Envisioning a sustainable future for space launches: a review of current research and policy

ABSTRACT

The global space industry is growing rapidly, with an increasing number of annual rocket launches. Gases and particulates are emitted by rockets directly into the middle and upper atmosphere, where the protective ozone layer resides. These emissions have been shown to damage ozone – highlighting the need for proper management of the upper atmosphere environment. We summarise the emission byproducts from rocket launches and discuss their involvement in chemical and radiative processes in the stratosphere, along with potential implications for the ozone layer due to an anticipated increase in rocket launch emissions in the future. We then present a potential vision for sustainable launches, including tractable pathways for both the aerospace industry and the ozone research community. We canvass international and domestic environmental regulation to consider how existing frameworks might be applied to rocket launches. We further identify gaps in aerospace industry practice where cooperation with environmental management and atmospheric science fields could lead to best-practise outcomes.

The global space industry is expanding rapidly

Driven by increased demand for space-based technologies and services, the space industry has grown rapidly into the twenty-first century. Global annual launches ranged between 90–130 in the past 5 years (Figure 1). Satellite telecommunications, remote sensing applications, and other scientific and commercial enterprises have spurred the need for space launch providers and supportive infrastructure. Furthermore, private and public partnerships in space operations have allowed renewed investment and mitigated risk for those interested in developing launch capability. Financial estimates indicate the global space industry will grow to USD 3.7 trillion by 2040 (Sheetz 2017). Investment into fledgling space industries also indicates widespread space technology adoption for socio-economic and industrial benefits (European Space Policy Institute 2021). This international investment is complemented by heavy financial backing of space-based startups. In the USA alone, new space companies received USD 8.9 billion in private funding (CNBC News 2021). Here, the public–private cooperation is further exemplified by contracted resupply missions until 2024 to the International Space Station by commercial companies SpaceX, Orbital ATK and Sierra Nevada Corporation. The New Zealand space industry is growing similarly: estimates indicate that the domestic space economy will be worth almost USD 1 billion by 2025 and involve work from over 100,000 New Zealanders (Deloitte Access Economics 2019).

Figure 1.

Growth of the space industry in the past two decades as represented by number of rocket launches annually (black) and number of satellites launched annually (blue). Launches show a steady increase over time, while satellites show exponential increase due to many factors including miniaturisation of payloads and constellation planning.

As the industry expands, logistical improvements and lowered financial barriers will allow for a higher frequency of launches worldwide. Twenty-first century reusable rockets dramatically reduce launch costs and have lowered launch frequency from quarterly in the Shuttle era to sub-weekly (Jones 2018). Space tourism initiatives from three commercial companies (Virgin Galactic, Blue Origin and SpaceX) also suggest an upwards trend in global launch totals (Ryan et al. 2022). Additionally, the theorised use of suborbital flights for international travel and shipping would also undoubtedly increase launch demands. Most dramatically, future plans for mega-constellation buildouts – satellite fleets on the order of tens of thousands of similar units – also require accelerated launch cadences to meet both orbital placement and replenishment demands (Cates et al. 2018). To satiate these needs, both the expansion of existing spaceports and the development of new launch sites continues internationally (Roberts 2019). These sites are almost entirely located in the Northern Hemisphere, across mid and tropical latitudes (Figure 2). And while this may not always be the case, currently most spaceports globally do not regulate launch frequency. As more nations and commercial entities enter into the space industry, logistical and supply chain constraints may be reduced; rapid-cadence launch is already a direct focus of several major launch providers.

Figure 2.

Layers of the atmosphere. The centre map displays existing global launch sites, with the insert of New Zealand displaying the existing Māhia launch site and a proposed site at Kaitōrete Spit.

Rocket launches put gases and particulates in the stratosphere

The launch industry today relies on four major fuel types for current rocket propulsion: liquid kerosene, cryogenic, hypergolic and solid. The combustion of these propellants creates a suite of gaseous and particulate exhaust products, including (but not limited to) carbon dioxide, water vapour, black carbon, alumina, reactive chloride and nitrogen oxides (Dallas et al. 2020). Individual rockets may contain component stages that include various propellant types, which makes quantifying these emissions essential to understanding their environmental impact. The scale of this emission, however, is still relatively poorly understood. In-situ measurements of exhaust plumes are limited, and most current data rely heavily on plume modelling or best estimates from combustion calculations. Even the most ubiquitous fuel, liquid kerosene, is still relatively poorly modelled in exhaust concentrations (Sheaffer 2021).

Rocket launches are unique anthropogenic emission sources in that they inject gases and particulates into multiple layers of the atmosphere (Figure 2). In contrast, emissions of other anthropogenic gases and particulates are either removed in the troposphere (the lowermost region of Earth’s atmosphere), or, if sufficiently long-lived, reach the upper layers of the atmosphere via natural circulation. Some estimates indicate that two-thirds of total rocket launch emissions are injected above 15 kilometres (Ross et al. 2000; Ryan et al. 2022), which is the approximate tropopause height and lower boundary of the stratosphere. Rocket emission products are also similarly removed by natural atmospheric processes in the troposphere, with fuel ‘afterburning’ – the secondary combustion of exhaust plumes – further reducing the importance of tropospheric launch emissions (Ross and Sheaffer 2014). However, the unique environmental characteristics of the stratosphere mean that the same emission products can be longer-lived. The increased lifespan means that smaller concentrations of an exhaust byproduct can have greater destructive effects.

Emissions products from the four principal rocket propellants currently in use and their impacts on the stratosphere are outlined in Table 1. Estimates from current launch behaviour yields roughly 10 kt of CO₂, 6 kt of H₂O, 0.5 kt of chlorine, and 0.05 kt of NO_x gases yearly into the stratosphere (Brown et al. 2022). Alumina particulate emissions are approximated at almost 1 kt, and black carbon estimates range around 1 kt annually (Maloney et al. 2022).

Table 1.

Emissions products from the four principal rocket propellants, and their potential impacts on stratospheric ozone.

Emission product	Propellant type	Impacts on stratospheric ozone
Carbon dioxide (CO2)	Kerosenea, solidb, hypergolicc	Greenhouse gas; cools the stratosphere leading to increases in ozone (Rosenfield et al. 2002; Jonsson et al. 2004; Portmann et al. 2012; Ross and Sheaffer 2014; Dhomse et al. 2018)
Water vapour (H2O)	Kerosene, cryogenicd, solid, hypergolic	Greenhouse gas; breaks down in the upper stratosphere to produce ozone-depleting hydrogen oxides (Kirk-Davidoff et al. 1999; Dvortsov and Solomon 2001; Tian et al. 2009; Solomon et al. 2010)
Nitrogen oxides (NOx = NO + NO2)	Kerosene, cryogenic, solid, hypergolic	Ozone-depleting (Crutzen 1970; Popp et al. 2002; Ross et al. 2004; Larson et al. 2017)
Hydrogen chloride (HCl)	Solid	Converts rapidly to Cl2, which is ozone-depleting (Molina and Rowland 1974; Farman et al. 1985; Prather et al. 1990; Zittel 1994; Jackman et al. 1996; Molina et al. 1997; Solomon 1999; Ross et al. 2000; Yang and Brasseur 2001)
Black carbon (soot)	Kerosene, solid, hypergolic	Heats the stratosphere, accelerating gas-phase ozone loss reactions and shifting stratospheric dynamics; heterogeneous reactions occur on its surface as it accumulates sulfate coatings (Ross et al. 2010; Yu et al. 2019; Maloney et al. 2022; Solomon et al. 2022)
Alumina (Al2O3)	Solid	Heterogeneous reactions may occur on its surface, potentially enhancing ozone depletion (Spencer 1996; Molina et al. 1997; Jackman et al. 1998; Danilin et al. 2001a, 2001b; Schmid et al. 2003)
Hydrogen gas (H2)	Cryogenic	Produces H2O (Tromp et al. 2003; Vogel et al. 2011; Larson et al. 2017)

^aAlso called RP-1. Contains kerosene/LOx.

^bComposed of Al/NH₄ClO₄ and HTPB.

^cComposed of N₂H₄/UDMH and N₂H₄.

^dContains LH₂/LOx.

Not only do rocket launches create emission products during ascent, but the re-entry of spent vehicle material into the atmosphere creates potentially significant amounts of nitrogen oxide gases (NO_x = NO + NO₂). Estimates of the ablative NO_x burden from re-entry can total up to 17.5% of material mass below 50 km (Larson et al. 2017). Some studies estimate 100% of re-entry material mass translated to NO_x production at altitudes above 50 km (Ryan et al. 2022). However, these NO_x burdens are heavily dependent on re-entry orientation, velocity and geometry, and thus difficult to accurately quantify (S. Kim et al. 2019; S.-H. Park et al. 2021).

Rocket launches are potentially damaging to the ozone layer

The defining characteristic of the stratosphere, the region approximately 15–50 km above Earth’s surface, is the ozone layer. Almost all atmospheric ozone (approximately 90%) resides in the stratosphere. By absorbing shortwave and high-energy ultraviolet (UV) radiation, the ozone layer is integral to protecting life on Earth. It is also sensitive to human activities. Most notably, chlorofluorocarbons and halons emitted in the late twentieth century from their use in refrigeration, solvents and various other industrial activities, led to widespread ozone losses. By the early 1990s, global annual average ozone was 5% lower than the 1960–84 average (Salawitch et al. 2019). The most significant losses occur over the Antarctic during springtime (Farman et al. 1985), and are commonly known as the ‘ozone hole’.

The Montreal Protocol of 1987 and its later Amendments and Adjustments have been enormously successful in protecting the ozone layer. Early indications are that the Antarctic ozone hole is beginning to recover (Solomon et al. 2016). In contrast, if the Montreal Protocol had not been implemented, it is estimated that two-thirds of the ozone layer would have been destroyed by 2065 (Newman et al. 2009). The amount of damaging solar radiation reaching the surface would have more than doubled, with serious consequences for all ecosystems and life on Earth. In order for the gains of the Montreal Protocol to be sustained long-term, ongoing scrutiny of new activities that potentially impact the ozone layer is essential.

The gases and particulates emitted by rockets as they punch through the stratosphere contribute to climate change and ozone depletion. Greenhouse gas emissions from present-day rocket launches are insignificant compared to emissions from all other human activities, but could grow to equal the size of the aviation industry in coming decades (Ross and Sheaffer 2014). Many emissions products from rocket launches are ozone-depleting, and the threat to the ozone layer could be significant if launch activities increase in future (Brown et al. 2022). The lack of comprehensive in-situ emission measurements for modern launch vehicles, however, limits the predictive power of atmospheric modelling. In Table 1, we summarise how the main emission products from the four most commonly used rocket fuels most commonly in use influence stratospheric ozone. Further details are given in Box 1.

Box 1. Impacts of emission products on stratospheric ozone.

Hydrogen chloride (HCl). In the stratosphere, HCl rapidly converts to form chlorine (Cl₂), which in turn is photolysed (split by light) during daytime to produce reactive chlorine (Cl and ClO). These destroy ozone catalytically via gas-phase reaction cycles as shown below.

$$\begin{array}{c}\mathrm{C}\mathrm{l}+{\mathrm{O}}_3\to \mathrm{C}\mathrm{l}\mathrm{O}+{\mathrm{O}}_2\\ \mathrm{C}\mathrm{l}\mathrm{O}+\mathrm{O}\to \mathrm{C}\mathrm{l}+{\mathrm{O}}_2\\ \mathrm{N}\mathrm{e}\mathrm{t}:\mathrm{O}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2\end{array}$$

Cl₂ is also produced from CFCs and other chlorine-containing halocarbons following reactions on polar stratospheric clouds during the Antarctic and Arctic winters. Cl₂ accumulates until sunlight returns at the end of winter. Photolysis of Cl₂, followed by gas-phase reaction cycles, leads to widespread ozone losses, particularly over the Antarctic where it is colder (relative to the Arctic winter), thus polar stratospheric clouds form for longer. The seasonal losses observed over the Antarctic each spring are known as the ‘ozone hole’.

Nitrogen oxides (NO_x = NO + NO₂). Nitrogen oxides destroy ozone catalytically via gas-phase reaction cycles. Such reactions are not confined to the polar regions, and can take place at all latitudes, year-round.

$$\begin{array}{rl}& \mathrm{N}\mathrm{O}+{\mathrm{O}}_3\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2\\ & \mathrm{N}{\mathrm{O}}_2+\mathrm{O}\to \mathrm{N}\mathrm{O}+{\mathrm{O}}_2\\ & \mathrm{N}\mathrm{e}\mathrm{t}:\mathrm{O}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2\end{array}$$

Carbon dioxide (CO₂). Although CO₂ warms the troposphere, it cools the stratosphere. Many gas-phase reactions in the stratosphere are temperature dependent, including the Chapman reactions that govern natural ozone formation:

$$\begin{array}{rl}& {\mathrm{O}}_2+\mathrm{U}\mathrm{V}\to \mathrm{O}+\mathrm{O}\\ & \mathrm{O}+{\mathrm{O}}_2+\mathrm{M}\to {\mathrm{O}}_3+\mathrm{M}\left(\mathrm{R}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}1\right)\\ & {\mathrm{O}}_3+\mathrm{U}\mathrm{V}\to {\mathrm{O}}_2+\mathrm{O}\\ & \mathrm{O}+{\mathrm{O}}_3\to {\mathrm{O}}_2+{\mathrm{O}}_2\left(\mathrm{R}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}2\right)\end{array}$$

Here M is a third body that carries away excess energy in the collision; UV represents ultraviolet light (ie sunlight), indicating photolysis. Reactions 1 and 2 are both temperature dependent. Reaction 1, which produces ozone, gets faster as the stratosphere cools. Reaction 2, which destroys ozone, gets slower as the stratosphere cools. The net effect of stratospheric cooling on the Chapman reactions is to increase ozone abundances.

Water vapour (H₂O). Similar to CO₂, water vapour is a greenhouse gas and cools the stratosphere. In the upper stratosphere, H₂O reacts with excited-state atomic oxygen or is photolysed to produce the hydroxyl radical OH, which destroys ozone catalytically via gas-phase reaction cycles such as:

$$\begin{array}{rl}& \mathrm{O}\mathrm{H}+{\mathrm{O}}_3\to \mathrm{H}{\mathrm{O}}_2+{\mathrm{O}}_2\\ & \mathrm{H}{\mathrm{O}}_2+\mathrm{O}\to \mathrm{O}\mathrm{H}+{\mathrm{O}}_2\\ & \mathrm{N}\mathrm{e}\mathrm{t}:\mathrm{O}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2\end{array}$$

Ozone loss may be intensified via cooling of polar regions, which enhances polar stratospheric cloud formation and thereby chlorine-induced ozone loss.

Black Carbon (soot). The warming effect of black carbon particulates serves to speed up other ozone-depleting reactions and enhance their destructive properties. Long residence times and stratospheric injection (versus natural lofting from the troposphere) yield efficient warming behaviour by black carbon through radiative forcing.

Alumina (Al₂O₃): Alumina particulates enhance stratospheric ozone destruction by acting as a reaction surface for reservoir chlorine to convert to ozone-destroying Cl₂:

$$\mathrm{C}\mathrm{l}\mathrm{O}\mathrm{N}{\mathrm{O}}_2+\mathrm{H}\mathrm{C}\mathrm{l}\to \mathrm{H}\mathrm{N}{\mathrm{O}}_3+\mathrm{C}{\mathrm{l}}_2$$

The greatest contributions are from sub-micron sized alumina, which are more efficient at promoting heterogeneous atmospheric chemistry.

Stratospheric chemistry is highly nonlinear (Dvortsov and Solomon 2001; Shindell and Grewe 2002; Stenke and Grewe 2005; Vogel et al. 2011), so the magnitude of impacts described in Table 1 depends on the background atmospheric composition and relative concentrations of other species (Röckmann et al. 2004; Portmann et al. 2012; Revell et al. 2015; Butler et al. 2016). Not included in Table 1 is methane (CH₄), which is emerging as a new rocket fuel; many rocket engines currently in development have opted for methane fuel, including SpaceX’s Starship, Rocketlab’s Neutron, and the ESA Arianespace Ariane Next. As yet, the emissions products of methane fuel are poorly understood and not experimentally quantified. Nonetheless, the upcoming popularity of this fuel type could drastically change estimates of the launch industry’s atmospheric impact. Further efforts to produce theoretically ‘greener’ fuels for future launch vehicles will also alter the emission profile of global launches – highlighting the need for further study.

The upper atmosphere is an environment to manage

The aspirational outcomes of space industrialisation, with its inspiring potential for economic and scientific gain, lead to a particularly fragile situation for fair and frank discussion of its impacts. Rockets are a perfect example of ‘charismatic technology’ (Ames 2015) – where the promise of what the technology can enable invokes the imagination and drives deep emotional investment – extending far beyond the actual material reality of the objects themselves. In common with other charismatic technologies, the utopian promises of space industrialisation can lead to a sense of inevitability of their future use (Ames 2015). This narrative power can flatten the potential for nuanced discussion of how rockets can best be forged into the tools that we wish to have to achieve certain outcomes – from exploration throughout the Solar System to a thriving near-Earth space economy.

A sustainable approach to launch requires a systems-led approach that encompasses both the atmospheric and near-Earth environments. Sustainability in this sense is the ability to maintain a biosphere-protecting ozone layer, relative to anthropogenic forcings. As a planet-wide volume that is both travelled within and acted upon, and where the outcomes of humanity’s actions are inherently global in effect, the upper atmosphere shares a relative lack of recognition as a near-Earth spatial environment (Lawrence et al. 2022). However, this is shifting. For instance, anthropogenic space debris is a focus of space sustainability, and an area where both industry and policy approaches (eg at the United Nations Committee on the Peaceful Uses of Outer Space ) have converged on pragmatic outcomes (Palmroth et al. 2021). Similarly, launching through the atmosphere is action with purpose and intent. Thus, the aerospace industry needs to consider both atmospheric and near-Earth environments as a contiguous whole when quantifying impacts, eg in life cycle development analyses, and fully accounting for impacts both in launch and in demisability.

Developing sustainable approaches in launch management requires attention to the ‘complex, interconnected, finite’ nature of the system (Meadows 1982) that encompasses the upper atmosphere. Such relationship-focussed approaches to environmental management are a fundamental philosophical principle in kaitiakitanga (Kawharu, 2000; Jackson et al., 2017; Stewart 2020) and in other Indigenous knowledge bases (cf. Australian fire and landscape management; Russell-Smith et al. 2009). There is precedent in aerospace: for instance, traditional ecological knowledge techniques can influence near-Earth environment space traffic and debris management (Jah 2019, 2021), and are key to the kaupapa of the nascent Project Tāwhaki (Ministry of Business, Innovation and Employment (MBIE) 2022a, 2022b).

In keeping with the aspirational nature of its guiding ambitions, the aerospace industry has a responsibility to approach its development in a way that does not privilege the paths that historically led to negative outcomes: avoiding neocolonialism. This is by nature even more challenging for a charismatic technology, whose allure acts to reinforce perceptions of implicit rightness in the approach that is taken to create it (Ames 2015). Actions which lead to responsibility for Earth system behaviour are critical for allowing aerospace to persist on longer timescales than immediate shareholder return: ozone recovery and change timescales are much better matched to multi-decadal planning, and match typical iwi planning horizons. Indeed, the increased rate of launches are poised to shift the technological charisma of rockets toward ‘quantity as quality’ (Rao 2019). Externalisation of costs in environmental damage is a common pattern of global industrialisation. Fortunately, there are many practical approaches from other areas of environmental impact to ensure potential ozone destruction can be avoided in launch development.

What does a sustainable future for the rocket launch industry look like?

To achieve a sustainable rocket industry into the future, the complex system of contributing factors across launch providers, environmental regulation, and atmospheric research must move forward together. This includes contributions from a rocket launch industry where vehicles with quantified emissions launch at rates (and with trajectories) that create a known and measurable impact on stratospheric ozone. It also includes international agreements on levels of ozone effect from launches that both enable industrial-level access to near-Earth and interplanetary space, but also preserve the critical protection of our current biosphere.

Environmental assessments need to account for emergent behaviour, as is standard with other environmental issues. Current environmental impact statements and risk assessments do not include stratospheric or ozone effects of rocket launches (National Aeronautics and Space Administration 2015; National Institute of Water and Atmospheric Research 2016; Federal Aviation Administration 2017; National Institute of Water and Atmospheric Research 2017; SpaceX 2019). Furthermore, most impact statements are not peer-reviewed and do not require new research to be done during assessment (Ross and Vedda 2018). Launches call for a ‘careful experiment and constant monitoring’ (Meadows 1982), due to the overall system complexity: coherent local- and international-level approaches to environmental regulation must include aspects of air quality, ozone impact, and overall climate effect.

A strong vision of sustainability across the integrated industry must be followed by specific, tangible action and decision-making. Understanding the policy landscape related to environmental regulation allows for identification of legislative frameworks and gaps. This, in turn, is addressed by changes in the aerospace industry and ozone research community, both detailed below.

A gap in international and domestic approach: how does the launch industry relate to environmental regulation?

Given the current absence of regulation, there will come a time in which the legislative and business worlds intersect, and the need for prescriptive regulation of the launch industry outweighs any economic benefit (Ross and Vedda 2018). The sooner this inevitable future is understood, the sooner a solution space can expand within commercial long and mid-term strategic planning. The aviation industry is currently grappling with this realisation, causing deadlocks between the airlines, traditional fuel providers, and other policymakers (Dodd and Yengin 2021). Here, we discuss international and New Zealand-specific legislation relevant to rocket emissions.

From an environmental budgeting perspective, New Zealand’s Ministry for the Environment compiles national greenhouse gas inventories in fulfilment of various agreement requirements (Ministry of Business, Innovation, & Employment 2021). Domestic rocket launches and the NZ aerospace industry are specifically mentioned in the 2020 inventory in section 1.A.3.e under ‘Other transportation’ (Ministry for the Environment 2020). Atmospheric effects are not accounted for; rather, methodological issues of where potential greenhouse gases (GHG) contributions from the space industry should be accounted for are highlighted. The document reiterates the Intergovernmental Panel on Climate Change’s exclusion of aerospace and space activities in its guidelines for emission factors, type of transport, and unclear international-domestic definition. In theory, rocket emissions should be more straightforward to account for in a domestic emissions budget as launches occur in a single country (in contrast to the aviation industry’s emissions from international travel).

As well as contributing to climate change via increasing radiative forcing (Ross and Sheaffer 2014), rocket emissions contribute to stratospheric ozone losses. Stratospheric ozone is protected by two global treaties. The first, the Vienna Convention for the Protection of the Ozone Layer (1985), established a global framework for monitoring ozone depletion. It led to the second treaty, the Montreal Protocol on Substances that Deplete the Ozone Layer (1987) and later Amendments and Adjustments. The Montreal Protocol established legally binding controls on the production and consumption of halogen source gases containing chlorine and bromine known to cause ozone depletion (Salawitch et al. 2019). Note that it is the halogen source gases themselves that are controlled, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and the bromine-containing halons. In the stratosphere, halogen source gases break down and release ozone-destroying chlorine and bromine. While HCl emitted from solid rocket motor fuel into the stratosphere causes ozone depletion (Table 1; Box 1), it is not controlled by the Montreal Protocol as it is not considered a source gas; if emitted directly into the troposphere it would not enter the stratosphere as its tropospheric lifetime is not long enough (on the order of days; Crisp et al. 2014).

The Vienna Convention and Montreal Protocol are the only environmental treaties to be universally ratified. In New Zealand the Ministry for the Environment handles policy relating to both treaties (Ministry for the Environment 2022), while the Environmental Protection Authority permits the import and export of ozone depleting substances (Environmental Protection Authority 2022). Sustainability of launch activities is noted in MBIE's Space Policy Review Consultation (MBIE, 2022b).

What actions could the aerospace industry take?

Much of the foundation in understanding and addressing a sustainable launch future lies in directed action from the aerospace industry; though this need not be detrimental or prohibitive for rocket development and design. In fact, early adoption of testing measures for environmental purposes could avoid overly strict regulation in the future. We believe these forward-thinking measures might include:

• Quantifiable emissions at both design and testing stages for launch vehicles. In optimisation of fuel efficiency and design, propulsion teams might also consider estimating emission factors due to design choices (Sheaffer 2021). This is especially important for novel fuel types, where industry is leading the way and scientific literature is largely absent for specific emissions characterisation. These design choices could be backed up by real-world emission measurements at the test-stand level – an easy value-add given the comprehensive data collection which already occurs during this process.

• Collaboration with researchers for in-situ measurements. The scientific community has tools and experience sampling rocket emissions in the atmosphere, which have partially formed the basis of literature on this topic (Jackman et al. 1996; Ross et al. 2000; Popp et al. 2002). Advancements in direct sampling technology and targeted campaigns could give strong credence to predictive modelling and future study. Stratospheric plume measurements could also serve as experimental confirmation for vehicle exhaust burden (cf. opportunities for New Zealand given domestic capabilities for in-situ stratospheric sampling).

• Consideration of stratospheric effects in the operation of launches. With increased focus on rocket reusability, recovery and payload, design teams can assess ablative NO_x creation from boosters and payload elements during atmospheric re-entry (S.-H. Park et al. 2021). Flight profile teams may also minimise time spent under thrust in the stratosphere.

• Promotion and normalisation of emissions data availability. Encouraging minimisation of harmful emissions as a key part of best-practise development in the industry could serve as a powerful benchmarking tool (and marketing differentiator for private industry competitors). This could be further incentivised by aerospace stakeholders or funders, aided by policy decisions to ensure adoption. More than anything, simply making emissions data publicly available greatly aids contextualisation and understanding of atmospheric impact. This could be in database form or required as part of licensing processes – though it must go further than current environmental impact assessments.

• Coordination with the stratospheric modelling community. The ozone research community has comprehensive tools to model atmospheric impacts given rocket launch inputs. This may be used to project impacts of new vehicles as well as confirm ongoing launch programmes to evaluate sustainability. Integration with this community also ensures adjustments based on current environmental needs are met. Feedback from modelling can also be used again for future aerospace design and operation stages, creating an informed, closed-loop system. Data stewardship and accountability of this nature is already successfully occurring in European aerospace circles, focused on entire life-cycle analysis (LCA) of space operations (ESA LCA Working Group 2016; Maury et al. 2020).

What actions could the ozone research community take?

The best available models to assess impacts of rocket propellants on stratospheric ozone are those that couple atmospheric chemistry, dynamics and radiation, known as chemistry-climate models (CCMs). CCMs contain a dynamical core that simulates temperature, pressure and winds. The dynamical core is coupled to an atmospheric chemistry model, such that chemical processes change the composition of the atmosphere, which may then affect radiative heating and consequently dynamics and transport. Chemical processes themselves are influenced by dynamics, transport and temperature, hence CCMs are fully coupled models (Morgenstern et al. 2010, 2017). Because the processes driving changes in stratospheric chemistry and composition are nonlinear (Box 1), CCMs – or larger Earth System Models containing the core components of a CCM, such as the New Zealand Earth System Model developed via the Deep South National Science Challenge (Behrens et al. 2020) – are the best available tool to assess the combined effects of rocket emissions.

The most robust way to assess impacts of rocket propellants and future growth in the industry on stratospheric ozone is via a coordinated multi-model intercomparison effort. In this way, individual model biases can be accounted for in a like-for-like comparison. Historically, the CCM community has worked on coordinated experiments via the Chemistry-Climate Model Initiative (CCMI) (Eyring et al. 2013), and its predecessor CCMVal (Eyring et al. 2010), which is supported by the World Climate Research Programme’s SPARC (Stratosphere-troposphere Processes and their Role in Climate) and Future Earth’s IGAC (International Global Atmospheric Chemistry). Simulations performed for CCMI are used to inform UNEP/WMO Scientific Assessments of Ozone Depletion.

The 2018 Scientific Assessment of Ozone Depletion reviewed a sparse body of literature on rocket emissions and their impacts on stratospheric ozone (Carpenter et al. 2018). Only one new CCM study had been performed since the 2014 assessment that focussed on stratospheric ozone changes associated with rocket emissions (Larson et al. 2017). At the time of writing, the 2022 Scientific Assessment of Ozone Depletion is being finalised and new studies are coming to light (eg Brown et al. 2022; Maloney et al. 2022; Ryan et al. 2022). These are single-model studies, each making different assumptions about future growth in the rocket launch industry. To date, different CCMs have been used to address individual aspects of the rocket launch industry in different ways (eg making assumptions about future launch rate growth and emission profiles). Not only are the number of studies performed too small to gain a comprehensive picture of how the ozone layer might be affected by future rocket launches, but the diversity in simulation design and model architecture makes it impossible to differentiate model versus scenario uncertainty.

Running future rocket emissions simulations with multiple CCMs would provide extensive value in assessing the safe operating space for the launch industry. To enable this, we believe the following needs to happen:

• Rocket launch operators would need to quantify their emissions products emplaced in the stratosphere, such that accurate emissions inventories can be compiled.

• CCMs would need to be configured to handle all emissions products and mechanisms (Table 1). For example, alumina is often routinely excluded in CCMs, even though it is an important emission product from solid rocket motor fuel. Similarly, CCMs would also require accounting of plume dispersion and high-concentration reactions for launches. Current treatment of plumes as point injection and scale mismatch introduce uncertainty via a lack of comprehensive impact for these launch events.

• Future launch scenarios would need to be constructed against the backdrop of changing atmospheric composition, given that greenhouse gas concentrations are projected to increase in future under most scenarios (Meinshausen et al. 2020) and stratospheric chlorine and bromine are projected to decrease. Nonlinearities in stratospheric chemistry mean that the rates at which certain species cause ozone destruction can be compounded or ameliorated depending on the concentration of other species (Revell et al. 2015). Similarly, focussing on all emissions products jointly will likely yield more realistic results than focussing on a single product in isolation.

• Contact points for industry could be established for those interested in further conversation, for example on potential effects of new propellant types. Similar structures have been used in the past for addressing CFCs – and with a current relatively small number of launch operators, promoting mutual contact points and research connections is still quite viable.

Conclusions

Rocket launches are shown to introduce gases and particulates into the stratosphere, where they are able to efficiently destroy ozone. Reactive chlorine, black carbon, and nitrogen oxides (among other species) are all emitted by contemporary rockets. As the global space industry expands rapidly, the destructive impact of these launches will grow larger. Furthermore, current gaps in policy from both the aerospace and environmental perspectives reinforce that greater consideration and quantification of these issues is paramount. Addressing stratospheric issues created by the rocket launch industry could benefit from a perspective shift: one focussed on treatment of the upper atmosphere as a managed environment. A more holistic analysis of emission, transit and deposition of foreign gases and particulates in this environment, with emphasis on source accountability and mitigation, could scale well as the aerospace industry expands. Environmental management at this level is not without precedent, especially in New Zealand spheres of partnership and regulation: New Zealand is well suited at policy- and thought-leadership in environmental management given the emphasis through kaitiakitanga.

Increased interest and funding through international agencies gives opportunity for forward-thinking behaviour in confronting stratospheric impacts of rocket launch emissions. A sustainable vision for the industry requires action, but does not have to be burdensome to have a positive impact – additions to existing design, testing, and life-cycle analysis regimes yields comparatively large benefits. Environmental impact reports might also adopt more rigour in their assessment of atmospheric impacts, just as they treat other ground-based effects. Quantification of these emissions can allow for increased cooperation, integration and study by the environmental modelling community. The ozone research community is well equipped to understand and give recommendations to these effects, and has existing frameworks to help develop sensible and non-restrictive regulation. Launch impact emissions present a ‘wicked problem’ for the world – a problem in which many changing, interdependent factors contribute. In turn, the mitigation of these effects must also be suitably complex, involving international cooperation across public and private entities, industries and research communities.

Disclosure statement

No potential conflict of interest was reported by the author(s).