Forecasting the Evolution of the Space Debris Population
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*Kostas Konstantinidis is a space systems engineer and has contributed multiple* [*essays*](https://www.metaculus.com/questions/?main-feed=true&status=active&has_group=false&type=notebook&username=Kostas&order_by=-activity) *to the Metaculus Journal.* This essay and [Our Life Underneath a Chaotic Orbital Environment: The Relationship Between Space Debris and Space Utilization](https://www.metaculus.com/notebooks/15408/our-life-underneath-a-chaotic-orbital-environment-the-relationship-between-space-debris-and-space-utilization/) by astrophysicist Emma Louden offer diverging perspectives and forecasts on space governance questions from the [Sagan Space Tournament](https://www.metaculus.com/tournament/sagan-tournament/). --- In a [previous essay](https://www.metaculus.com/notebooks/10025/space-debris-and-the-kessler-syndrome/) we discussed the hazards of space debris and the measures being taken to reduce risks of collision with active spacecraft. Space is a surprisingly crowded place, with many active spacecraft navigating a large population of orbiting space debris that range in size from flecks of paint to decommissioned satellites. This [space debris population is growing](https://www.esa.int/Space_Safety/Space_Debris/Space_debris_by_the_numbers) and may one day reach a tipping point scenario known as the [Kessler syndrome](https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.394.6767&rep=rep1&type=pdf) where consecutive impacts cause a chain reaction of collisions, thus making key orbits unusable. Agencies and other space users are beginning to take precautions, such as [monitoring the space debris population](https://www.euspa.europa.eu/european-space/space-situational-awareness) and engaging in spacecraft [avoidance maneuvers](https://en.wikipedia.org/wiki/Collision_avoidance_(spacecraft)) when debris nears. [New regulations](https://www.esa.int/Space_Safety/Space_Debris/Mitigating_space_debris_generation) are being implemented to ensure that satellite operators remove their spacecraft from sensitive orbits after the end of their operational lifetimes, and more active solutions are being investigated for debris removal, such as launching [dedicated spacecraft](https://clearspace.today/) to capture and remove larger pieces of debris, or using [lasers](https://elib.dlr.de/105165/1/Laser-Based%20Space%20Debris%20Removal.pdf) and other Earth-based methods to remotely push smaller objects from orbit. ## Modeling space debris populations Mathematical and computational models play a key role in investigating the evolution of the debris population, as they can be used to gain insights into the mechanisms affecting this evolution while identifying possible measures to control the growth of the debris population. In its simplest form, the evolution of the population of orbiting bodies can be modeled in terms of sources and sinks: Sources increase the amount of debris and sinks decrease it. Let's take the case of a satellite reaching the end of its operational lifetime—either by design or by accident. This type of object can either remove itself from orbit or can remain as space debris. If it remains, it can orbit for decades or more, can reenter the atmosphere due to atmospheric drag if the orbital altitude is low enough, or it can one day be removed by a dedicated debris removal spacecraft. There are other more dramatic examples of sources and sinks: An explosion aboard a spacecraft can create a cloud of smaller explosion fragments that spreads throughout Earth orbit over time. Such an explosion can result from overheating unspent batteries and, propellant tanks on decommissioned spacecraft, or via an anti-satellite weapon. Further, a collision between objects can break them up, resulting in two debris clouds. All these small fragments remain in orbit until their orbits also naturally decay into the atmosphere or they are removed from orbit artificially. Researchers try to predict the evolution of the overall population by investigating these sources and sinks, and we can use this framework to break forecasts down into logical chunks. Consider the number of spacecraft launched and scenarios that [anticipate the](http://www.frontiersin.org/articles/10.3389/frobt.2018.00054/full) number of launches in the future: In the last few years, the number of launches has been increasing constantly, with [2163 objects placed into orbit](https://ourworldindata.org/grapher/yearly-number-of-objects-launched-into-outer-space?country=OWID_WRL~USA~RUS~CHN~GBR~JPN~FRA~IND~DEU~European+Space+Agency) by [180 launches](https://www.nature.com/articles/d41586-023-00048-7#:~:text=2022%20was%20a%20record%20year%20for%20space%20with%20180%20successful,the%20Chinese%20government%20and%20businesses.) in 2022. In a conservative, business-as-usual scenario, the number and mass of spacecraft launched in the coming decades would stabilize and remain more or less as it is today. In an aggressive scenario, both space activity and miniaturization technologies would grow significantly. In this scenario, the current trend continues, and an increasingly large number of smaller spacecraft are launched over the years. **Metaculus Community forecasts on the** [**Annual Number of Objects Launched Into Space**](https://www.metaculus.com/questions/12740/annual-number-of-objects-launched-into-space/) To forecast debris removal, we'll need to consider several parameters. First, [post-mission removal guidelines](https://www.esa.int/Space_Safety/Space_Debris/Mitigating_space_debris_generation) are currently in place, dictating that operators must “passivate” their spacecraft after the end of their lifetime to avoid explosions and then remove them to a naturally decaying or [graveyard orbit](https://en.wikipedia.org/wiki/Graveyard_orbit). Currently, the self-removal success rate stands at [around 50%](https://www.sdo.esoc.esa.int/environment_report/Space_Environment_Report_latest.pdf), and some have [argued](https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.394.6767&rep=rep1&type=pdf) that a perfect success rate will be necessary for low Earth orbit to remain usable in the long term. It remains to be seen whether operators will meet these guidelines or be compelled to by stricter future regulations. Secondly, the first active debris removal missions are [scheduled to fly soon](https://www.esa.int/Space_Safety/ClearSpace-1), and there is currently [great](https://www.surrey.ac.uk/surrey-space-centre/missions/removedebris) [interest](https://astroscale.com/astroscale-forges-ahead-with-uk-active-debris-removal-mission-with-support-from-uk-space-agency/) in further developing these concepts. [There](https://www.scientificamerican.com/article/space-junk-removal-is-not-going-smoothly/) [*is*](https://www.scientificamerican.com/article/space-junk-removal-is-not-going-smoothly/) [some disagreement](https://www.scientificamerican.com/article/space-junk-removal-is-not-going-smoothly/) about the viability of such future missions, and the amount of debris they will be able to clear up, however. [Estimates indicate](https://www.esa.int/Space_Safety/Space_Debris/Mitigating_space_debris_generation) that 5-10 pieces of debris must be removed every year from key orbits in order to stabilize the growth of the debris population. [Earth-based removal methods](https://spectrum.ieee.org/laser-could-clear-space-junk-from-orbit) can play an important role here—especially in removing smaller pieces of debris—but these methods are less mature. A direct estimate of the rate of collisions and explosions can be made based on historical data. Or, [physical models](https://www.lpi.usra.edu/meetings/lpsc2003/pdf/1828.pdf) and [simulations](https://www.emi.fraunhofer.de/en/business-units/space/research/numerische-untersuchungen-satellitenkollisionen-im-orbit.html) can calculate the probability and impact of such events. Since the advent of the space age, [more than 640 break-ups](https://www.esa.int/Space_Safety/Space_Debris/Space_debris_by_the_numbers), explosions, collisions, or anomalous events resulting in fragmentation have taken place. **My prediction**: I foresee a business-as-usual scenario: Approximately 600 events have taken place historically so far. Assuming the same average rate of events per year over the average number of operational spacecraft, we reach 600 more events for a total of 1200 by 2030. Most satellites by 2030 will be relatively new, e.g. the Starlink constellation, so there is a thick tail to the left. High impact, low probability events like the Kessler syndrome or heavy ASAT use give a thin long tail to the right. This is in contrast with astrophysicist [Emma Louden's calculations, which indicate double the number of events](https://www.metaculus.com/notebooks/15408/our-life-underneath-a-chaotic-orbital-environment-the-relationship-between-space-debris-and-space-utilization/). Still, I believe that since the majority of satellites in 2030 will be relatively new and operational, they will apply more advanced traffic management and post-mission disposal methods, resulting in a relatively lower number of events. The effect of the use of debris-producing anti-satellite weapons (ASAT) can also be investigated in scenarios anticipating e.g. an increase in or ban of ASAT testing, or conflicts where ASAT weapons are used. Several [ASAT tests](https://en.wikipedia.org/wiki/Anti-satellite_weapon) have taken place in the recent past that [contributed significantly to the debris population](https://www.ucsusa.org/sites/default/files/2019-09/debris-in-brief-factsheet.pdf). **My prediction**: I foresee a 10% chance of ASAT use (relatively low) due to the highly escalatory nature of these weapons (especially of the kinetic sort), as much of the infrastructure in space (e.g. navigation satellites like GPS) is entangled with nuclear deterrent and other military functions. **My prediction**: I assume that the US moratorium on ASAT testing will influence others, and that there will be a relatively low number of ASAT tests: around five, with a long tail in case of low probability, high impact scenarios. My prediction is lower than Louden's as I believe that other countries will follow the example of the US, either by declaration or de facto. As we have seen, the evolution of the debris population depends on the evolution of the relevant sources and sinks of debris, each of which in turn is driven by several uncertain parameters. Breaking the problem down along these components can make the task more tractable, and help us answer the key questions about the evolution of the population of space debris and the probability that a Kessler Syndrome will be triggered in the coming years. **Here are my forecasts on the** [**total quantity of space debris f**](https://www.metaculus.com/questions/13744/total-quantity-of-space-debris-by-year/)or 2025, 2030, and 2035**. (My forecasts are in orange, and the Metaculus community's are in green.)**    **My prediction**: I project an increasing debris population, at a reducing rate at each 5-year increment. This reduction in the rate of increase will be driven by post-mission disposal measures. Further, there is an increasingly long tail to the right of the distribution to account for the probability of a Kessler syndrome.