2. A legal definition of space debris

Definitions of space debris have been adopted in a number of instruments. The ILA Draft Convention focuses on the fact that space debris is not an ‘active or otherwise useful’ man-made object,Footnote ¹⁵ while the IADC Guidelines focus on lack of functionality, which is defined as the ability of a spacecraft to ‘fulfil its intended mission’.Footnote ¹⁶ Using the IADC Space Debris Mitigation Guidelines as a starting point, the UNCOPUOS Scientific and Technical Subcommittee drafted its Debris Mitigation Guidelines and adopted the IADC debris definition focusing on the lack of functionality. Although the function of the UNCOPUOS definition is to limit the scope of application of the Guidelines, it is likely to have an impact on future international practice and to become a benchmark in this domain. Indeed, with resolution 62/217 the UN General Assembly endorsed the UNCOPUOS Guidelines,Footnote ¹⁷ agreeing that they reflected existing practices as developed by a number of national and international organizations and invited member states to further implement them through relevant national mechanisms.Footnote ¹⁸

The UNCOPUOS definition is, on the one hand, narrower and on the other hand, broader than that in the IADC Guidelines. It is narrower because it generally states that space debris is non-functional without referring to the inability of the object to fulfil ‘its intended mission’.Footnote ¹⁹ This is worth noting because it is not uncommon for a satellite to be repurposed after having surpassed its estimated useful life or after a technical anomaly renders it unfit for its primary function. On the other hand, the UNCOPUOS definition could be deemed broader because it solely refers to the actual status of non-functionality, without considering that a space object can stay in standby mode for a long time awaiting possible re-activation. By contrast, both the ILA Draft and the IADC Guidelines positively consider this eventuality and specify that an object is not regarded as space debris if reactivation is possible.Footnote ²⁰ To avoid absurd conclusions – for instance that a temporary glitch in functionality downgrades a space object to space debris – the UNCOPUOS definition should be interpreted consistently with these guidelines. In summary, in order to account for the eventualities of temporary inactivity and repurposing, a space object should be considered space debris as soon as it is no longer of practical use and a change to this status cannot be reasonably expected in the foreseeable future. No doubt, as long as these requirements are met even an intact satellite can be classified as space debris, notwithstanding the etymological origin of this term in the French word ‘débriser’ (to break up).Footnote ²¹

One might argue that dwelling on a definition of space debris is a mere academic exercise because whether a particular ‘space object’ is functional or conversely ‘space debris’ ultimately depends on an assessment by the operating state. However, operating states do not enjoy an unlimited margin of discretion in this regard and have to apply the definition provided by the Guidelines in good faith, especially in cases of a collision between a functional and a non-functional object.Footnote ²² Active satellites and manned space stations arguably take precedence over debris in cases of collision. Article I OST can, in fact, be interpreted as recognizing states’ freedom to use outer space only to the extent that a space activity is aimed at and sustained by a purpose of exploration or use. Considering that space debris is by definition non-functional, it increases the risk of collisions for no justifiable reason and poses a threat to the exploration and use of outer space by other states without serving anybody’s interests.Footnote ²³ The ITU regime on the management of radio frequencies is governed by the same principles. Protection of assigned frequencies and orbital positions is contingent on their actual and continued use.Footnote ²⁴ Therefore, in cases of collision the fact that one of the two objects is a piece of space debris should be considered when assessing the fault requirement for liability to arise under the terms of Article III of the Liability Convention.Footnote ²⁵ Fault by the launching state of the non-functional object should be presumed. This conclusion is further supported by the IADC and UNCOPUOS Guidelines, which warn against the dangers of inactive space objects and recommend their removal.Footnote ²⁶

The Registration Convention requires a launching state to provide information on the ‘general function’ of a space object and its orbital parameters to the UN Secretary-General.Footnote ²⁷ Nevertheless, the lack of specific data on the functional status of space objects, the vague requirement to provide registration information ‘as soon as practicable’ and to discretionally update it ‘from time to time’ and the failure to properly empower the UN Secretary-General to demand updates to this effect render the Registration Convention mostly ineffective in the fight against debris.Footnote ²⁸ Several instruments coherently emphasize the need to enhance the practice of registering space objects. In particular, GA Res 62/101 usefully recommends providing the Secretary-General with more specific information, including ‘any change of status in operations (inter alia, when a space object is no longer functional)’ and ‘the approximate date of decay or re-entry, if States are capable of verifying that information’.Footnote ²⁹ The legal framework in place is more stringent for satellites in geostationary orbit. For these, the ITU registration procedure contains a number of useful elements for an objective functionality assessment. Article 11 of the Radio Regulations requires that the ‘date of bringing into use’ and the nature of service of a satellite system be clearly marked in the registration notice. Moreover, any alteration of the basic characteristics of a registered satellite must be notified to the Bureau if the frequency assignment is to retain its original date of entry and the corresponding level of international protection. Therefore, any subjective assessment by the registering state might be countered by an objective assessment by the ITU.

Indeed, in most cases the Bureau relies on information provided by the registering state to periodically review its master register and the registering state may not be likely to admit that one of its satellite systems is non-functional. However, according to Article 13.6 of the Radio Regulations, the Bureau may also initiate inquiries ‘whenever it appears from reliable information available’ that a frequency assignment is not being used. If it is found that the frequency assignment does not correspond to an operating satellite, the assignment will be deleted from the register. Removal from the register of a previously recorded assignment should trigger a presumption that the corresponding satellite is no longer considered valuable by the registering state, as harmful interference might impair its functioning without any protection by the ITU.

Therefore, mandatory provision of information to the ITU would be helpful in establishing the debris status of inactive satellites in geostationary orbit and provide the basis for further action. Admittedly, such a suspicion of non-functionality rests on the presupposition that the relevant satellite was actually registered in the first place, which is not a legal requirement for launching a space object but a technical requirement for obtaining international protection from harmful interference by other users. Nevertheless, whether or not registration has taken place or been withdrawn seems worthy of consideration.

3. The liability approach

The doctrinal debate on space debris often develops from a liability standpoint and hence from the question of whether space debris falls within the definition of ‘space object’ or ‘component parts’ so as to trigger the application of Article VII OST and the Liability Convention in cases of damage.Footnote ³⁰ However, focusing on the liability regime seems neither sufficient nor opportuneFootnote ³¹ – not because space debris cannot be included in the notion of ‘space object’Footnote ³² but instead because of other legal limitations inherent in the liability approach and practical considerations. First of all, and this is the legal limit, the very notion of damage in Article VII OST and the Liability Convention is limited to damage to a state party or private persons.Footnote ³³ Damage to the environment would be included on the sole condition that a state or a person was affected – a hypothesis which is negligible in outer space.Footnote ³⁴ However, even from a practical point of view a liability approach causes some perplexity. Indeed, a key preoccupation driving the debate on space debris is to protect the fragile space environment against potentially irreversible harm, not to compensate victims of personal injury or property damage. Furthermore, in most cases it is impossible to trace a fragment back to the object from which it originated and therefore to identify the state liable.Footnote ³⁵ Establishing an ‘International Outer Space Clean-Up Fund’ aimed at providing compensation for damage caused by unidentifiable space debris has so far remained nothing but a doctrinal proposal.Footnote ³⁶ Therefore, the high road to take is arguably not that of liability for damage but that of prevention of space debris, focusing on the nexus between preventive obligations in the conduct of space activities and responsibility. This does not mean that the liability regime in this sector can be ignored, of course. Quite the contrary, it constitutes a deterrent to space debris proliferation and is arguably the main approach to take in cases of collisions between identifiable space objects.Footnote ³⁷

4. The duty to avoid harmful contamination

In its opening sentence Article IX OST specifies that states’ space activities are to be guided by the principle of co-operation and mutual assistance and conducted ‘with due regard’ to the corresponding interests of all other states parties.Footnote ³⁸ It then continues in a symmetrical fashion, referring to the terrestrial environment, which must be protected from any ‘adverse changes’ resulting from the introduction of ‘extra-terrestrial’ matter, on the one hand, and to the possible ‘harmful contamination’ of outer space and celestial bodies arguably deriving from terrestrial matter, on the other.

The admonition against adverse changes to the terrestrial environment refers to so-called ‘back contamination’ and is not applicable to debris. It is true that debris might fall to Earth and contaminate the terrestrial environmentFootnote ³⁹ but debris is, by definition, of human not extra-terrestrial origin. What is relevant to our reasoning is instead the other part of the rule, which requires states ‘to avoid … harmful contamination’ of outer space and celestial bodies (this is so-called ‘forward contamination’) and to ‘adopt appropriate measures for this purpose’. According to the wording of Article IX, these obligations apply solely to the performance of studies and the conduct of exploration without mentioning the use of outer space. However, a combined reading with the first part of Article IX, where the due regard rule is set out, leads to the applicability of the obligations ‘to avoid their harmful contamination’ and to ‘adopt appropriate measures for this purpose’ even to the use of outer space.Footnote ⁴⁰ Indeed, the due regard rule refers to both ‘exploration’ and ‘use’ and the latter obligations are nothing but a specification of the due regard rule.Footnote ⁴¹

As far as the notion of ‘harmful contamination’ is concerned, two paths of reasoning are available. One might argue that any contamination is per se harmful and should be avoided insofar as it implies by definition a modification of the environment through human agency. On the other hand, and this is the second path, one might emphasize that Article IX OST does not prohibit any kind of contamination unless it can be considered ‘harmful’.Footnote ⁴²

This is the preferable interpretation. Indeed, if any contamination were harmful the use of this adjective in Article IX would be redundant. The French version of the OST points to ‘les effets préjudiciables de leur contamination’, and therefore assumes more explicitly than the English version that contamination per se is not harmful. This interpretation can also be confirmed by a combined reading of Article IX with Article I, which declares that outer space and celestial bodies are free to be explored and used. If contamination were generally harmful, taking into consideration that space activities inevitably result in debris and hence contamination, the only viable way of avoiding it would be to ban space activities tout court. However, this would be a paradoxical conclusion as the purpose of the Outer Space Treaty is to promote the exploration and use of outer space, not to restrict them.Footnote ⁴³

Article IX OST seems to be nothing but the expression and application to outer space of the international rule on the prohibition of transboundary harmFootnote ⁴⁴ and as such should be interpreted accordingly in virtue of Article 31, paragraph 3 of the VCLT.Footnote ⁴⁵ Indeed, the so-called no harm rule also applies if places beyond the national jurisdiction are affected, including the high seas, Antarctica and indeed outer space. That the territory of another state does not need to be involved was specifically clarified by the ICJ in its Nuclear Weapons Advisory Opinion,Footnote ⁴⁶ shifting the focus of international protection from the territorial integrity of a particular state to the protection of the environment per se.

Just like the general no harm rule, Article IX OST also seems to be interpreted as imposing on states not an absolute obligation of result but one of diligent conduct.Footnote ⁴⁷ Therefore, as long as states adopt appropriate measures to avoid harmful consequences of their space activities they will be released from any responsibility even if harm is caused.Footnote ⁴⁸ This is, of course, without prejudice to liability for damage eventually caused to other states if the requirements laid down in Article VII OST are met.

5. Standards for assessing harmfulness

Assuming that the harmfulness of a certain activity must be measured by its potential contribution to the accumulation of debris, the issue is to establish the level beyond which contamination becomes harmful/préjudiciable. In this regard, a joint reading of the avoid harmful contamination rule and the due regard principle in Article IX suggests that a space activity produces harmful contamination when it disrespects the interests of other states and undermines their freedom to explore and use outer space.Footnote ⁴⁹ A space activity which significantly increases the risk of collision and thus of space debris in a crowded orbit, for instance, hampers the freedom of other states to explore and use outer space and causes harmful contamination. There is arguably an intimate connection between harmful contamination of outer space and potential harmful interference with present and future space activities of other states, in that potential harmful interference with space activities can be used as a parameter to assess whether a certain type of contamination reaches the threshold of harmfulness and must therefore be avoided. It is not a coincidence that the drafters of the OST decided to address in the same Article IX both the issue of harmful contamination of outer space and that of harmful interference with other space activities.Footnote ⁵⁰

The UNCOPUOS Guidelines play a fundamental role in harmfulness assessment. They interact with the binding rules in Article IX in that, although not compulsory in themselves, they complement and complete the general obligations it contains. Having been adopted by consensus both by the UNCOPUOS and the UN General Assembly, the UNCOPUOS Guidelines encompass a common understanding by all the parties to the OST as to their proper interpretation. They can be considered ‘subsequent practice’ of the state parties according to Article 31(3) of the Vienna Convention and can, as such, provide elements to be taken into account in the specification of the appropriateness of the measures to adopt. In more concrete terms, they offer the standards of due diligence against which a state’s conduct should be assessed.Footnote ⁵¹

Due to the fact that any introduction of matter into outer space increases collision risk and so threatens the interests of other states, new matter should be limited to what is strictly necessary for the purpose of its introduction.Footnote ⁵² Article 44 of the ITU Convention accordingly defines radio frequencies and any associated orbital positions, including the geostationary satellite orbit, as ‘limited natural resources’ and imposes on member states the obligation to use them rationally, efficiently and economically.

Given that the consequences of a single launch in terms of its contribution to increasing the amount of debris are not foreseeable with absolute certainty, the precautionary principle assumes a prominent role in determining what standards and regulations states must adopt to prevent harmful contamination.Footnote ⁵³ Coupled with the due diligence requirement, the precautionary principle implies that a state undertaking a space activity must adopt the best available practices and standards.Footnote ⁵⁴ These are arguably the only ‘appropriate’ measures for minimizing, if not avoiding, harmful contamination.Footnote ⁵⁵

While the standards to adopt and the degree of appropriateness evolve in step with technology,Footnote ⁵⁶ they also depend on the capability of the state concerned, on the overall state of the orbit in which the activity is carried out and, in general, on the level of risk involved in a specific operation.Footnote ⁵⁷ Therefore, standards are characterized by a certain degree of dynamism, but this does not affect the verifiability of the degree of appropriateness in a particular context on a case-by-case basis. Moreover, states do not have complete discretion in defining the ‘measures’ to adopt. Indeed, the duty to adopt appropriate measures in Article IX OST is substantiated in further duties, namely the duties to carry out an environmental impact assessment of the planned space activity, to notify and to consult.

6. Environmental impact assessment

The consistent jurisprudence of the ICJ leaves no doubt as to the customary status of the obligation to perform an environmental impact assessment (EIA) before commencing activities that may have a significant adverse impact in a transboundary context. This was declared in the Pulp Mills caseFootnote ⁵⁸ and confirmed in the joint cases Certain Activities Carried Out by Nicaragua in the Border Area and Construction of a Road in Costa Rica along the San Juan River.Footnote ⁵⁹ In the latter case, the obligation to perform an EIA was invoked by both parties. According to Costa Rica, Nicaragua should have conducted such an assessment before carrying out dredging worksFootnote ⁶⁰ whereas according to Nicaragua, Costa Rica breached the same obligation in relation to the construction of the road.Footnote ⁶¹

The fact that most judicial decisions relate to the transboundary application of EIAs should not lead to exclusion of the application of this obligation to areas beyond national jurisdiction. Indeed, there is a symmetry between the scope of application of this obligation and of the general no harm rule, which it derives from.Footnote ⁶² Likewise, the fact that Article IX OST does not explicitly contemplate an obligation to carry out an EIA has no relevance. As the perhaps pleonastic Article III OST clarifies, states are to carry out space activities ‘in accordance with international law’.Footnote ⁶³ As a general requirement of international law, the EIA is therefore also applicable to space activities.

The real issue here is to establish the role that ascertaining risk might have in this context. One might pragmatically argue that an EIA should be undertaken whenever a planned activity entails a significant risk in a transboundary or international context, while it should simply be skipped if the risk is icto oculi low or even absent as it would unnecessarily overburden domestic administrations. It is, on the contrary, submitted that, in line with the rationale of an EIA, ascertaining risk should be the outcome of the process rather than one of its constitutive elements. The performance of an EIA for each planned activity might enable the consideration and choice of more environmentally sound options. The current status of Earth’s environment calls for the inclusion of environmental concerns in each planned activity, not only in those entailing an icto oculi significant risk. Even the reduction of low environmental impacts can have an overall benefit for the environment. However, also from a legal point of view, making the performance of an EIA contingent on the existence of a risk unnecessarily complicates the issue by triggering further and difficult-to-solve problems. In particular, it would be hard to establish which risk thresholds to apply in the context of the preliminary assessment and the EIA. These thresholds would arguably not correspond to that required to prove that significant transboundary harm has actually been caused.Footnote ⁶⁴

Therefore, the point is not whether or not an EIA is compulsory but instead to identify the depth of the EIA a state should perform.Footnote ⁶⁵ This assessment should be carried out diligently taking into account the characteristics of the planned activity and in compliance with the precautionary approach.Footnote ⁶⁶ For activities which empirically entail a very low risk of harm being done it will be sufficient to carry out a ‘light’ EIA, while an extensive one will be required for activities which empirically entail a significant risk.

The International Law Commission cites space activities as an example of ultra-hazardous activities,Footnote ⁶⁷ and one might argue that a risk of harmful contamination is presumed in any space activity. However, the depth of the EIA will depend on concrete assessments to be carried out on a case-by-case basis. A cubesat and a space station, and objects with and without nuclear power sources on board, arguably do not pose the same risks.

This leads us to the last point, i.e., the content of the EIA and the existence of a noyeau dur of information that the EIA has to obtain. In the Pulp Mills case, the Court left the choice of the modality of the EIA to the state.Footnote ⁶⁸ In this respect, Judge Bhandari posited in his separate opinion that, despite its regional nature, the Espoo Convention could be used as a ‘standard that nation States should strive toward’.Footnote ⁶⁹ Based on this idea, he suggested what he deemed could constitute a ‘lowest common denominator’Footnote ⁷⁰ for conducting an EIA, including descriptions of the activity and of potential alternatives.Footnote ⁷¹

Such a detailed approach seems difficult to reconcile with the general nature of obligations that characterizes customary norms, and in any case with current practice and the opinio juris of states. In the current stage of development of international law, the content of an EIA can be solely determined by the international treaties that states are party to or by non-binding guidelines such as the UNCOPUOS Guidelines on Space Debris Mitigation.Footnote ⁷² The latter pragmatically consider the entire life cycle of the space object and in each phase from design to disposal contemplate safeguards to curtail the production of mission-related space debris. These include proper planning of passivation measures to avoid break-ups,Footnote ⁷³ specific designs to increase the trackability of space objects,Footnote ⁷⁴ a minimum de-orbiting capability to avoid collisionsFootnote ⁷⁵ and periodic controls to detect malfunctions.Footnote ⁷⁶ An EIA for space activities should accordingly carefully consider all these aspects along the entire life cycle of the space object about to be launched. For this purpose, a uniform metric could be developed for the environmental footprint of each new mission to evaluate its impact on the space environment.Footnote ⁷⁷

The obligation to undertake an EIA should induce states to draw up a debris mitigation plan for each new space activityFootnote ⁷⁸ and to impose it on private entities as a licensing condition with the aim of demonstrating the projected space activity’s compliance with the debris mitigation requirements.Footnote ⁷⁹ Indeed, under Article VI OST, states have to ensure that national private activities are carried out in conformity with the Outer Space Treaty, including its Article IX. States could also more carefully explore the option of regulatory incentives.Footnote ⁸⁰ For instance, a link could be legally established between the adoption of more stringent debris mitigation measures and a lower required level of insurance. In this manner, far from being an end in itself, an EIA could well bring improvements in the design of space projects and the integration of environmental considerations in them.

7. Notification

If the EIA confirms that there is a risk of harmful contamination or interference, the state planning to undertake the space activity is required to notify potentially affected states in conformity with its due diligence obligation with the aim of determining the appropriate measures to prevent or mitigate this risk.Footnote ⁸¹ It is admittedly true that the obligation to notify in cases of potential harmful interference with other space activities is not explicitly contemplated in Article IX OST. This is nonetheless a necessary assumption because the rule enabling other state parties to request consultation in cases of potential harmful interference with their space activities would otherwise be ineffective.Footnote ⁸² The rationale of the obligation to notify is clearly to lead the parties to consult and create the conditions for successful co-operation, enabling them to assess the impact of their planned activities on the environment and on other space activities with the fullest possible information and, if necessary, to negotiate the adjustments needed to avoid or at least minimize the risk.Footnote ⁸³ The duty to notify amounts to a legal obligation and, in order to implement it, spacefaring nations should at least issue an official statement at the international level before a potentially harmful space activity takes place. This results in overall benefits in terms of transparency and public participation.

As long as the obligations to perform an EIA and to notify are assessed independently, a separate application would allow the judge to hold states responsible for two distinct breaches.Footnote ⁸⁴ This could be particularly useful in those circumstances (as in the Pulp Mills case, and in the OST) where applicable treaty provisions are specific about other obligations – notification or consultation – but more vague or even silent in relation to the performance of an EIA.

8. Consultation

The obligation to undertake consultations in cases of potential harmful effects being foreseen as a consequence of a particular space activity is the only procedural obligation which Article IX explicitly contemplates. Article IX admittedly limits the application of this obligation to potential harmful interference between space activities while the potential harmful contamination of outer space is not considered. However, the intimate connection between harmful contamination of outer space and potential harmful interference with the space activities of other states has already been emphasized, in the sense that the former necessarily entails the latter, and vice versa. The effectiveness of even this part of Article IX has nonetheless been found to be curtailed by uncertainties in its interpretation. It has been noted that no deadline is mentioned for consultations, with the consequence that talks could go on and on and in the meantime serious – and possibly irreversible – damage could be caused to the Earth or the space environment.Footnote ⁸⁵ To add to the obscurity underlying Article IX, it has been claimed that the requirement for a state having ‘reason to believe’ that its activity may cause damage to request consultation is left entirely to the discretion of that state. Indeed, it may have no ‘reason to believe’ and yet the activity could entail harmful consequences.Footnote ⁸⁶ Finally, it has been noted that Article IX does not provide any clue to the legal consequences in cases in which the states concerned are not able to reach an agreement.Footnote ⁸⁷

All these interpretations trying to demolish the compulsory value, and ultimately the useful effect, of Article IX OST are untenable.Footnote ⁸⁸ First of all, although no deadlines are established, one point is clear: consultations are to take place, or at least start, before a ‘planned’ space activity takes place, both because of the wording ‘before proceeding with any such activity or experiment’ and because they would otherwise be largely deprived of their utility. Second, although it is true that potentially affected states have no right to veto a project with a risk of significant harm, while consultations are underway the project should not be carried out precisely because the aim of consultations is to arrive at an equitable and satisfactory solution for all.Footnote ⁸⁹ Finally, dwelling on the subjectivity of the assessment seems to be out of place. Given the lack of a mandatory jurisdiction in international law, the legal dynamics of the process of interaction between states mainly involve them self-interpreting legal rules.Footnote ⁹⁰ Therefore, although the expression ‘has reason to believe’ could seem to make self-interpretation ‘more unilateral’ than the interpretation of other international law rules, self-interpretation is far from being a prerogative of Article IX OST. At the same time, the principles of good faith and reasonableness spontaneously restrain and mitigate the unilateralism and discretion underlying self-interpretation of international law.

First of all, treaty interpretation and performance are guided by the good faith principle,Footnote ⁹¹ which implies that the legal arguments underlying a state’s claims today cannot be refuted tomorrow when relied on against that state by other states. States need to be aware that rules of law and material situations are reversible, in the sense that a country blamed for being a polluter today could very well end up being a victim of space debris tomorrow. This should lead them not to interpret Article IX OST arbitrarily. Finally, in addition to being unconvincing for an international judge, an unreasonable and legally unfounded claim would be unacceptable and hardly credible for the other states concerned, the international community and public opinion, even outside the judicial context.Footnote ⁹² This is particularly relevant to our purposes because the obligations relating to the preservation of the space environment have an erga omnes character. Therefore, any state is entitled to invoke the responsibility of the polluting state even if it is not directly affected by the breach.Footnote ⁹³ In conclusion, one cannot really support the view that Article IX does not provide any definitions concerning the time and features of consultations.Footnote ⁹⁴

Article IX admittedly does not provide for a formalized dispute settlement procedure leading to a binding decision by any particular body should the parties not be able to agree on a settlement. However, as long as an EIA confirms that a planned space activity may cause harmful interference with the space activities of other states, each involved state has the right to trigger the consultation mechanism. Refusing to hold consultations amounts to a breach of the OST, and so would any unjustified breach of the talks and systematic refusal to take into consideration adverse proposals.Footnote ⁹⁵ Against this background, it is finally possible to assess the most concerning action regarding debris proliferation.

9. The intentional destruction of space objects

Clearly enough, being generally carried out to test anti-satellite weapons, the intentional destruction of space objects is intertwined with the broader and topical issue of the lawfulness of military use of outer space.Footnote ⁹⁶ It is well-known in this respect that the crucial interpretation of the adjective ‘peaceful’, which is used several times in the OST to qualify the purposes space activities are to be carried out for, is not univocal. This term could be interpreted as either ‘non-military’ or ‘non-aggressive’.Footnote ⁹⁷ If the first interpretation were to be adopted, the solution would be straightforward: any military use of outer space, including anti-satellite weapon tests, would be prohibited. However, all the arguments in support of either interpretation seem rebuttable and international practice seems to support the narrower one.

As far as anti-satellite weapon testing is concerned, UNCOPUOS Guideline 4 states that ‘the intentional destruction of any on-orbit spacecraft and launch vehicle orbital stages … should be avoided’Footnote ⁹⁸ except in cases where break-ups are ‘necessary’. Within UNCOPUOS, some delegations argue that the intentional destruction of space objects should be legally prohibited.Footnote ⁹⁹ Others emphasize the need to submit such destruction to criteria and procedures.Footnote ¹⁰⁰ International practice is not univocal. Some spacefaring nations continue to carry out anti-satellite weapon tests,Footnote ¹⁰¹ yet any new test triggers stronger protests and calls for consultations by the international community.

Against this background, two paths of reasoning are available. One might argue that Article IX OST in combination with Guideline 4 supports the argument that anti-satellite weapon testing is per se inherently unlawful, except for cases where break-ups are ‘necessary’.Footnote ¹⁰² Indeed, the energy converted by the impact of anti-satellite weapons, even in low-Earth orbits, would render any resulting space debris uncontrollable and hence increase the risk of collision. Under this perspective, contamination stemming from anti-satellite weapon testing is by definition harmful and the only way to avoid it is to refrain from anti-satellite testing.

On the contrary, and this is the more convincing path, one might emphasize that a specific prohibition is needed to be truly effective.Footnote ¹⁰³ After all, even states claiming that the intentional destruction of space objects should be legally prohibited implicitly but unequivocally assume that it is not prohibited so far.Footnote ¹⁰⁴ However, this does not mean that states carrying out anti-satellite testing are free from any limits and obligations. In this regard, a parallel might be drawn with the contiguous field of nuclear weapon testing. It is well known that the ICJ has twice decided not to rule on the merits on whether such activities are contrary to international law. While in the first application the arguments of the parties were based on the existing norms of international law setting the traditional rules on mutual respect of sovereignty by states,Footnote ¹⁰⁵ several years later the Court decided to address the issue of nuclear testing from a different perspective, focusing on obligations of states to respect and protect the environment. As mentioned, even in this latest case, the Court did not rule on the merits, thus avoiding applying any rules involving these obligations.Footnote ¹⁰⁶ However, by way of obiter dicta, the ICJ referred to ‘the development of international law in recent decades’Footnote ¹⁰⁷ and indicated that its order was ‘without prejudice to the obligations of states to respect and protect the natural environment, obligations to which both New Zealand and France have in the present instance reaffirmed their commitment’.Footnote ¹⁰⁸ Indeed, both France and New Zealand had recognized the existence of a general obligation to prevent environmental damage, and in particular obligations relating to EIA.Footnote ¹⁰⁹ The relevance of this reasoning to this work is obvious. Even admitting that anti-satellite testing is not per se contrary to international law, it must in any case be submitted to the environmental obligations outlined above.Footnote ¹¹⁰

In particular, while a risk of harmful contamination is presumed in any space activity,Footnote ¹¹¹ this presumption is not rebuttable in the case of anti-satellite weapon tests. As a result, notification to the international community before testing is always compulsory, as is a consultation on request of other states within the framework of an EIA.Footnote ¹¹² Moreover, as Guideline 4 recommends, destruction should be performed only at sufficiently low altitudes to limit the orbital lifetime of resulting fragments.Footnote ¹¹³ This part of Guideline 4 is binding via Article IX as long as it is the only appropriate measure to avoid long-lasting harmful contamination.

10. Debris de-orbit and removal

Debris mitigation instruments contain specific guidelines for low earth orbits (LEOs) and the geostationary orbit (GEO) with the final objective of limiting the presence of spacecraft in these protected areas after the end of their operational phase and therefore to avoid collisions. Because of their particular characteristics, these orbits are indeed the most crowded.Footnote ¹¹⁴

The UNCOPUOS Guidelines contemplate two separate rules for LEOs and the GEO.Footnote ¹¹⁵ Guideline 6 states that non-functional space objects that pass through the LEO region should be removed from orbit in a controlled fashion. This consists in re-entering the space object into the Earth’s atmosphere.Footnote ¹¹⁶ If removal is not possible, satellites ‘should be disposed of in orbits that avoid their long-term presence in the LEO region’.Footnote ¹¹⁷ Guideline 7 states that non-functional space objects that pass through the GEO region should be left in orbits that avoid ‘long-term interference’ with the GEO region.Footnote ¹¹⁸ Disposal to a higher orbit normally is the only viable way to remove objects from the GEO. Natural forces such as atmospheric drag do not exert sufficient power for re-entry.

The two alternatives outlined – disposal to a dedicated higher orbit and re-entry – of course entail some side-effects and risks. Disposal to a higher orbit has side-effects because it leads to the creation around the Earth of a belt of debris like those around Saturn. Even re-entry is not without risks. Although objects usually burn up with the heat of friction with atmospheric gases, the most compact pieces of debris reach the ground. Think of the uncontrolled re-entry of Soviet Cosmos 945 in 1978, that of the US space laboratory Skylab in 1979, and that of the Chinese Long March 5B rocket in 2020. It is precisely because of these side-effects and risks that technologies for debris docking and removal, and for on-orbit servicing are in development.Footnote ¹¹⁹

In virtue of a combined reading of Article IX OST and UNCOPUOS Guidelines 6 and 7, and taking into account the technology available, states are already required to design and produce their satellites so that, once they have completed their operational phase, they can be de-orbited. If this is not possible for technical reasons,Footnote ¹²⁰ they are required to plan them to be injected into an orbit in which natural decay would be possible or ‘long-term interference’ with the LEO and GEO region would be avoided. French legislation explicitly contemplates these requirements and is a model to follow. Indeed, the French Decree on Technical Regulation provides that in order to operate in low earth orbit a space object must be designed and produced so that once it has completed its operational phase it can be de-orbited.Footnote ¹²¹ If this is not possible for technical reasons, it is to be produced and placed so that it is no longer present in the LEO region more than 25 years after the end of its mission.Footnote ¹²²

In the current stage, studies on active debris removal are mostly theoretical but, as mentioned above, technologies necessary for debris docking and removal will soon be available. Therefore, a further issue to assess is whether a state has an obligation to actively remove its no longer functional space objects or, at least, to consent to their removal by other states. Article VIII OST clearly provides that the state of registry has exclusive jurisdiction over its own space objects and that the permanence of an object in space does not affect its ownership. Therefore, states should have an obligation not to interfere with the space objects of other states, regardless of whether they are functional or not.Footnote ¹²³

In virtue of Article IX OST, other states should first of all request consultations and eventually obtain consent to remove an object. At the same time, a duty can be identified for the launching state to not arbitrarily withhold its consent. Indeed, non-functional satellites occupy valuable orbital positions that could be used by other states. Therefore, arbitrary refusal by a state to remove its non-functional objects or at least to give its consent for them to be removed by other states constitutes abuse of the freedoms guaranteed by Article I and a violation of the due regard principle in Article IX.Footnote ¹²⁴ Since a mere containment approach will soon no longer be sufficient to meet the preservation needs of the space environment, active removal or consent to removal are arguably the only ‘appropriate measures’ to avoid harmful contamination under Article IX.

The above is all the more true if non-functional objects are situated in areas particularly protected due to unrepeatability of their characteristics.Footnote ¹²⁵ Much like paper satellites, a refusal would also contravene some of the most fundamental principles underlying the ITU regime.Footnote ¹²⁶ After all, despite their differences, the space regimes installed by the UN treaties and the ITU instruments work towards the same end: both aim to ensure the actual and efficient exploration and use of outer space and its natural resources by states and their nationals in order to safeguard the corresponding rights of other users to engage in similar space undertakings.

Article 5, paragraph 4 of the Rescue and Return Agreement represents a further suitable legal basis supporting the existence of an obligation to actively remove space debris or to give consent for it to be removed, at least when it is ‘of a hazardous or deleterious nature’.Footnote ¹²⁷ Under this Article, a contracting party which has:

Such ‘effective steps’ cannot but be to remove or, when the launching state does not have at its disposal the necessary technology, not to arbitrarily refuse to accept the removal of such space objects.

Neither the OST nor the Rescue and Return Agreement specify what remedies are available if a state fails to eliminate possible danger of harm and at the same time arbitrarily refuses to accept the removal of its non-functional satellite by other states. Of course, when a refusal by a state determines a breach of its obligations it will be held responsible. Moreover, since the obligations relating to the preservation of the space environment have an erga omnes character, any state is entitled to invoke the responsibility of the polluting state even if it is not directly affected by the breach.Footnote ¹²⁹ Nevertheless, this obviously does not mean that other states have a right to remove a non-functional object without the consent of its state. What is certain is that if consent to remove is not given and the requirements of necessity are met a state could invoke this justification and unilaterally remove another state’s space debris, for instance when a potential collision threatens the functionality of its space object, the safety of its manned spaceflight or the safety of the population on the ground.Footnote ¹³⁰

11. Final remarks

The existing legal framework applicable to space debris has its limits. Article IX solely considers adverse changes in the Earth’s environment resulting from the introduction of ‘extra-terrestrial matter’, while the possibility that adverse changes could result from the re-entry of man-made debris is not even considered and deserves greater attention. The UNCOPUOS Guidelines correctly recognize that ‘due consideration should be given to ensuring that debris that survives to reach the surface of the Earth does not pose an undue risk to people or property, including through environmental pollution caused by hazardous substances’.Footnote ¹³¹

It has been pointed out that, from a combined reading of Article I and Article IX OST and Article 5 of the Rescue and Return Agreement, the existence of an obligation can be deduced on the state parties to these treaties to actively remove their space debris or at least to consent to its removal by other states. Nevertheless, reasons of legal certainty would suggest adopting an explicit binding provision stating an obligation on operating states to actively remove their no longer functional space objects coupled with a mechanism to react in cases of arbitrary refusal.Footnote ¹³² Moreover, Article 5 of the Rescue and Return Agreement qualifies the object to be removed as being ‘of a hazardous or deleterious nature’. Unless one accepts that any space debris is hazardous or deleterious, this restrictive requirement could severely limit the application of the rule to space debris. A new binding provision could be modelled on Article 60, paragraph 3 of the Montego Bay Convention, which requires states to remove any installations or structures which are abandoned or disused ‘to ensure safety of navigation’ in the exclusive economic zone with no further requirements.Footnote ¹³³

Then, in consideration of the fact that it is in most cases impossible to trace a fragment back to the object from which it originates and to identify a responsible and liable state, the proposal to establish an ‘International Outer Space Clean-Up Fund’ aimed at both providing compensation for damage caused by unidentifiable space debris and paying the cost of its removal before damage is done could be renewed. Each state should contribute to the Fund according to its past and actual use of the space ‘environment’ and a useful indicator for this purpose could be the profit made from or the cost involved in launches. Last but not least, a specific prohibition of anti-satellite weapon testing would be needed as it is the most concerning action in the field of debris proliferation.

Time will tell whether these proposals will be implemented. However, one point is certain. Regardless of any further legal development, the basic provisions against debris proliferation are already at hand in the Outer Space Treaty. Against the common interpretation of Article IX OST, I have tried to demonstrate that the final aim – to avoid harmful contamination and interference with the space activity of other states – cannot be the object of a reductionist interpretation. The obligations in Article IX OST likewise cannot be downgraded to a general pledge or a mere political commitment. Although Article IX OST admittedly does not specify which ‘appropriate’ measures to adopt in order to avoid harmful contamination of outer space and celestial bodies, the rule is supplemented by general international law rules and by the UNCOPUOS Guidelines, which provide for the application by states of the most advanced debris mitigation standards along the entire life cycle of a space object. The relationship between the obligation to carry out an EIA and the obligation to notify and consult other states has consistently been ascertained. As these are obligations of conduct, compliance with them does not depend on the level of a state’s development or of the financial resources at its disposal. The technical standards at hand, and consequently the required level of due diligence, may admittedly vary from state to state, but the fact that a state does not have the most advanced technology at its disposal does not equate to a licence for recklessness. On the contrary, the lower the available technical standards, the greater the potential for the activity to be harmful, and the more stringent the obligations to perform an EIA, to notify and to consult become.

Fruitfully combined with other rules and standards, Article IX OST already provides a set of important limits and constraints for states carrying out space activities. Failure to co-operate and minimize the risk of harmful contamination may have legal implications in terms of responsibility, and even liability in cases where these risks materialize. Consequently, it is difficult to deny that a refusal by space powers to even start any consultation in cases of potential harmful contamination/interference – no matter how serious or visible this might be to public opinion – constitutes a material breach of Article IX OST. This is particularly true in the deplorable case that a state intends to perform an anti-satellite weapon test. International lawyers would render a disservice to the universal human cause of space exploration and use if – merely focusing on the general character of the OST provisions – they tried to demolish the limits already contained in them.