This post draws on the work of the International Institute for Space Law (IISL) Working Group on Pluralistic Governance and International Space Law (Chair, Elena Cirkovic).

The 2024–2025 climate advisory opinions confirm that neither specialised treaties nor voluntary frameworks can displace customary environmental obligations or operate in isolation from general international law. The opinions reinforce the systemic integration mandated by Article 31(3)(c) VCLT. Building on this reasoning, we apply this systemic integration to outer space treaty interpretation in light of the opinions.

The ITLOS Advisory Opinion on Marine Pollution from Greenhouse Gas Emissions (21 May 2024) concluded that “the Paris Agreement is not lex specialis to the Convention” and that compliance with the Paris Agreement does not exhaust states’ obligations under UNCLOS (paras 223–224). The ICJ Advisory Opinion on State Obligations in Respect of Climate Change (23 July 2025) stated that “the principle of lex specialis does not lead to a general exclusion by the climate change treaties of other rules of international law” (paras 162–173). The IACtHR Advisory Opinion on Climate Emergency and Human Rights (29 May 2025) concluded that preventing irreversible environmental harm constitutes jus cogens (para 269).

Article III of the Outer Space Treaty (OST; adopted 27 January 1967, entered into force 10 October 1967) contains a renvoi to international law. The provision incorporates general international law by reference. Because the reference is unqualified, it encompasses international law as it develops over time, including the customary environmental obligations confirmed by the 2024–2025 advisory opinions. Article III also excludes any reading of the OST as a self-contained regime. Article IX imposes specific obligations: due regard, consultation, harmful contamination avoidance, and prevention of adverse changes to Earth’s environment. These do not exhaust the applicable environmental law. Article III provides the basis for supplementing them with general international environmental obligations.

The reasoning of the advisory opinions provides a counterargument to several narratives prevalent in the outer space sector: that international space law is insufficient to address environmental and climate issues; that existing treaties are being superseded by instruments such as the Artemis Accords; or that the Lotus principle provides legal grounding for commercial exploration, exploitation, and appropriation of extraterrestrial resources. Some interpretations invoke the Lotus principle alongside OST Article I’s freedom of exploration to argue that Article II’s non-appropriation rule prohibits territorial sovereignty but not ownership of extracted materials. National legislation reflects this interpretation: for example, the U.S. Commercial Space Launch Competitiveness Act (2015), the Luxembourg Law on the Exploration and Use of Space Resources (2017), the UAE Federal Law No. 12 on the Regulation of the Space Sector (2019), and the Japan Act on the Promotion of Business Activities Related to the Exploration and Development of Space Resources (2021). Critics counter that the approach treats space as res nullius rather than res communis omnium, potentially undermining Article I’s common benefit principle.

Against this background, we present the available science on atmospheric impacts from spacecraft re-entry, establishing the factual basis for harm prevention obligations under Article IX OST and customary international law. We then demonstrate how the existing treaty structure incorporates evolving customary environmental law through systemic integration; analyse the due diligence standards, precautionary approaches, and environmental impact assessment obligations clarified by the advisory opinions; and finally, address how these principles challenge political coordination frameworks for outer space activities that lack substantive environmental constraints.

Outer Space Activities and the Earth’s Atmosphere

Recent research demonstrates measurable atmospheric impacts from spacecraft re-entry, with considerable increase over the past five years. In 2019, anthropogenic mass influx reached 2.8% of natural meteoric input, projected to increase with large satellite constellation deployment. A 2025 reassessment revealed space debris mass influx had reached 7% of meteoroids, with metals at 14%. In 2024, anthropogenic aluminium influx exceeded several hundred tonnes—more than double the average injection in 2015–2020 and nearly three times meteoric input. Aircraft-based measurements confirm that a measurable fraction of stratospheric sulfuric acid particles contain aluminium and other elements from spacecraft re-entry.

In 2024, approximately 1,600 metric tons of human-made orbital objects re-entered the atmosphere, with a considerable share ablating and fragmenting at high altitudes. Falcon 9 Heavy upper stages contributed 23% of the mass; Starship, 7.5%. Starlink satellites alone contributed more than 200 metric tons of re-entered material between 2020 and 2024, half in 2024 alone. Large constellation satellite re-entries now account for a significant share of anthropogenic aluminium emissions. Anthropogenic aluminium is now the dominant source to the middle and upper atmosphere, exceeding natural meteoroid contribution severalfold. Anthropogenic inputs have already been detected in stratospheric aerosols, raising concerns about effects on ozone chemistry, radiative balance, and climate. Uncertainties remain concerning partitioning between vaporised and condensed material during re-entry, formation and size distribution of sub-micron particles, their chemical and optical properties, and their residence time and global transport.

Aluminium oxide and aluminium hydroxide species, together with nitrogen oxide emissions, participate in heterogeneous reactions that enhance ozone depletion. Metals released during spacecraft vaporisation can become incorporated into the stratospheric aerosol population, modifying particle composition and surface area and potentially influencing polar stratospheric cloud formation.

We argue that the documented atmospheric transformation establishes grounds for interpreting Article IX through contemporary environmental law. Article IX—drafted with planetary protection in mind—addresses “adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter”. And further, systemic integration with evolving customary environmental law supports extending the provision to cover terrestrial environmental harm from re-entry ablation, given the common atmospheric medium affected.

Article IX is silent on the threshold of harm required for “harmful contamination”. Contemporary jurisprudence addresses the gap by linking harm thresholds to the significance of environmental impact rather than to irreversibility. In Pulp Mills, the ICJ recognised that even probabilistic or low-certainty risk may engage the duty of prevention. The ICJ’s 2025 Advisory Opinion reaffirmed the position (paras 274, 229, 75). Atmospheric alterations from re-entry ablation need not be proven catastrophic or irreversible to fall within Article IX. Measurable alteration of stratospheric composition, regardless of short-term severity, meets the significant risk standard sufficient to activate due diligence duties. This approach accords with ITLOS’s view expressed in its 2024 advisory opinion that necessary measures “should be determined objectively, taking into account, inter alia, the best available science” (para 243).

The Outer Space Treaty and Systemic Integration

Article III OST contains a renvoi to international law (“in accordance with international law, including the Charter of the United Nations”). Article 31(3)(c) VCLT requires consideration of relevant rules of international law applicable between the parties, treating treaties as instruments within the prevailing legal system at the time of interpretation. ITLOS endorsed the principle: treaties are to be “interpreted and applied within the framework of the entire legal system prevailing at the time of the interpretation” (ITLOS 2024: para 135, citing the ICJ’s Namibia Advisory Opinion, para 53).

Article I’s object and purpose—that space activities benefit all countries as “the province of all mankind”—supports interpreting the OST to prevent environmental harm to global commons. The purposive framework enables interpretation through customary environmental law regardless of participation in specific environmental treaties.

Due Diligence, Precaution, and Environmental Impact Assessment

The three courts emphasised that customary harm prevention obligations exist independently of treaty commitments. The advisory opinions converge on the role of science, the standard of due diligence, and the duty to cooperate. The ICJ found that the duty to prevent significant environmental harm applies to anthropogenic emissions, with due diligence as the required standard of conduct (paras 140, 273).

The ICJ observed that “where there are plausible indications of potential risks, a State would not meet its obligation of due diligence if it disregarded those risks” and that “the precautionary approach is an integral part of the general obligation of due diligence” (para 294).

The documented atmospheric effects constitute “plausible indications of potential risks” within the framework. Research demonstrates aluminium oxide accumulation at rates projected to reach 90% metal injection and 94% aerosol injection under worst-case megaconstellation scenarios, with potential ozone depletion in the upper stratosphere and polar mesosphere. Risk assessment objectively determines both probability and severity of potential harm (ICJ 2025: para 275, citing ITLOS 2024: para 239). The ICJ confirmed that “the standard of conduct evolves over time in light of new scientific or technological knowledge” (para 284).

The advisory opinions further confirm that environmental impact assessment (EIA) has crystallised as a customary procedural obligation. The ICJ affirmed that any activity posing risk of significant transboundary harm triggers an EIA duty. Space operations—especially large-scale constellation deployments, frequent re-entries, and launch activities—fall within this category by virtue of their global atmospheric effects (ICJ 2025: paras 295–298). States authorising non-governmental space operators under Article VI OST cannot rely on domestic licensing frameworks that omit EIAs. States are required to ensure ex ante assessment, ongoing monitoring, and adaptive management. Failure to do so may constitute an internationally wrongful act attributable to the state.

The Artemis Accords

The Artemis Accords, launched in 2020, establish non-binding principles for civil space exploration. As of November 2025, 60 states have signed. Section 10.2 provides: “The Signatories affirm that the extraction of space resources does not inherently constitute national appropriation under Article II of the Outer Space Treaty.” The Artemis Accords establish procedural coordination mechanisms—transparency requirements, registration, and “safety zones” for deconfliction—but not substantive prohibitions grounded in precautionary assessment of systemic environmental risk.

The Accords are non-binding political commitments, designed to avoid treaty ratification while establishing operational principles for the Artemis Program. Section 12 commits signatories to “limit, to the extent practicable, the generation of new, long-lived harmful debris”—a qualification permitting economic or operational considerations to override debris limitation. Section 11’s “safety zones” framework addresses harmful interference between specific operations but not cumulative systemic effects.

The ICJ’s reasoning on UNCLOS applies by analogy to the OST. The Court stated that UNCLOS “forms part of the most directly relevant applicable law” for climate obligations (ICJ 2025: para 124), confirming that specialised treaty frameworks operate within, not apart from, general international law. The same logic governs outer space: the OST, read through systemic integration, incorporates customary environmental obligations that non-binding instruments cannot displace.

By asserting consistency with the OST, signatories to the Artemis Accords may contribute to state practice. Whether the assertions generate opinio juris remains contested given the Accords’ explicit non-binding character. If states begin to treat Artemis principles as legally required rather than merely permitted, the principles could eventually harden into customary international law—but the current framework lacks the substantive environmental constraints that binding custom would require.

Conclusions

The 2024–2025 climate advisory opinions confirm that specialised treaty regimes and non-binding frameworks cannot displace customary international law. Systemic integration under Article 31(3)(c) VCLT requires the OST to be interpreted within the contemporary international legal order. Articles III and IX OST therefore incorporate evolving customary environmental law, including the duty to prevent significant environmental harm.

Documented anthropogenic alteration of the stratosphere from spacecraft re-entry constitutes potential risk sufficient to trigger due diligence and precaution, irrespective of irreversibility or extensive scientific certainty. Environmental impact assessment has crystallised as a customary procedural obligation where activities pose risk of significant adverse effects beyond the acting state’s jurisdiction, including large-scale space operations. States authorising private space activities remain internationally responsible under Article VI OST for failure to implement the duties.

Non-binding instruments such as the Artemis Accords cannot derogate from the obligations and risk inconsistency where they lack substantive environmental constraints. Outer space governance is already legally constrained by binding customary environmental law applicable to the Earth’s atmosphere as a shared natural resource.