Since the dawn of the Space Age, States have sought to use space for a myriad of reasons. One motivation was exploration—the desire to know more about the cosmos. Another was national defense, with both the USSR and the United States in a competition to leverage space for their own defense and the political rewards to be won through prestige achievements. All of these activities have led to the extensive use of space around the Earth and the Moon. While this has led to numerous benefits for the nations of Earth which have ventured into space, it has also created an increasingly congested and competitive arena that challenges space governance models (Khan and Ullah, 2020, p. 405). One negative consequence of the expanded use of space is the proliferation of derelict or otherwise useless objects, including orbital debris, all of which pose potential hazards to the continued use of space. One of the largest challenges facing States in the near future is how to expand their usage and exploration of space without adding to the mass of debris orbiting the planet, and how to prevent a similar problem from accumulating near other celestial objects explored by nations deeper into the solar system. Sustainability of space has therefore emerged as a key issue for space States, and is increasingly attracting the attention of scholars, diplomats, and international lawmakers, as a balance is sought between national defense, exploration, and commercialization of the space environs.

There is increasing consensus amongst States through both international law and space diplomacy that the sustainability of space is critical for the contemporary and future interests of all. Multiple efforts are underway internationally to determine best practices, goals, and both technological and legal solutions to perceived or actual threats to continued sustainable use of space. This essay aims to demonstrate that as States acknowledge the critical importance of maintaining the useability of the space environment, they have and will continue to open up new opportunities for peaceful cooperation in space. Presenting an exemplar of this principle, Japan and the United States are modelling one possible model for future bilateral or multilateral cooperative agreements to fulfill extant political or legal obligations in space.

As States look for ways to use space sustainably, they must consider the not only the impacts of current and past missions, but also future needs that will include more and more satellite constellations, space stations, and other objects launched into space. One possibility for successfully managing the growing use of space is to establish best practices that can not only work within, but also compliment, the international legal regime currently governing space. This essay aims to showcase one possible model for establishing these best practices, using the relationship between Japan and the United States as an example of diplomatic relations that have led to legal and geopolitical arrangements intended to respect the needs and interests of all States in space.

Japan and the United States, through a series of diplomatic exchanges that have led to a series of bilateral agreements, have demonstrated one method for using space sustainably that could be adopted, modified, and expanded to cover other spacefaring States’ needs and interests in the utilization and scientific exploration in space. A key utility for States is to craft framework agreements that create geopolitical scaffolding which permit the generation of further implementing arrangements that enhance ties between the parties, create clarity of purpose, and solidify current and future State actions and obligations towards one another, as well as the space environment. Such implementing agreements fulfill State obligations under the Outer Space Treaty regime, and fertilize the ground for bilateral and multilateral space-based exploratory projects.

Before delving into the specifics of the Japan-U.S. space agreements that can be seen as a model for sustainable space cooperation, it is incumbent on any spacefaring State to know the foundation of the rules applicable to space, as well as the problem of proliferating debris. Consequentially, a brief recitation of the technical problem, and legal parameters governing space, is in order.

Many objects launched into space around a celestial body will remain there long after their useful life has expired (NASA, Orbital Debris Program Office, FAQ 12). Depending on the dynamics of the orbital parameters associated with each object, some may remain in space for decades, or even centuries, before gravity is able to pull them back down to the planet. Once the object no longer serves a purpose, it essentially does little more than take up space that could be used by another object and provide an opportunity for a future collision or break up. These sorts of useless orbiting objects, especially those incapables of rehabilitation through future refueling or repair missions, are often called orbital, or space, debris (Tan, 2000, p. 151). Debris travels quickly through space and is a genuine danger to the continued use and exploration of space. It has been noted that debris as small as five millimeters could pierce through the crew cabin of a spacecraft, and debris as small as one tenth of a millimeter could penetrate the suit of an astronaut (Mirmina, 2005, p. 653-54). With the recent instances of Chinese (2007), Indian (2019), and Russian (2021) ASATs, the debris situation is space has become more perilous (Chelsea Gohd, Russian Anti-satellite Test, 2022). It was estimated that China’s shootdown of its FY-1C satellite generated up to 5,000 pieces of space debris (NASA, 2010). Former NASA Administrator Jim Bridenstine noted that the Indian ASAT had generated over 400 pieces of trackable debris (Space News, Bridenstine). Further, the U.S. Space Command has reported that the 2021 Russian ASAT “so far has generated more than 1,500 pieces of trackable orbital debris and will likely generate hundreds of thousands of pieces of smaller orbital debris” (Space Command, 2021). With direct Ascent Anti-Satellite tests, the debris situation is becoming more and more tenuous, generating many thousands of new debris pieces, and compromising the sustainability of space. All told, with debris of any kind surrounding the Earth, the ESA has estimated that there are more than 29,000 pieces of debris larger than 10cm, and potentially millions of exceptionally small pieces of debris (1mm or below) (ESA, 2024).

One issue with controlling space (or orbital) debris is that there is no universally agreed upon definition (Reynolds & Merges, 1997). Despite this, serious efforts have been made to build a useful definition by organization studying the problem of orbital debris, including the work of the Inter-Agency Space Debris Coordination Committee (IADC), which is an international body comprised of several space agencies, including NASA, JAXA, ROSCOSMOS, CNSA, the ESA, and others. The IADC has promulgated a series of guideline that assist States in planning their space missions, including methods and strategies for reducing the likelihood of generating orbital debris during the use and exploration of space. The organization has noted that “Space debris are all human made objects including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional” (IADC Guidelines, Art. 3.1, 2021). Under this definition, the element of non-functionality serves to distinguish desirable space objects from objects that need to be removed or otherwise mitigated.

The United Nations has recognized the dangers associated with orbital debris and have created their own debris guidelines to address the issue (UN Space Debris Guidelines, 2010). The UN Debris Guidelines, which were inspired by the work of the IADC and that body’s own Guidelines (UN Space Debris Guidelines, 2010, p. iv), provide the views of a collective of States working collaboratively to identify both the problem and its potential solutions. They note that:

In 2019, the United Nations expanded its work in preventing a catastrophe in the space environment by creating the U.N. Sustainability Guidelines. This work, generated through the efforts of the Legal Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space, addresses many aspects of the sustainability of space, though debris management and prevention is clearly a priority. The Sustainability Guidelines were “welcomed with appreciation by the UN General Assembly in 2019” (Awareness-raising, UNOOSA, 2024) and have continued to influence the conversation over the various aspects of space sustainability ever since. They note that among other factors impacting space use, space debris “may affect the long-term sustainability of space activities” (UN Sustainability Guidelines, at I, Background 1). The Sustainability Guidelines recommend that States and international organizations implement debris mitigation through the use of measures like the U.N. Debris Guidelines, as one action among many that can be pursued to reduce the threats to space sustainability (UN Debris Guidelines A2, 2(b)).

The efforts of the United Nations, as well as the studies of scholars, has demonstrated that sustainability in space is critically important, if ill-defined, and that there is a risk that this phrase has become a “buzzword” (Bartoki-Gonczy et al., Space Sustainability, 2024, p.261). Even if a universally agreed upon definition of sustainability has yet to materialize, it is clear the concept is an important one to states, given efforts such as the U.S.-Japan cooperative agreements, and larger endeavors like the Artemis Accords.

Despite the efforts of international organizations to mitigate the dangers of orbital debris, guidelines are not binding on spacefaring states. While individual states may act to create domestic laws that restrict their own citizens’ or corporations’ activities in space, guidelines are examples of best practices for space activities. Given the investment by numerous states, it is clear that orbital debris guidelines are taken seriously by the spacefaring community. By themselves, these documents cannot prevent nations from generating debris. However, when contextualized within the international space law regime, these guidelines take on additional force as soft law implements.

Both Japan and the United States are States Party to the ISS IGA (ISS IGA, Preamble). Through this arrangement, both nations have operated cooperatively and peacefully for the duration of the International Space Station’s current lifespan. While the continued operation of the ageing Station is unknown, all signs indicate that both nations will continue to operate there pursuant to both the framework treaty itself, and any implementing agreements between one another since that time. One such MOU was concluded between Japan and the United States to specify each state’s obligations to the Station (Japan-U.S. MOU, 1998). Similarly, the United States has MOU arrangements with other partners, such as the European Space Agency (ESA-U.S. MOU, 1998). Decades of operation of the International Space Station have demonstrated that the initial framework agreement successfully enabled subsequent diplomatic interchanges between states which provided necessary specifics for contributions.

In addition to the ISS IGA, Japan and the United States have continued to engage one another through other instances of space diplomacy, some of which have resulted in additional bilateral and multilateral agreements. Indeed, as states grapple with the challenges of maintaining the sustainability of space amidst its expanding, Japan and the United States have concluded a series of framework and implementing agreements that model the ability to work cooperatively towards areas of mutual interest. Collectively, these agreements provide a demonstration of how diplomatic relationships can bind states together for space activities while simultaneously developing best practices for environmental stewardship.

One such framework agreement is the Artemis Accords. Whereas the ISS IGA took the form of a legal agreement between Station partners, the Artemis Accords are more akin to a geopolitical agreement between like-minded states. As of January 2026, sixty states have signed the Accords, indicating general agreement on a host of issues, including peaceful purposes in space, the registration of objects, and the acquisition and use of space resources (NASA, The Artemis Accords). Importantly for efforts in space sustainability, the “signatories are committed to the open sharing of scientific data” (Artemis Accords, 2020, sec. 8). Such a data sharing arrangement can assist signatories with sharing of discoveries, and with data concerning threats to the use of the space environment. Specifically, the signatories “commit to plan for the mitigation of orbital debris, including the safe, timely, and efficient passivation and disposal of spacecraft at the end of their missions, when appropriate, as part of their mission planning process” (Artemis Accords, 2020, sec. 12). Not only does this provide for state intentions regarding the continued sustainability of space, but it also leaves room for successive agreements designed to implement each state’s plans, especially in instances where joint projects to use or explore the Moon or asteroids.

More specifically, Japan and the United States have executed a number of agreements through decades of diplomatic exchanges on topics as diverse as national security (Japan-U.S. Defense Treaty, 1960), to the provision of lunar landers for exploration of the Moon (NASA, Japan Lunar Rover, 2024). In 2020, Japan and the United States initiated the Memorandum of Understanding on the Lunar Gateway, noting the desirability and benefits of operating a “human outpost in the lunar vicinity” (Civil Lunar Gateway MOU, 2020, preamble). The agreement further details the goals, objectives, and operational expectations for cooperation in the design and utilization of the Gateway project between the United States and Japan. Recognizing that the lunar outpost is “the next step to enable sustainable exploration and use of the Moon and Mars” (Civil Lunar Gateway MOU, 2020, preamble), the parties agreed that the project be operated “in a manner that is safe [and] sustainable….” (Civil Lunar Gateway MOU, 2020, art. 1(1.3)(d)). Further, the Gateway is described as a platform that will be used for multiple purposes, including “enabling lunar surface activities and sustainable exploration” (Civil Lunar Gateway MOU, 2020, art. 4(4.1)(e)(4)), which itself ties in to both states’ legal obligations under Art. IX of the Outer Space Treaty.

In 2023, Japan and the United States concluded the Exchanges of Notes on the Martian Moons exploration project, a sample return project intended to determine the origin of Mars’ satellites (Exchange, 2023). The agreement was signed within the greater context of Japan’s signing of the Artemis Accords, and the plans for continued cooperation on lunar exploration (Exchange, 2023).

Further, 2023 also saw the states conclude the Japan-U.S. Framework for Cooperation in the Exploration and Use of Outer Space. This agreement creates a comprehensive basis for continued cooperation in space between the parties, including for space and lunar science, space technology, the use of spacecraft and research facilities on Earth, the exchange of scientists and scientific data, and other cooperative endeavors (Japan-U.S. Framework, 2023, art. 1(A)(B)). True to its name, the Framework envisions future arrangements and diplomatic exchanges between the parties in order to fulfill its terms. Additionally, the increasing pressure on states to explore space sustainably was recognized by the parties, where they note that:

The Parties have also agreed to use planetary protection measures consistent with the Committee on Space Research (COSPAR). Planetary Protection Policy and Implementation Guidelines, which concern themselves primarily with the sustainability of the space environment, as well as the protection of the Earth (COSPAR Guidelines). A key feature of these Guidelines is to ensure that in their exploration and use of outer space, states do not harmfully contaminate other celestial objects or introduce harmful materials to the environment of Earth (COSPAR Guidelines, Preamble).

Collectively, these agreements have demonstrated both Japan and the United States’ decades-long commitment to peaceful, sustainable exploration and use of space, in conformity with obligations under the Outer Space Treaty and Registration Convention, and consistent with larger multilateral agreements like the ISS IGA and the Artemis Accords. Additionally, these agreements are consistent with the national space policies of both states, indicating that domestic and foreign policy are aligned in the cooperative and sustainable approach to the use of outer space. While only one example of international relations that have led to space cooperation, the exchanges and agreement-building between Japan and the United States has provided an example of state-to-state space diplomacy that showcase consistency with international space law, the Charter of the United Nations, and an awareness of the pressures on the increasing usages of the outer space environment. As space coalitions continue to grow, alliances like Artemis and the Japan-U.S. Framework will require greater specificity with regards to their foundational norms. Japan and the United States, alongside other Artemis partners, will have to work together to create best practices respectful of all member states. With both states’ open desire to work collaboratively on a host of space activities, Japan and the United States have therefore created an opportunity to lead the international discussion surrounding best practices and legal obligations regarding space sustainability and data sharing incumbent on future space activities for future State cooperatives.

It remains to be asked how non-aligned nations view these activities. One wonders if these sorts of state-to-state bilateral agreements deepen divisions globally, or provide opportunity for adoption of best practices internationally. Since the issue of environmental management is, in theory, an issue of deep importance for all states wishing to continue their use and exploration of outer space, any cooperation in this direction could be seen as both a fulfillment of international law, and a responsible practice for continued use and exploration. To date, no substantial objections to U.S.-Japan cooperation on these matters has arisen in international legal circles. Further, arrangements such as those between Japan and the United States present, in actual practice, an apolitical model (i.e., the functioning of these sustainability arrangements, while political in origin, are not inherently associated with one geopolitical direction or governance model over another). This allows other nations wishing to further their operations in outer space without needing to see themselves as adopting a “U.S.” or “Japan” model for space activity, but rather as adopting and adapting useful behavioral paradigms for their own needs. States like the Russian Federation and China would be able to mimic the elements of the U.S.-Japan model that suit their own needs, and discard what they do not need, without losing face in diplomatic circles or kowtowing to rival state global politics, since the efforts to declutter and sustain outer space is a benefit shared by states of all persuasions. Potential rivals could also adopt similar strategies to those of the U.S. and Japan in open furtherance of their own international obligations under the Outer Space Treaty system, further obviating the concern about agreements such as those between the U.S. and Japan fracturing international relations between states.

Because the U.S.-Japan agreements are bilateral in nature, they most directly impact those two states, even though a model for sustainability could be adopted by other states as needed. This bilateral nature partially insulates these sustainability efforts from more obvious political posturing, such as is seen in ongoing geopolitical alignment between growing space organizations like the Artemis Accords and the International Lunar Research Station (ILRS). Where there is both a growth of international cooperation through willingness to be bound to one of those organizations, there is also an apparent fracturing effect seen by diving the states into one camp or another. Such a division is part of a larger conversation about geopolitics, political posturing, and international relations. However, it need not be seen as the inevitable result of what has been accomplished by the U.S.-Japan agreements, nor should states politically opposed to U.S. or Japanese interests need feel acquiescence in adapting similar methods for their own states, if they so choose. Under international law, states are free to adopt whatever methods they choose to address global issues, so long as these are consistent with their own international obligations (i.e., ratified treaties, jus cogens norms, etc.). In short, there is more to encourage international cooperation in these agreements than there is to divide states against one another.

As states grow their uses of the space environment, they will need to consider how to expand implementing agreements that complement the frameworks they have designed. Agreements like the Artemis Accords are clearly open to additional membership, and have been expanding steadily since their creation in 2020. There is even a possibility, however remote, that China, Russia, and the United States might join together in a future project. However, there is a U.S.-initiated legal gulf preventing some forms of cooperation between China and the United States—known as the Wolf Amendment—which prevents NASA from cooperating with China to “plan, promulgate, implement, or execute a bilateral policy, program, order, or contract of any kind to participate, collaborate, or coordinate bilaterally in any way with China” absent consultation with the Federal Bureau of Investigations (NASA Reauthorization Act, 2024, sec. 310(a)(1) and (b). Despite this, at the 2024 International Astronautical Congress in Milan, Italy, a senior NASA official opined that “China is free to sign the Accords any time they want to.” (NASA, n.d.)

No matter what may happen between the United States and China, ongoing diplomatic relations between Japan and the United States have provided a platform on which further international collaborations may be modelled. Still, it is critical to note that while this essay focuses on the impact that Japan and the United States may have on developing international best practices, standards, and policies, these two states do not operate in a geopolitical vacuum. Voices from all partners, and ideally from representatives of alternative proto-hegemons like that of the ILRS, must be included to secure sufficient buy-in and meaningful participation from the states likeliest to use space in the near and long-term.

Given the intersection of the Japan-U.S. Framework within the broader context of international relations in space activities between these states, especially within the matrix of space agreements that contains related arrangements like the ISS IGA and the Artemis Accords, future joint ventures portend activities not only consistent with current and developing best practices, but the expansion of internationally beneficial data sharing and sustainability schemes amenable to myriad space operators. In essence, the diplomatic interchanges between Japan and the United States not only fulfill obligations incumbent upon both states under the Outer Space Treaty to give due regard to one another, but also open up opportunities to create meaningful exploration and use case scenarios that provide more than lip service to the legal obligation to sustainably use and explore space.

One of the key challenges to internationally recognized and effective means to maintain space sustainability is state buy-in for specific behavioral norms in space. Agreements such as the Framework, the Artemis Accords, and the ILRS agreement can be leveraged to move states forward in a positive direction for reducing risks in space, such as the proliferation of debris, interference with the use of radiofrequency spectra, and protocols for planetary protection impacting space exploration. Increasingly, diplomatic solutions to these problems have emerged in the form of multilateral resolutions such as the UN Sustainability Guidelines, geopolitical arrangements like the Artemis Accords, and bilateral assurances such as the Framework between Japan and the United States. This suggests that spacefaring states, despite continuing to prioritize space for their own national defense and other interests, nevertheless recognize that unsustainable space is a cognizable threat to necessary current and future uses, and therefore a challenge that requires a solution which, minimally, considers the needs and interest of all.

Given that the United States has codified into law that “the aeronautical and space activities of the United States shall be conducted so as to contribute materially to…the preservation of the role of the United States as a leader in aeronautical and space science” (51 U.S.C. Sec. 20102(d)(5)), forming cooperative agreements with other states towards enhancing sustainable space activities provides an opportunity to lead the development of best practices. United States national space policy also comports with this legal mandate, noting that the U.S. will “continue leading the development and adoption of international and industry standards and policies, such as the Guidelines for the Long-term Sustainability of Outer Space Activities and the Space Debris Mitigation Guidelines of the United Nations…” (U.S. National Space Policy, 2020, p. 14). Cooperative endeavors like the Framework and its progeny lend credence to the notion that these states may yet help create more widely adopted sustainable norms for spacefaring states.