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With Due Diligence: a Guiding Principle in International Space Law

On 23 July 2025, the International Court of Justice (ICJ) handed down perhaps one of the most consequential advisory opinions of our time. The Advisory Opinion () was hailed by the United Nations (UN) Secretary-General as 鈥�.鈥�

The focus of the advisory opinion is on various matters of environmental law and human rights law. However, the questions put to the Court by the also offered the ICJ the chance to clarify, among other issues, the relationship between customary and conventional law, as well as the role of certain 鈥済uiding principles鈥� in the interpretation and application of treaties. In particular, the Court took the opportunity to clarify the duty of due diligence, which has been a subject matter in various decisions and opinions of international courts and tribunals,[[i]] and the focus of study by international law practitioners and experts,[[ii]] as well as the (ILC).

As will be argued in this brief commentary, the obligation of due diligence is of great relevance to the governance of activities that involve a high degree of risk, and where there is an obligation on States to prevent harm emanating from activities under their jurisdiction and control. This is of direct application to the conduct of space activities, particularly in light of the growing privatisation of space activities.

A landmark opinion

In its advisory opinion, the Court in paragraph 137 underlined that the risks of significant harm to the climate system by anthropogenic activities are 鈥渋ndisputably established,鈥� and 鈥減oses a quintessentially universal risk to all States.鈥� The unanimous Court also underlined the consensus of limiting global average temperature increase to 1.5掳C above pre-industrial levels is an obligation, and not merely an aspiration, when interpreted in light of the customary obligation to prevent significant harm to the environment (paragraph 242). Further, the Court also underscored the inseparable relationship between environmental degradation and human rights (paragraphs 389 & 393).

General Principles and Due Diligence

After elaborating on the applicable conventional rules relevant to the issue of climate change, the ICJ turned its attention to the applicable customary international law. Notably, the Court unequivocally recognised the role that 鈥済uiding principles鈥� play in the interpretation and application of treaty rules (paragraph 146). Among the principles highlighted is the principle of due diligence. Notably, the Court connected the obligation to act with due diligence to the duty to prevent significant harm to the environment, which they held to be a customary duty, which has its origins in the 鈥済eneral nature of the no harm principle鈥� (paragraph 133). , the Climate Change Advisory Opinion shed further light on this age-old but nebulous concept of due diligence, which is said to lie 鈥渁t the nexus between the responsibility of States as members of the international community and the sovereign right of States to act within their territory.鈥漑[iii]]

The Court previously underlined in that it is 鈥渆very State鈥檚 obligation not to allow knowingly its territory to be used for acts contrary to the rights of other States.鈥� Due diligence, therefore, demands that a State 鈥溾�� to avoid activities taking place in its territory, or in any area under its jurisdiction, from causing significant harm (in case of the advisory opinion, to the environment).[[iv]] In turn, due diligence can also be extrapolated to demand that States exercise vigilance over activities of non-governmental entities taking place in its territory or under its jurisdiction.

Elements of Due Diligence

Due diligence has been invoked in a number of contexts, including international , , as well as international environmental law as this advisory opinion shows. The standard of due diligence differs and varies depending on the context, and the nature of the activity in question.[[v]]

In general, the riskier an activity, the more stringent the level of due diligence to prevent significant harm is expected of States (paragraph 138). When there is a risk of significant harm, as is the case with climate change, then the standard of due diligence is considered stringent, and in turn requires States to have 鈥渁 heightened degree of vigilance and prevention.鈥� Failure to exercise due diligence, as the Court confirms in its advisory opinion, constitutes an internationally wrongful act and entails legal consequences (paragraph 444).[[vi]]

In practice, the duty of due diligence and what exactly is required of a State contains both substantive elements and procedural elements (paragraph 289), and is assessed based on what is considered reasonable under the circumstances (paragraph 137).[[vii]] Substantively, due diligence requires States to undertake 鈥渢o the best of their ability, appropriate and, if necessary, precautionary measures鈥� that would not cause significant harm to other States (paragraph 136). Such measures may take into account the scientific and technological information available (paragraphs 284 & 293), the relevant 鈥渞ules and international standards鈥� that are applicable, as well as the capabilities of each State (paragraphs 136 & 292). The Court also highlights that due diligence requires States to undertake an impact risk assessment (paragraph 295),[[viii]] notify as well as consult with other potentially affected States (paragraph 136).

Space Due Diligence

Given the ultrahazardous risks involved in the conduct of space activities, the principle of due diligence is adept to aid in the interpretation and application of treaty obligations for outer space activities. For example, Professor Setsuko Aoki, writing about the difficulty of defining and imputing 鈥榝ault鈥� in case of a collision between space objects in outer space, argues that due diligence can be a useful concept to bridge gaps in international law where binding rules may not be easily pointed to or as clear-cut.[[ix]]

Placed in an outer space context, due diligence requires that States adopt appropriate measures to prevent significant harm. This dovetails with the obligation on States to authorise and continue to supervise space activities of their non-governmental entities as required by Article VI of the .

Due diligence would also require that states undertake a risk impact assessment, which may be termed a 鈥渟pace risk impact assessment,鈥� as well as notify and consult with other potentially affected States. Such obligations already exist under the Outer Space Treaty. Article IX of the said treaty contains the obligation to have due regard to the corresponding interests of other States, as well as the obligation to consult, and the related right to seek consultation, in the event of activities that may potentially cause harmful interference with the activities of other States in the peaceful exploration and use of outer space.

Due diligence, as the Court points out, is an evolving standard, which is more demanding in the light of new scientific or technological knowledge. This again would be well suited to the regulation of space activities, which is characterised by rapidly evolving technological progress and applications that may have a tremendous impact on the space environment, as well as the lives of people on Earth. As more standards and guidelines are being adopted at the international level, as opposed to binding law, the regulation of space activities can greatly benefit from the minimum standard of vigilance and prevention that due diligence places on States, which in turn must be translated into the authorisation and supervision of activities of non-governmental actors in space.

Conclusion

In the Climate Change Advisory Opinion, the ICJ rightly underscores that the 鈥渃onsequences of climate change are severe and far-reaching鈥� (paragraph 73). The effects of climate change affect all States, peoples, and if unmitigated through States continuing to pretend that their commitments under various conventional and customary frameworks remain voluntary, will be felt by present and future generations.

This pivotal moment in international law began in earnest as a campaign by a group of law to persuade leaders of the Pacific Islands to take the issue of climate change and human rights to the World Court. These would culminate in an opinion with potentially far-reaching legal repercussions that may be felt in all areas of international law, including international space law.

Though advisory , they do 鈥�.鈥� Like previous opinions proffered by the Court in its advisory capacity, the Climate Change Advisory Opinion can shed light and clarity on areas of issues that have confounded States, practitioners and scholars alike. This is all the more pertinent in light of rising nationalistic sentiments and isolationist foreign policies, which threaten the coherence and integrity of the .

In light of the ICJ鈥檚 latest advisory opinion, the age-old principle of due diligence arguably has continued relevance in new-age activities in outer space, including on celestial bodies. Together with other fundamental principles of space law, due diligence will serve to ensure that States, as well as private space actors, adhere to minimum standards of behaviour that will ensure the long-term peaceful and sustainable exploration and use of outer space.