Space sustainability through atmosphere pollution? De-orbiting,atmosphere-blindness and planetary environmental injustice

Abstract

Space debris is a major issue for space safety as any collision of a space object with even a small piece of debris can have catastrophic consequences. The most recent dramatic increase of the number of satellites in Earth’s orbit is clearly exacerbating the risks. In this context there is a growing norm of disposal of orbital debris through atmospheric re-entry: space debris is to ‘burn up harmlessly in the atmosphere’, in order to provide for a ‘clean space’. Unfortunately, there is very little data on the environmental impact of deorbiting on atmospheric chemistry and in-situ data collection is practically impossible. The few existing studies – our own modelling included – agree that while the current impact of deorbiting is likely negligible, the projected exponential growth of satellites in LEO could exacerbate the risks. In consequence, space sustainability may come at the expense of damaging the health of the middle and upper atmosphere, with potentially unforeseeable consequences. Against this backdrop, we argue that in order to manage LEO sustainably, we must overcome this – what we call – ‘atmosphere-blindness’: our limited understanding of space-Earth system links through orbital disposal practices and their atmospheric impacts. While there is growing environmental consciousness with regard to outer space, we need to acknowledge that space sustainability is embedded in a wider context of outer space geopolitics, where the benefits and risks of the space infrastructure are distributed highly unequally. In our view it is thus crucially important to undertake more interdisciplinary research on the issue of de-orbiting, as it is not merely a technical environmental problem to be fixed but also an inherently political matter of planetary scale environmental justice.

Cleaning up space debris?

Space or orbital debris are ‘man-made objects, including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional’ (Space Safety Coalition, 2019: A-5). Travelling at extremely high velocities in Low Earth Orbit (LEO), space debris consisting of non-functional payloads, rockets bodies and fragments is a major issue for space safety as any collision of a space object with even a small fragment (of the size of 10 cm; cf. Esmiller et al., 2014) can have catastrophic consequences, possibly even an unstoppable ‘cascade of orbital debris that could potentially hinder humanity’s space ambitions and activities’ (Wall, 2022). While there is growing evidence (cf. Phipps and Bonnal, 2022) that this Kessler syndrome (Kessler, 1991) might already be underway, the recent boom of private sector space activity and the dramatic increase of the number of satellites in Earth’s orbit is clearly exacerbating the risks (Ziegler, 2023). In addition to space safety, space debris has negative impacts on ground-based astronomy (Barentine et al, 2023). This light pollution is caused by sunlight reflected from orbital objects, and amounts to around 10% of the natural sky brightness (Kocifaj et al., 2021). Space debris, or space junk, is a salient example for an Anthropocene problem: human-made, of planetary scale, and with non-linear and unforeseeable risks.

There are several methods for orbital debris removal, from the physical removal of selected space debris through, for example, robots to various systems that enable re-entry into the Earth’s lower atmosphere, such as thrusters or solar sails on new spacecraft (cf. Mark and Kamath, 2019). The ultimate goal is the disposal through atmospheric re-entry: space debris is to ‘burn up harmlessly in the atmosphere’ (European Space Agency [ESA], 2016) in order to provide for a ‘clean space’ (ESA, 2023a). The practice of de-orbiting is becoming a global norm for space sustainability. For instance, the US Federal Communications Commission (FCC), the governmental agency responsible for licensing satellite and space-based communications and activities in the US, adopted a new orbital debris rule in late 2022, demanding that satellites will soon have to be deorbited within 5 years of end-of-mission instead of the previously mandated 25 years (Federal Communications Commission, 2022). The latest release of the European Space Agency’s (ESA) Zero Debris Charter also features this 5-year-rule. Clearly, deorbiting is emerging as the main strategy to improve space safety and space sustainability.

The problem is that satellite deorbiting may have polluting effects on the middle atmosphere. There is very little data on the environmental impact of deorbiting on atmospheric chemistry and in-situ data collection is practically impossible. The few existing studies agree that while the current impact of deorbiting is likely negligible, the projected exponential growth of satellites in LEO (Ciocca et al., 2021) and the ensuing need to deorbit these objects could exacerbate the risks of damaging the ozone layer (Pultarova, 2021), as well as lead to a runaway, uncontrolled solar radiation management experiment on a planetary scale (Boley and Byers, 2021). While there are currently around 9000 satellites in orbit (ESA 2023b), the United Nations Office for Outer Space Affairs (UNOOSA, 2023a) reports that ‘35 per cent of all satellites launched over the past three years’ (p. 3) and that there is the potential of 100,000 satellites being launched over the next decade. The International Telecommunication Union, which is responsible for allocating radio frequencies to satellites, has registered around 1.75 million satellites for launches up to 2029, however (UNOOSA, 2023b: 5). If these concerns turn out to be true, improvements in space sustainability come at the expense of damaging the health of the middle (stratosphere and mesosphere) and upper (thermosphere) atmosphere, with potentially unforeseeable consequences. Alarmed scientists have thus already called for a precautionary approach, advocating for fewer launches of satellites with longer operational lifetimes (Gaston, 2023).

Against this backdrop, we argue that in order to manage LEO sustainably, we must overcome this – what we call – ‘atmosphere-blindness’: our limited understanding of space-Earth system links through orbital disposal practices and their atmospheric impacts. Many of the highly non-linear processes in the middle atmosphere are not or not as well-known as in the troposphere and stratosphere. For example, in the stratosphere NOx has a negative impact on ozone, that is, NO destroy the ozone (Crutzen, 1970; Johnston, 1971). In contrast, in the troposphere the response is positive (Chameides and Walker, 1973; Crutzen, 1973; Crutzen, 1974). Thus, the atmospheric photochemical system is in the mesosphere could respond strongly or weakly, positively or negatively, or have zero response to external forcing.

While there is growing environmental consciousness with regard to outer space (Lawrence et al., 2022; Newman and Williamson, 2018), we need to acknowledge that space sustainability is embedded in a wider context of outer space geopolitics, where the benefits and risks of the space infrastructure are distributed highly unequally. In our view it is thus crucially important to undertake more interdisciplinary research on the issue of de-orbiting, as it is not merely a technical environmental problem to be fixed but also an inherently political matter of planetary scale environmental justice.

Overcoming atmosphere-blindness

When a spacecraft re-enters Earth’s atmosphere, it breaks up and burns up completely or to a large degree with possible toxic, ozone-depleting and radiative forcing impacts (ESA, 2022). Larger pieces will eventually find their way back to the Earth’s surface, in the best case in a controlled and planned way. While there are several little researched environmental impacts here, such as the impact of toxic space debris on marine life in the deep sea (Rebay, 2022), we focus on the atmospheric impact of de-orbiting on the middle atmosphere. The middle atmosphere has remained curiously out of sight in space sustainability debates. ESA’s Space Environment Report 2023, for example, states that ‘Earth’s orbital environment is a finite resource’ (ESA, 2023c), and shows concern for the potential damage done by uncontrolled re-entry on the Earth’s surface. So, space sustainability here refers only to spacecraft in orbit and then on Earth’s surface again – what is happening in-between, in the atmosphere, is neither really seen nor known.

The European Space Agency seems aware of the problem, however, as it commissioned two studies in 2019 which focused on the ozone-depleting impact of spacecraft re-entries. Both studies find ‘that the atmospheric impact of spacecraft reentries is relatively low, there are still high-level uncertainties on aerothermodynamics and atmospheric chemistry-transport modelling and a lack of observational (in-situ) data to evaluate assumptions and models’ (ESA, 2022). Some other studies conclude that ‘[t]he predicted strong increase in anthropogenic injection [of deorbited space derbis] will make it significant in comparison to the natural injection [meteoroids] which can have yet unknown effects on Earth’s atmosphere and the terrestrial habitat’ (Schulz and Glassmeier, 2021), that satellites could soon become the dominant source of high-altitude alumina (Boley and Byers, 2021), and that ‘radiative forcing from space debris aluminium, interactions with high altitude cloud formation and possible consequences on measurement techniques of the middle atmosphere appear to be feasible consequences of space debris re-entries’ (Jain and Hastings, 2022). Recently, a study by Murphy et al. (2023) revealed that ‘[a]bout 10% of stratospheric sulfuric acid particles larger than 120 nm in diameter contain aluminium and other elements from spacecraft reentry’. Accordingly, there is a clear need for more simulation and measuring efforts.

Our own modelling ¹ undertaken at the Leibniz Institute of Atmospheric Physics in Rostock, Germany, together with OHB systems also finds that in the mesosphere, at altitudes of about 50–85 km, nitric oxide (NO) produced by the combustion of space debris objects has no significant effect on ozone, that is, the response is close to zero and may have either a positive or negative sign. The reason is that the lifetime of the odd-nitrogen family NO_x (N, NO, NO₂, NO₃) is only a few weeks and vertical mixing is hindered because the characteristic times of vertical turbulent and advective transport are of the order of a month or more. Nitric oxide released at altitudes above 55 km is thus more likely to be converted to other forms of odd-nitrogen (e.g. N), then to the less reactive N₂O, and then back to molecular nitrogen. Therefore, in this altitude region there are strong losses of NO_x, and even with an increase of NO from combustion, there is no accumulation of NO_x in the mesosphere, so that the effect is almost the same for all scenarios. These processes release one or more atoms of oxygen, however, so we can observe a slight positive effect on ozone in this region. Therefore, in order to minimise the effect of re-entry of space objects on stratospheric ozone, we find that their re-entry into the atmosphere should take place in such a way that burnup takes place above 55–60 km. It remains to be noted that current model estimates are based on present knowledge by chemical and physical simulations. Verification through long-term observations is pending and thus there needs to be an enhanced effort in developing an observational program for impacts beyond ozone-depletion, such as interactions with high altitude cloud formation and the potential for an uncontrolled solar radiation management.

Acknowledging planetary environmental injustice

De-orbital pollution will be the first anthropogenic material injection of significant scale into the middle atmosphere (Schulz and Glassmeier, 2021). While this layer of the atmosphere has been free from human interference for most of human history, this has changed in the planetary age of spaceflight. Crucially, however, it is not humanity as a whole that is responsible for the space debris problem and for de-orbital pollution but rather a small set of early space powers, consisting of developed industrial nations exclusively from the Global North. As of late 2022, according to the Union of Concerned Scientists (2023), 4529 out of around 6700 satellites belonged to the US. Of these almost 4000 are commercial, 260 governmental and 247 are from the US military. Overall, China comes second with 590 satellites in space and Russia third with 174. Almost 90% of all satellites are located in LEO. As Klinger (2019) noted, ‘[a]ccess to these [satellite] technologies is deeply uneven within and across countries, reflecting and retrenching existing geopolitical arrangements of power through the differential capacity to sense, monitor, and access information generated by space-based and space-linked technologies’ (p. 20).

Further building on Klinger’s work on the environmental geopolitics of outer space, we argue that de-orbital pollution is another manifestation of environmental injustice on a planetary scale. Environmental benefits of and responsibility for orbital satellite infrastructures and the environmental risks and vulnerabilities related to its pollution – whether in orbit as space debris or as burned-up traces in the middle atmosphere – are unequally distributed. It is due to the prevailing atmosphere-blindness that the environmental risks of de-orbiting are practically invisible while the environmental benefits of, first, large constellations of orbital satellites and, second, the space debris solution of de-orbiting seem so compelling at first sight. But as explained earlier, there are many unknowns about the precise environmental risks of de-orbiting pollution. The environmental and health impacts of a deterioration of the ozone layer for Earth’s biosphere are well known, as is its unequal impact on human and animal populations living in Southern hemisphere, and an uncontrolled solar geoengineering scenario would dramatically affect the living conditions of future generations.

Certainly, Earth observation satellites have provided many environmental benefits (Curnick et al., 2022) and have led to the development of forms of satellite-based activism for human rights causes, albeit within the confines of the ‘exclusive and closed nature of the satellite image complex’ (Rothe and Shim, 2018: 435). Rothe and Shim (2018: 436) wonder whether a ‘further opening and democratisation of satellite technologies (including the development of commercial micro-satellites)’ could provide for more counter-hegemonic potential. This democratising potential would also have to be weighed against the environmental trade-offs of orbital and atmospheric pollution, as ‘the polluting activity of one subset of users reduces accessibility for subsequent parties’ (Klinger, 2019: 21), in Earth’s orbit as well as the middle atmosphere.

Conclusion

Space sustainability is facing a dilemma: space debris is the most serious challenge to the future use of space, but the most promising counter-measure, deorbiting, may induce further risks into the Earth system through impacts on the middle and subsequently the whole atmosphere. We contend that the twofold ‘atmosphere blindness’ – in research as well as in the sustainability debate – needs to be overcome urgently. Further research is clearly needed, on atmospheric impacts as well as non-pollutant options, such as for example, the Japanese LignoSat probe which has been constructed from magnolia wood (McKie, 2024) as well as on the potential of in-space recycling, which tries to see space debris not as waste, but as a source of valuable raw materials (cf. Koch, 2021).

Crucially, however, the environmental impacts caused by US dominated space industries raise questions about the geopolitical context of this much-needed middle and upper atmospheric observational program: who will get to monitor whom and what, and who regulates whom? Do states continue to be in charge, as are they now according to in space law (Outer Space Treaty, 1966), and will they charge private space companies for the monitoring costs in a ‘polluter pays’-model (Adilov et al., 2023)?

In order to find sustainable and equitable ways of satellite use in orbit, we have to first acknowledge the geopolitical structures which underpin the current status quo arrangement and second work from there towards alternative, sustainable satellite uses on the basis of atmosphere-centred research and in-situ-measuring. But even then, there remain further political questions: once there is a clearer scientific picture about deorbiting risks through atmosphere pollution, who then – for example, nation states, space companies, the United Nations, new international institutions, or the scientific community – should have the authority to weigh up whether to proceed with current New Space Age developments and the ‘sustainability norm’ of deorbiting ‘for humanity’ (UNOOSA, 2023b)?