a gradual decay) out of orbit. The reduced operational time in orbit could lead to more replenishment of satellites and, thus, increased launch activity. The increased traffic going into and out of orbit may involve more spacecraft maneuvering which can challenge current collision avoidance capabilities and operators. This suggests the need for more sensors (in orbit and ground) with the goal towards achieving continuous custody. This also suggests the need for automation and efficiency to reduce the manual effort involved with mitigating close approaches. Additionally, the operator community will need to work together to develop effective and efficient means of communication and coordination for the increased congestion in orbit. This series of steps are just some of the advances and maturation that could occur with a push towards shorter disposal timeframes. The ideal approach is for a spacecraft to be removed immediately upon mission end so as to minimize the risk of collisions and explosions. However, the tradeoffs to these advances are likely in the form of increased cost, which may disincentivize the development of some satellites. What remains unclear, and is a potential future area of study, are the mission requirement and cost implications to pursuing the ideal approach. How might a higher standard impact future satellite operators and launch vehicle development? Could new technologies and approaches be developed in response? For both the launch vehicle and the spacecraft there will be cost implications. A 2021 paper cited a list of top 50 most concerning objects in LEO. This list of concerning objects includes 39 rocket bodies, which are derelict objects in an uncontrolled state with high mass. The primary factors used to create the list were “mass, encounter rates, orbital lifetime, and proximity to operational satellites.” (It is worth noting is that “concerning” objects can also include small, untrackable debris. Differing perspectives exist on whether high mass, trackable objects that could produce a lot of debris are more concerning than mission ending, untrackable debris. The ability to track an object creates an opportunity to avoid the object and, thus, reduce collision consequences to impacted satellites and the environment. However, high mass objects could result in the creation of more debris, whether trackable or not. This would have a significant impact on the environment. Both perspectives have merit and pursue the same goal of a sustainable environment. Ultimately, measurement of both parameters, trackability and mass, will present a more accurate assessment of the environment.) NASA’s reputation for technical rigor presents an opportunity to study the individual cases without concern for neglect of details or a lack of competence in the assessments leading to decision making. Legacy launch vehicles release debris in orbit and are estimated to cost $300–500 million to upgrade. Drag devices may allow spacecraft to maximize use of their fuel while maintaining reasonable decay time frames in LEO. These devices will need to be high reliability, otherwise they risk a mission ending failure due to accidental deployment. A better understanding of the impacts and consequences may educate opinions on the appropriate balance for disposal timeframes. Below is a review of multiple NASA spacecraft that were decommissioned while in Earth orbit. NASA spacecraft were chosen for the large amount of publicly available information, avoiding the need for concern over proprietary or otherwise sensitive information that might occur with commercial or defense spacecraft. Furthermore, NASA’s reputation for technical rigor presents an opportunity to study the individual cases without concern for neglect of details or a lack of competence in the assessments leading to decision making. The scope of this effort is spacecraft in LEO. The sample of case studies will allow the observation of any patterns between spacecraft, but also permit the clarification of individual findings regarding any unique aspects. The research question under consideration is “What are the obstacles to ODMSP compliance?” This explanatory study will assess the decision making process, the decisions made, and the effect of those decisions. illustration Image source: NASA Case Study: EO-1 NASA’s Earth Observing-1 (or EO-1) satellite was launched November 2000 from Vandenberg Air Force Base on a Delta II rocket . In 2011, EO-1 reached “critical fuel levels and suspended orbital maintenance, beginning a slow decline in orbital altitude. A small fuel reserve was used for debris avoidance maneuvers…” Multiple senior review panels assessed the scientific value and availability of funding for operations. The 2011 Senior Review panel specifically cited EO-1’s instruments as providing a “gap-fill between Landsat 7 and LDCM (Landsat 8), gap-fill for ASTER SWIR bands ” and “prototyping for HyspIRI.” The satellite was in good health and “predicted to function through 2015.” These parameters ultimately led to a decommissioning date in late 2016 due to the degraded orbit reducing the value of the science. On March 30, 2017, EO-1 was “transitioned into a permanently safed configuration with fully emptied propellant tanks and drained batteries. NASA forecasts the satellite’s re-entry for 2056…” Below is a schedule of tasks leading up to passivation of the spacecraft: illustration Image source: NASA How does this relate to the postmission disposal practices? EO-1 will remain in LEO for greater than 25 years beyond mission completion. At the time of design and launch, the solar activity estimates predicted an orbit decay design within the 25 years of postmission completion. However, updated solar cycle information increased the time frame originally estimated. This information was not known until well into the lifetime of the spacecraft. Estimating the impact of drag (which is influenced by solar activity) on a LEO satellite is part of the design process—something which typically occurs decades in advance of the satellite disposal. In this situation, the updated estimates meant that the satellite would experience less drag than originally expected at the time of design and will remain in orbit longer. Once the new estimate of the decay time frame was known, nothing could be done to accelerate the satellite’s demise. Instead, NASA made the most of the situation by continuing operations for as long as possible. This allowed more science data to be collected and accommodated the need for any collision avoidance maneuvers until passivated. What’s the takeaway from this case study? Space weather is challenging to predict, especially far into the future. Considering the lengthy design, build, and fly process for a satellite, the space weather predictions will have limited accuracy in the crucial design phase. The design of a maneuverable (propulsive) satellite involves estimating the fuel necessary to maintain orbit and mission (i.e. science) location, as well as the fuel reserve necessary for postmission disposal. In other words, the low atmospheric drag experienced in LEO requires the occasional reboost of the satellite to maintain the stringent requirements for science needs. Fuel for collision avoidance maneuvers must also be estimated for the lifetime of the satellite. All of these fuel estimates are based on predicted solar activity levels that can influence the amount of drag experienced by each object in LEO. Further complicating the situation is the satellite area exposed along the velocity vector influences the amount of drag each object experiences. This means that different objects can experience different levels of drag. As our understanding of space weather improves, the process of designing a satellite and estimating lifetimes will also improve. illustration Image source: NASA Case Study: TRMM The Tropical Rainfall Measuring Mission (TRMM) was launched in November 1997 from Tanegashima Island on NASDA’s H-II rocket. The TRMM mission was known early on to be a valuable data source for disaster efforts. "The data were being heavily used for tropical cyclone monitoring and forecasting," said TRMM Project Scientist Scott Braun at Goddard. "It was being used for flood detection and monitoring. It was also used for drought monitoring, disease monitoring — where diseases are most prevalent in areas of heavy precipitation and flooding.” As a result of the valuable social contribution from the TRMM spacecraft, NASA approved a mission extension in 2001. The mission extension allowed for the orbit to be raised slightly to 402.5 kilometers. This orbit allowed the mission to continue, while minimizing fuel consumption, and extending the mission. In 2005, NASA again reviewed the status of the TRMM mission and determined the spacecraft was healthy, the mission continued to be a valuable social contribution, and operating costs were adequate. The 2005 review resulted in another mission extension until the satellite ran out of fuel. “Rationale was that TRMM, through its hurricane tracking and other capabilities, had the potential to save lives, out-weighing the risk of human casualty from uncontrolled reentry.” As part of the 2005 review, TRMM’s ODMSP compliance was considered. TRMM was in a low 350-kilometer circular orbit where atmospheric drag requires regular reboosts. At this orbit, the spacecraft was low enough to minimize on orbit collision risk. However, the use of the fuel for mission extension meant that an uncontrolled (versus controlled) reentry was necessary. The TRMM spacecraft had a dry mass of 2,630 kilograms and was estimated to have 12 pieces (with a total mass of 112 kilograms) survive reentry to the surface of the Earth. The human casualty risk was estimated as 1 in 4,200, which is slightly higher than the ODMSP risk. NASA decided to waive the controlled reentry requirement in favor of extending the mission and utilizing an uncontrolled reentry. The issue here was not the amount of time the satellite would take to decay from orbit. At such a low altitude, atmospheric drag provides a natural cleansing process and (solar activity withstanding) will remove the satellite from orbit within a reasonable period of time. In the case of TRMM, NASA needed to consider the risk of an uncontrolled reentry against that of mission extension. The replacement satellite for TRMM was the Global Precipitation Measurement (GPM) mission which was planned to launch in 2010 (at the time of analysis) and ultimately launched in 2014. Thus, NASA was considering the human casualty risk of an uncontrolled reentry against that of the lives saved from TRMM’s data (and future GPM data) for disaster support. “Rationale was that TRMM, through its hurricane tracking and other capabilities, had the potential to save lives, out-weighing the risk of human casualty from uncontrolled reentry.” illustration TRMM represents a situation where the science data provided a social value that was deemed greater than the level of risk exceeding the ODMSP. As the orbit decayed, the satellite’s reentry was closely monitored. TRMM ultimately reentered in June 2015 without any issue. illustration Image source: NASA Case Study: Van Allen Probes NASA’s Van Allen Probes were launched in August 2012 from Cape Canaveral Air Force Station . This unique mission consists of two satellites in highly elliptical Earth orbits exploring the Van Allen radiation belts. The mission was extended in 2017 and then ended operations in 2019 . In February 2019, shortly before decommissioning, the satellites executed a well-coordinated perigee-lowering campaign intended to decay the orbit of the satellites within 15 years of decommissioning. The campaign was designed around the limited thruster performance capability and consisted of a series of maneuvers over two weeks. Although complying with the 25-year postmission disposal rule, a 2021 NASA inspector general report cites this mission as being an explosive risk due to the inability to disconnect and drain the battery sources. “Originally, the mission planned to reserve sufficient fuel in order to lower the spacecraft to an orbit that would enable reentry within 5 months and in which the spacecraft could be reoriented so that its solar arrays would be away from the Sun preventing recharging of the batteries. However, the subsequent mission extension led to insufficient fuel to lower the spacecraft’s orbit per the original plan and disconnect the solar array from the battery. Instead, the batteries will continue to recharge which could eventually result in an explosion and creation of additional orbital debris. ” This situation highlights how both postmission disposal and passivation contribute a challenging, but valuable role to sustainment of the orbital environment. illustration Image source: NASA Case Study: Terra The Terra mission was launched December 1999 on an Atlas-Centaur IIAS launch vehicle from Vandenberg Air Force Base. Terra is a flagship mission that is currently flying an extended mission due to the highly desirable contribution of the mission’s science data (discussed below in the Senior Review section). In 2020, Terra performed its last inclination adjust maneuver ; a stationkeeping maneuver that maintains the satellites mean local time equatorial crossing (for science data quality). The mission orbit was lowered in 2022 to reduce potential close approaches with other satellites in the Earth Sciences Constellation . Maneuvers will continue for the purpose of avoiding debris. Once all of the fuel is expended, the satellite will passivate and continue an orbital decay to an uncontrolled reentry. The predominant challenge appears to be the use of limited fuel capacity for the continuation of science versus lowering the orbit of the satellite for disposal and reentry. A 2021 NASA inspector general report expressed concern over the Terra spacecraft’s ODMSP compliance indicating the “…Terra spacecraft, a 10,000 pound satellite observatory … is scheduled to be decommissioned in 2026. Terra will not be able to deorbit within 25 years, its batteries cannot be disconnected, and the propellant system cannot be depressurized, increasing the possibility for a debris-generating explosive event. Furthermore, Terra is expected to be in orbit for 50 years after the end of its mission, also increasing the likelihood of collision with other objects in orbit.” The public information available on Terra decommissioning dates varies and suggests uncertainty in orbital lifetime estimates. A variety of presentations exist that indicate many options were considered for both the ODMSP compliance and the maximization of science. Studies were accomplished to estimate risk during uncontrolled reentry, fuel usage, time remaining on orbit, etc. These presentations demonstrate the dynamic nature of operations and the challenges in precisely estimating time on orbit. Terra’s significant contributions resulted in another mission extension from the 2020 Earth Science Senior Review. The review highlighted the uncertainties surrounding Terra’s time on orbit and encouraged more deliberate analysis of the predicted orbit and impacts to science. illustration Image source: NASA Case Study: QuikSCAT NASA’s Quick Scatterometer (QuikSCAT) satellite was launched June 1999 on a Titan II vehicle from Vandenberg Air Force Base. A 2011 Earth Science Senior Review Panel extended the QuikSCAT mission due to the strong national interest of the science data and fulfillment of NASA’s ocean winds science objectives . The panel did note concerns about the spacecraft’s technical health stating “the QuikSCAT spacecraft is approaching 12 years in operation (design life was three years), has suffered several faults and degraded components, and seems unlikely to survive the next several years without incurring a mission-ending failure. There is no trending data or component health analysis presented to support the assertion that this 12-year old spacecraft is capable of operating through a 2-year mission extension. There is a risk that the spacecraft will be unable to achieve the planned decommissioning orbit.” However, the addendum (included in the report) indicated the benefits outweighed the risks: “Although the QuikSCAT spacecraft has suffered several faults and degraded components (other than scatterometer's antenna spin mechanism) and may incur a mission-ending failure, it is also probable that the spacecraft can operated at this level for the next 2 years; therefore the Senior Review panel feels that this is a worthwhile risk.” QuikSCAT continued to operate until its decommissioning in 2018 , many years after the difficult decision made by the 2011 Earth Science Senior Review Panel. According to the 2021 NASA inspector general report, the spacecraft will significantly exceed the 25-year disposal timeframe and is not fully passivated. Many of the satellites studied were launched shortly before the establishment of ODMSP. However, NASA continues to consider the ODMSP and to pursue efforts to minimize risks to the sustainment of the orbital environment. The predominant challenge appears to be the use of limited fuel capacity for the continuation of science versus lowering the orbit of the satellite for disposal and reentry. A look at the Senior Review process, in the second part of this essay, can provide insight into the assessment of a mission’s science value and the decision of whether to extend a mission. Disclaimer: the views expressed in this paper are solely those of the authors, not of NASA or of the Federal Government. Dr. Marissa Herron is currently a Senior Space Policy Analyst with NASA. She previously served as the Program Executive for the Sustainable Land Imaging (Landsat) Program. She has over 20 years of experience as a NASA Aerospace Engineer in space situational awareness, mission operations, and flight dynamics analysis for human spaceflight and robotic missions.