C. Sources of Space Law
4 Space law includes five core multilateral treaties. The first, most comprehensive and fundamental of these is the 1967 Outer Space Treaty, which has 113 States parties (as of May 2023). However, several issues that were only briefly addressed in this broadly gauged treaty were soon seen as requiring elaboration, leading to the rapid negotiation of more focused treaties. The 1968 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space (‘Rescue Agreement’) elaborated obligations concerning astronauts and the return of space objects found beyond the territorial limits of the State that launched them (Spacecraft, Satellites, and Space Objects). The liability regime applicable in space (Outer Space, Liability for Damage) was refined in the 1972 Convention on International Liability for Damage Caused by Space Objects (‘Liability Convention’), while the registration of spacecraft was addressed in the 1975 Convention on Registration of Objects Launched into Outer Space (‘Registration Convention’). Finally, the 1979 Agreement Governing the Activities of States on the Moon and Other Celestial Bodies (‘Moon Agreement’) began to address the issue of space resource utilization (‘space mining’) on the Moon and other celestial bodies. However, it explicitly deferred the negotiation of an international space mining regime until such time as this activity became feasible. The Moon Agreement does recognize space resources on celestial bodies as the common heritage of mankind.
5 Quite a few other treaties are relevant to space. For example, the 1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water (‘Partial Test Ban Treaty’) was concluded by the United States, the United Kingdom, and the Soviet Union just one year after they discovered that nuclear explosions in space posed an indiscriminate threat to all satellites. The 1992 Constitution and Convention of the International Telecommunication Union, along with its binding Administrative Regulations (particularly its Radio Regulations), have provided a mechanism for allocating radio spectrum to all satellites, as well as locations (‘orbital slots’) for communications satellites in geostationary orbit (International Telecommunication Union [ITU]). Several regional instruments are also of importance, notably the 1975 Convention for the establishment of a European Space Agency (European Space Agency [ESA]), which presently has 22 Member States. Some space treaties have limited numbers of States parties for practical reasons only. For example, the 1988 International COSPAS-SARSAT Programme Agreement has just four satellite-operating States parties (see further Section E.4 below) and 30 ‘ground segment providers’, but distributes information on the location of activated search-and-rescue beacons to authorities in more than 200 States and territories. The 1998 Intergovernmental Agreement Concerning Cooperation on the Civil International Space Station established a governance framework among the States involved in the station (see further Section E.6 below).
6 It is widely accepted that some of the rules set out in the Outer Space Treaty are also customary in character, including the prohibition on national appropriation, the freedoms of exploration, use and scientific investigation, and the demilitarization of celestial bodies. This means that as customary international law, these rules bind all States and not only those which are party to the treaty. While State practice and opinio iuris over a long period are normally required for the development of a new rule of customary international law, this process has been accelerated within space law due to the rapid development of space technologies. The earliest instance of this concerns the right of satellite overflight, which developed very quickly after the first satellite was launched in 1957 and no State objected to the repeated orbital passes above its territory. A similarly rapid process of customary international law-making resulted from the adoption by the UN General Assembly (‘UNGA’) of the 1963 ‘Declaration of Legal Principles Governing Activities of States in the Exploration and Use of Outer Space’, leading Bin Cheng to coin the term ‘instant customary international law’ (Cheng ; United Nations, General Assembly). These rules were then codified in the 1967 Outer Space Treaty.
7 Soft law is playing an increasingly important role in outer space, including in the form of UNGA resolutions whose provisions have not yet achieved the status of customary international law. Such resolutions include declarations on direct television broadcasting (1982), remote sensing (1986), nuclear power sources in outer space (1992) (Nuclear Energy, Peaceful Uses), international cooperation in space for the benefit and interest of all States (1996) and the concept of the ‘launching State’ (2004). More recent examples include the ‘UN Space Debris Mitigation Guidelines’ endorsed by the UNGA in 2007 and a 2014 resolution on ‘No First Placement of Weapons in Outer Space’. Non-legally binding but still highly influential instruments are also produced by other bodies. The Inter-Agency Space Debris Coordination Committee (‘IADC’), which is made up of representatives from 13 national space agencies, produced in 2007 its own set of ‘IADC Space Debris Mitigation Guidelines’, and the Committee on Space Research (‘COSPAR’), a quasi-governmental international organization, since 1963 has regularly released updated versions of its ‘COSPAR Policy on Planetary Protection’ (with planetary protection focusing on preventing the cross-contamination of lifeforms between celestial bodies).
D. Principal International Institutions in Space Law
8 In 1957, the Soviet Union launched Sputnik 1, the world’s first artificial satellite. The following year, the United Nations (UN) established an ad-hoc Committee on the Peaceful Uses of Outer Space (‘COPOUS’), which it then made a permanent committee of the UNGA in 1959. For more than six decades COPUOS has served as the principal international forum for the development of space law, with all five core multilateral space treaties and most space-related resolutions having been negotiated within it before being considered by the full UNGA. In addition to its main committee, COPUOS contains two subcommittees, the Scientific and Technical Subcommittee and the Legal Subcommittee, with the latter being normally responsible for the drafting of multilateral space law instruments. From an original membership of 18 States, COPUOS has grown to 102 members (as of May 2023). Both the main committee and the two subcommittees operate on the basis of consensus.
9 Several other UN bodies have some outer space-related responsibilities, for example the World Meteorological Organization (WMO) in relation to remote sensing data. However, it is the UN’s oldest specialized agency, the ITU that, aside from COPUOS and the UNGA, is the multilateral body most involved in the development of space law (United Nations, Specialized Agencies). Alongside its role in allocating radio spectrum (Broadcasting, International Regulation), including for satellites, the ITU assigns locations (‘orbital slots’) for communications satellites (Satellite Broadcasting) in geostationary orbit. The ITU Constitution and Convention provide the legal authority for these tasks, with implementation coming through the ‘Radio Regulations’ which are renegotiated and revised at the ITU’s World Radiocommunication Conference every three to four years. The ITU is presently dealing with filings for radio spectrum for more than 1 million satellites, most of them planned as part of ‘mega-constellations’ of communications satellites (systems made up of hundreds or thousands of individual satellites) in low Earth orbit (orbits below 2,000 km altitude). In an attempt to reduce the number of filings, the ITU recently adopted a ‘milestone’ approach that requires companies to launch certain percentages of their approved satellites within set times.
10 To date, no international space law disputes have been heard before an international court or other international adjudicatory body. A dispute settlement process was initiated in 1978 when radioactive debris from the re-entering Cosmos 954 satellite landed on Canadian territory, with Canada filing a claim through diplomatic notes against the Soviet Union under the Liability Convention. However, the two States negotiated a settlement without an ad-hoc Claims Commission being established pursuant to that treaty, with the Soviet Union voluntarily paying a portion of the requested compensation. More recently, some international dispute settlement bodies have taken steps to prepare themselves for the eventuality of space law disputes, with the Permanent Court of Arbitration (PCA) adopting in 2011 Optional Rules for Arbitration of Disputes Relating to Outer Space Activities (Optional Rules for Arbitration of Disputes Relating to Outer Space Activities: Permanent Court of Arbitration [PCA]).
E. Practical Cooperation Generating or Necessitating Space Law
11 Sputnik 1, the first artificial satellite, passed above the territory of numerous States but elicited no protests (Protest). This absence of international protest quickly cemented an accepted right of satellite overflight, which has been described as an example of ‘instant customary international law’. As of 2022, there are about 7000 operational satellites in orbit, as well as thousands of defunct ones. The Registration Convention requires that the ‘launching State’ of all space objects, which includes all satellites, must enter each launched object on a registry it maintains and must further inform the UN Secretary-General of specified particulars regarding each object for inclusion on a UN register (Art. II (1); United Nations, Secretary-General). Due to the prevalence of military reconnaissance, intelligence and other classified satellites, this treaty commitment is not universally observed, with the United Nations Office for Outer Space Affairs (‘UNOOSA’) estimating that only 88 percent of launched objects are registered with the UN. Registration is important because it determines which State holds ‘jurisdiction and control’ over an object in space and who is liable in cases of accident.
12 Under Art. I Liability Convention, the ‘launching State’ includes a State ‘which launches or procures the launching of a space object’ and a State ‘from whose territory or facility a space object is launched’. As a result, two or more States can be jointly and severally liable for damage caused by a single space object.
13 Since 1963, the ITU has allocated radio frequencies for space communications, subsequently assuming responsibility for the allocation of geostationary orbital slots and most recently instituting a milestone-based regulatory approach for non-geostationary satellites, predominantly relating to mega-constellations in low Earth orbit. Satellites in geostationary orbit, at 35,786 km above Earth, orbit in synchronization with the Earth’s rotational speed so that they always remain above the same location on the Earth’s surface. Since geostationary orbit is a finite international resource, States are required to file applications for orbital slots with the ITU on behalf of government and private entities operating under their jurisdiction and control. These longstanding procedures have seen some possible flags of convenience behaviour, whereby States attract foreign companies by providing ITU registration at lower cost and with less stringent national regulations and enforcement. For example, in the early 1990s, Tonga requested and received geostationary orbital slots and radio frequencies from the ITU before leasing them to satellite operators from other States. In 2021, Rwanda requested frequency allocations for 327,320 satellites in low Earth orbit, the largest filing ever made, despite only having launched one satellite previously.
2. Human Space Flight
14 Article V Outer Space Treaty declares that States ‘shall regard astronauts as envoys of mankind’. This recognition of shared humanity is backed up by a requirement that States render astronauts ‘all possible assistance in the event of accident, distress, or emergency landing on the territory of another State Party or the high seas’. Furthermore, if the landing is on the territory of another State party, the astronauts ‘shall be safely and promptly returned’. This requirement, which differs from air law, was largely included because of the experience of a 1960 incident where a US plane was shot down while undertaking a secret intelligence mission over the Soviet Union. The pilot was captured, tried, and imprisoned, before being released in a US-Soviet prisoner exchange.
15 The assistance and protection afforded to ‘personnel of spacecraft’ was subsequently the focus of a dedicated treaty, the 1968 Rescue Agreement, which elaborated upon these guarantees and extended the duty of return to ‘space objects’, ie, spacecraft, satellites, and space objects launched by a State into outer space. The Rescue Agreement also expanded the geographic scope of the duty to rescue, from the territory of a State and the high seas to ‘any other place not under the jurisdiction of any State’ (Art. 1). Accordingly, the duty to rescue exists in Antarctica and most importantly, within outer space itself. Given the broad scope given to the duty to rescue in other treaties, including the Chicago Convention (1944), the 1979 International Convention on Maritime Search and Rescue, and the 1982 UN Convention on the Law of the Sea, it can be further concluded that the duty extends to all persons in the space domain, including commercial astronauts and passengers.
16 The status of astronauts as ‘envoys of mankind’ may have contributed to space being prominently used as an opportunity for cooperation between otherwise antagonistic States. During the 1975 Apollo-Soyuz Project, US astronauts famously shook hands with Soviet cosmonauts in low Earth orbit. From 1993–1998, Western astronauts visited the Russian space station Mir, while cosmonauts travelled aboard the US Space Shuttle. This led to Russia’s full partnership in the International Space Station, which has proven immune from political tensions on Earth, including those resulting from the 2003 US invasion of Iraq (Iraq, Invasion of ), the 2014 Russian annexation of Crimea, and the 2022 Russian invasion of Ukraine.
3. Arms Control
17 The 1963 Partial Test Ban Treaty was the first of a series of arms control treaties directed at the demilitarization of space. Now ratified by 125 States, it has a record of perfect compliance. The prohibition on nuclear explosions in space is likely therefore now also part of customary international law.
18 In 1967, the Outer Space Treaty banned the deployment and use in space of ‘any objects carrying nuclear weapons or any other kinds of weapons of mass destruction’ (Art. IV; Weapons of Mass Destruction). To enable a successful conclusion to the negotiations, which occurred at the height of the Cold War (1947–91), the treaty avoided the issue of nuclear weapons transiting space on intercontinental ballistic missiles (‘ICBMs’). The Outer Space Treaty further requires that the Moon and celestial bodies be used ‘exclusively for peaceful purposes’ (Art. IV; Peaceful Purposes). Neither of these limits constrain the use of artificial satellites to support military activities on Earth’s surface since that use was already well underway at the time.
19 In 1972, the Strategic Arms Limitation Talks (SALT) between the United States and the Soviet Union led to the SALT I treaty, which limited the number of silos and launch tubes available for ICBMs (Missile Warfare). These limitations were revised and superseded by the 2010 New START treaty, which was extended in 2021 and was expected to remain in force until 2026 (Strategic Offensive Arms, Treaties on Reduction and Limitation [START]) until Russia ‘suspended’ its participation (or in the US view, illegally ceased participating) in February 2023. The SALT and START treaties also prohibit interference with ‘national technical means of verification’, namely reconnaissance satellites used for verifying compliance with arms control commitments (Art. X New START Treaty; Art. V SALT I Treaty).
20 Another result of the 1972 US–Soviet talks was the Treaty on the Limitation of Anti-Ballistic Missile Systems (‘ABM Treaty’) which sought to preserve the status quo of ‘mutually assured destruction’ by, among other things, prohibiting space-based anti-ballistic missile systems. In 2001, President George W Bush announced the United States’ withdrawal from the ABM Treaty, leaving all States free to pursue space-based anti-ballistic missile technology. In response to this and the increased militarization of space generally, China and Russia introduced in 2008 and again in 2014 a proposed Treaty on the Prevention of the Placement of Weapons in Outer Space, the Threat or Use of Force Against Outer Space Objects (‘PPWT’). The US has blocked negotiations on the PPWT at the Conference on Disarmament, a permanent forum linked to but not formally part of the United Nations (Disarmament), based on concerns that it fails to address direct ascent (ie, ground-based) anti-satellite (‘ASAT’) weapons. The proposed PPWT is however consistent with the ‘Prevention of an Arms Race in Outer Space’ (‘PAROS’) resolution of the UNGA which is passed annually and with near unanimity. It is also consistent with a resolution on ‘No First Placement of Weapons in Outer Space’ adopted by the UNGA on five occasions since 2015.
4. International Cospas-Sarsat Programme
21 Around the globe, individuals venturing into the remote wilderness are encouraged to carry satellite search-and-rescue beacons, while most ships and aircraft are required to be equipped with such beacons by law. The International COSPAS-SARSAT Programme (‘Cospas-Sarsat’) coordinates the detection and location of activated beacons and ensures that this information is promptly sent to the relevant authority responsible for search-and-rescue in the territory or maritime zone from which the distress signal is received (Byers and Boley  at 363–65). It uses a network of satellites that provide coverage of the entire planet, including five satellites in low Earth polar orbit, 17 in geostationary orbit, and more than 50 in medium Earth orbit. The satellites in the network are owned and operated by the United States, Russia, France, Canada, India, the European Union, and EUMETSAT, the European Organization for the Exploitation of Meteorological Satellites. Dozens of ground stations, including at least one in China, track the satellites and receive signals relayed by them. Information about distress signals and their locations is distributed to search-and-rescue centres in over 200 countries and territories, at no cost to the owners of the beacons or to the authorities that conduct the rescues.
22 Cospas-Sarsat was created by Canada, France, the Soviet Union, and the United States in 1979. In 1988, the four States grounded the system in a treaty: The International COSPAS-SARSAT Programme Agreement. Cospas-Sarsat is now an international organization with 43 Member States and a permanent secretariat located in Montreal, Canada (International Organizations or Institutions, Secretariats). Over four decades, it has helped rescue at least 45,000 people by guiding more than 13,000 search-and-rescue missions worldwide.
5. Disasters Charter
23 The Charter on Cooperation to Achieve the Coordinated Use of Space Facilities in the Event of Natural or Technological Disasters (‘Disasters Charter’) is a nonbinding agreement that has, since its inception in 2000, saved many thousands if not millions of lives (Non-Binding Agreements). It began as a partnership of the Canadian, French, and European Space Agencies, which made their Earth imaging satellites available to the new system. It is open to signature by any space agency or private company wishing to cooperate in achieving a simple goal: to provide emergency responders in any country with imagery showing the location or extent of damage caused by a ‘natural or technological disaster’, and to do so quickly and without charge. The Disasters Charter is primarily a coordination mechanism. When a disaster strikes, the affected State only needs to make one telephone call. Analysts are on duty 24 hours a day to receive requests and determine which satellites are best suited to provide the relevant imagery. Yet there is no permanent secretariat: coordination of the system is rotated among member governments, who take on the responsibility for six months at a time.
24 Imagery is currently (as of May 2023) supplied by 270 satellites, including from the United States, Russia, China, Canada, and the European Union, alongside several satellite companies that participate directly. No matter the recipient, the imagery is provided for free, with the supplying entities absorbing the costs of tasking their satellites and downloading, processing, and distributing the relevant data. The Disasters Charter represents an implementation of the general duty to rescue, as discussed above in the context of the Rescue Agreement. Its humanitarian motivation is confirmed by the fact that, seven months after the Russian invasion of Ukraine, the United States and France responded to a Russian request after floods in its eastern region by sending imagery. The Disasters Charter further demonstrates that non-binding agreements can sometimes be extremely effective—if States are willing to cooperate, as they are in space.
6. International Space Station
25 The International Space Station (‘ISS’) is the most expensive and complex object ever built by humanity. It has been occupied continuously, by various combinations of astronauts and cosmonauts, since November 2000. The ISS is governed by the International Space Station Intergovernmental Agreement, which was signed in 1998 by the United States, Canada, Japan, Russia, and 11 Member States of ESA (with ESA participation expanding since then). The treaty provides a governance framework for the ISS and covers issues as diverse as intellectual property and criminal jurisdiction. Russia remains a full partner in the ISS notwithstanding the tensions created by its invasion of Ukraine. In July 2022, Russia and the United States signed a new agreement on integrating flights to the ISS. This will continue the practice of some US astronauts travelling on Russian Soyuz spacecraft, while some Russian cosmonauts travel on the SpaceX Crew Dragon.
F. Current Issues
1. Space Debris on Earth
26 In May 2020, a 20-tonne core stage of a Chinese Long March 5B rocket re-entered the atmosphere from low Earth orbit in an uncontrolled manner after being used to launch an experimental capsule. Debris from the rocket body, including a 12-metre-long pipe, struck two villages in Côte d’Ivoire, causing damage to several buildings. One year later, another 20-tonne core stage of a Long March 5B rocket made an uncontrolled re-entry after being used to launch part of China’s Tiangong space station into low Earth orbit. This time, the debris crashed into the Indian Ocean. Then, in August 2022, after another Long March 5B re-entry, debris was found near villages in Malaysia and Indonesia. These three rocket stages were the heaviest objects to re-enter in an uncontrolled manner since the Soviet Union’s Salyut-7 space station in 1991 (Byers and Boley  at 114–29).
27 China received criticism for imposing the re-entry risks of its rockets onto the world. However, there is no international consensus on the acceptable level of risk, and other spacefaring States make similar choices concerning uncontrolled reentries. In 2016, the second stage of a SpaceX rocket was abandoned in orbit; it reentered one month later over Indonesia, with two refrigerator-sized fuel tanks reaching the ground intact. In 2020, over 60 percent of launches to low Earth orbit resulted in a rocket body being abandoned in orbit. These remain in orbit for days, months or even years, before they eventually return to Earth in an uncontrolled manner, with a substantial fraction of their mass surviving the heat of atmospheric re-entry. Many of the surviving pieces are potentially lethal, posing serious risks on land, at sea, and to aircraft.
28 In the United States, the Orbital Debris Mitigation Standard Practices (‘ODMSPs’) apply to all launches and require that the risk of a casualty from a re-entering rocket body is below a 1-in-10,000 threshold. However, the US Air Force waived the ODMSP requirements for 37 of the 66 launches conducted for it between 2011 and 2018, on the basis that it would be too expensive to replace non-compliant rockets with compliant ones. NASA waived the requirements seven times between 2008 and 2018, including for an Atlas V launch in 2015 where the casualty risk was estimated at 1 in 600. In addition to being frequently waived, the 1-in-10,000 threshold makes little sense in an era when new technologies and mission profiles enable controlled re-entries. It also fails to address low-risk, high-consequence outcomes, such as a piece of a rocket stage crashing into a high-density city or a large passenger airplane. In the latter case, even a small piece could cause the fatalities of all those aboard. A similar issue exists concerning satellites, the hardened components of which often survive atmospheric re-entry. There can also be a risk associated with fuel on board a re-entering satellite or rocket body, with the radioactive debris from Cosmos 954 in 1978 (mentioned above) being the most prominent example.
29 Internationally, there is no clear and widely agreed casualty risk threshold. The 2010 UN Space Debris Mitigation Guidelines recommend that re-entering spacecraft not pose ‘an undue risk to people or property’, but do not define what this means (Guideline 6). The 2018 UN Guidelines for the Long-term Sustainability of Outer Space Activities call on national governments to address risks associated with the uncontrolled re-entry of space objects, but do not specify how. There is no binding treaty that addresses rocket body re-entries, apart from the 1972 Liability Convention, which stipulates that a ‘launching State shall be absolutely liable to pay compensation for damage caused by its space object on the surface of the earth or to aircraft in flight’ (Art. II). Although the possibility of liability often induces responsible behaviour, on this issue national governments have apparently chosen to bear the slight risk of having to compensate for one or more casualties, rather than to require launch providers to make expensive technological or mission design changes.
2. Space Debris in Orbit
30 There are more than 10,000 pieces of ‘tracked debris’ in orbit. This term refers to pieces of space debris large enough to be catalogued and reliably reacquired through observation, typically with diameters of 10 cm or greater. Such tracked debris has grown exponentially over the six decades of the Space Age, with the number of smaller, untracked but still potentially lethal debris estimated to be 100 million or more. This proliferation of debris will likely continue as the number of satellites in low Earth orbit grows, increasing the risks of failures and collisions. Rocket bodies abandoned in orbit can also fragment into thousands of pieces after either a collision or the explosion of residual fuel. In 1978, NASA scientists Donald Kessler and Burton Cour-Palais identified that a collisional cascade would occur when the density of objects in low Earth orbit, arising largely from debris, reaches a point where collisions create more debris than can be removed by the natural process of gas-drag. Simulations of the long-term evolution of debris suggest that we are already seeing the early and still slow-moving stages of what is now called the Kessler-Cour-Palais Syndrome.
31 Active debris removal involves capturing space debris and either de-orbiting it or manoeuvring it into a ‘graveyard orbit’ where there is less risk of a collision, at least in the short term. This emerging technology could be legally contentious, should one State seek to retrieve debris launched by another State without having its permission. This is because States retain jurisdiction and legal responsibility over spacecraft that have stopped functioning or even fragmented, with the Liability Convention defining ‘space object’ as including ‘component parts of a space object as well as its launch vehicle and parts thereof’ (Art. I (d)).
32 Finally, there is the issue of liability itself. Under the Liability Convention, any damage caused by a space object ‘on the surface of the Earth or to aircraft in flight’ involves absolute liability (Art. II), whereas damage caused ‘elsewhere’—ie, in space—involves fault-based liability (Art. III). Establishing causation can, however, be very difficult for damage resulting from secondary or tertiary effects, for example, damage caused to a satellite by debris from a previous collision, or damage caused to governments, companies, or individuals on Earth from the loss of the services provided by satellites. Advances in space situational awareness, which involves tracking space objects and identifying their orbits, will reduce some of these uncertainties, strengthening the role of liability as an incentive for responsible behaviour in space as ‘fault’ becomes easier to determine.
3. Space Security
33 It has long been accepted that States may use satellites to support military operations at sea, in the air, and on land. The legal issues regarding space security therefore centrally concern whether, and under what circumstances, international law would allow satellites to be targeted, either in self-defence or during an armed conflict, and whether the reasons for disallowing the use of some types of anti-satellite (‘ASAT’) weapons also require a prohibition of their testing (Outer Space, Military Uses of).
34 In November 2021, Russia struck a defunct Soviet-era satellite with a ground-based missile. Although the test was (in retrospect) clearly part of Russia’s buildup to the invasion of Ukraine, it was hardly unprecedented. All the major spacefaring States have tested similar ‘kinetic’ ASAT weapons, which use violent strikes to destroy their targets, including the United States in 1985, China in 2007, and India in 2019. These States desire a kinetic ASAT weapon capability because modern militaries are dependent on satellites for communications, navigation, reconnaissance, situational awareness, and targeting. Furthermore, because satellites are fragile pieces of equipment that travel on predictable paths, they are easy to attack and very difficult to defend.
35 Yet no State has ever used a kinetic ASAT weapon against another State’s satellite. Such weapons involve high-energy collisions at relative speeds of up to 35,000 km/hr. They consequently create large amounts of debris and send it on a wide range of orbits. This debris then poses a collision risk to any further satellites crossing those orbits. This could lead to a collisional cascade (the Kessler-Cour-Palais Syndrome) as one collision leads to more collisions, and so on. The physics of orbital dynamics thus creates a situation of ‘mutually assured destruction’ with regard to satellites. Any State that initiates armed conflict in outer space risks losing more—its own satellites, and possibly safe access to valuable regions of orbit—than it could hope to gain.
36 A further, legally important consideration is that striking satellites could have disastrous consequences for the billions of civilians who depend upon them for communications, weather forecasting, food production, and disaster relief. A proportionality requirement (Proportionality and Collateral Damage) is part of the right of self-defence, which in turn is part of the ius ad bellum governing the recourse to force. A similar requirement is found in international humanitarian law (Humanitarian Law, International), within the ius in bello governing the conduct of armed conflict. For these reasons, it is difficult to imagine a lawful use of a kinetic ASAT weapon due to the indiscriminate and disproportionate nature of its effects.
37 The recent, rapid development of low Earth orbit has led to renewed efforts at arms control. In 2021, the UNGA established an ‘open-ended working group’ with a mandate to consider ‘current and future threats by States to space systems, and actions, activities and omissions that could be considered irresponsible’ and make ‘recommendations on possible norms, rules and principles of responsible behaviours’ (UNGA Res 76/231 ). In 2022, the United States, Canada, New Zealand, Japan, Germany, South Korea, the United Kingdom, and Australia each made a unilateral declaration, thereby committing themselves under international law to never conduct kinetic ASAT weapon tests (Unilateral Acts of States in International Law). These steps, combined with the general restraint concerning the use of kinetic ASAT weapons and the generally negative responses to the few tests that have recently occurred, indicate that these weapons are widely regarded as a threat to the freedom of exploration and use of Space. As a result, the accepted interpretation of the second paragraph of Art. I Outer Space Treaty guaranteeing this freedom could be evolving to prohibit any tests that create long-lasting space debris. A parallel rule of customary international law might also be emerging from the same State practice and accompanying opinio iuris.
38 While preserving safe access to low Earth orbit could be the most important issue of space security today, the militarization of ‘cis-lunar space’ is also emerging as a challenge. Cis-lunar space extends from Earth’s geostationary orbit to the Moon, its orbits, and the Earth-Moon Lagrange Points, which are locations where the gravitation forces are in balance and spacecraft can maintain their position without consuming much fuel. Lunar orbits and the Lagrange Points are all potentially very useful locations that are restricted in size. This creates a potential for competition, militarization, and conflict, and therefore a need for new rules or other coordination mechanisms. The development of the lunar surface could also raise legal problems. In 2021, the US Defense Advanced Research Projects Agency (‘DARPA’) released a diagram showing planned military facilities on the surface of the Moon, in apparent disregard of Art. IV Outer Space Treaty, which prohibits the ‘establishment of military bases, installations and fortifications’ on the surface of celestial bodies.
39 Astronomers started calling for reductions in the number and brightness of satellites in 2019 after a telescopic image from Chile was ruined by some of the first satellites in SpaceX’s Starlink mega-constellation. Several companies are now working on brightness mitigation for their satellites, with only moderate success. Others are concerned that, if some licensing States impose rules on brightness and others do not, this could put them at a competitive disadvantage. Seen from a broader perspective, these sorts of concerns are hardly new: comparable ones have been voiced in nearly every industry that operates internationally, with a common response being multilateral negotiations leading to internationally agreed rules. Done well, these rules ensure that every actor is subject to the same standards, thus discouraging ‘free riding’ and the emergence of flags of convenience (Byers and Boley  at 98–99).
40 In the meantime, it is possible that international law already requires States to prevent or at least reduce the interference caused to astronomical observatories. Article I Outer Space Treaty recognizes that space is free for ‘exploration and use by all States without discrimination of any kind’. When this provision is interpreted according to the customary international law rules on treaty interpretation that are codified in the VCLT, it becomes clear that mega-constellations and astronomical observatories are two competing exercises of the freedom of ‘exploration and use’. Astronomy has since ancient times utilized space for understanding natural phenomena and discovering and testing physical laws. Indeed, for many States, including spacefaring States, astronomy remains the primary means by which space is explored.
41 Further to this, Art. IX Outer Space Treaty requires that States ‘conduct all their activities in outer space … with due regard to the corresponding interests of all other States Parties’. Since the treaty does not elaborate on what due regard entails, this provision must also be interpreted per the same rules of treaty interpretation. This process quickly takes us to the now well-established rule of customary international law set out in Principle 21 1972 Stockholm Declaration: ‘States have … the responsibility to ensure that activities within their jurisdiction or control do not cause damage to the environment of other States or of areas beyond the limits of national jurisdiction’ (Stockholm Declaration  and Rio Declaration ). Many multilateral environmental treaties now include this obligation, and the International Court of Justice (ICJ) has referred to it on numerous occasions (Environment, Multilateral Agreements).
42 The duty of due regard in the Outer Space Treaty, interpreted in accordance with developments since 1967, also engages the precautionary principle (Precautionary Approach/Principle). Principle 15 1992 Rio Declaration reads:
Today, the precautionary principle entails a responsibility, on the part of a State, to conduct an environmental impact assessment prior to authorizing an activity that could cause damage to the environment of areas beyond national jurisdiction (Environmental Impact Assessment). Yet mega-constellations have not to date been subject to such environmental impact assessments.
5. Space Tourism
43 Seven space tourists visited the International Space Station on Soyuz spacecraft between 2001–2009, paying the Russian space agency for the privilege. In 2021, three US-based space companies began using their own rockets for tourism. Blue Origin and Virgin Galactic offer sub-orbital flights, although only the former reaches an altitude of 100 km, the so-called Kármán Line. Virgin Galactic instead attains the US Air Force’s definition of space, which is 50 miles (approximately 80 km), and the failure to go higher engages questions of where outer space begins and whether these tourists meet the definition of astronauts. The question of where space begins could also have legal consequences in the event of an accident, determining whether absolute or fault-based liability applies.
44 In the event of an accident during a suborbital flight, the spacecraft will almost inevitably come down on the territory of the ‘launch State’, and as a result, the 1968 Rescue Agreement will not be engaged. But the question of whether space tourists trigger the duty to rescue does arise in orbital tourism, which SpaceX began offering in 2021. Nothing in the Outer Space Treaty or the Rescue Agreement indicates that the duty to rescue in space is limited to astronauts employed by a State. Indeed, the generic term ‘personnel of spacecraft’ is used throughout the Rescue Agreement, which, when the rules of treaty interpretation are applied, clearly includes nongovernmental spacecraft. Furthermore, the various terms adopted in the four space treaties concluded between 1967 and 1974—‘astronauts’, ‘personnel of a space object’, ‘personnel of a spacecraft’, and ‘persons on board a space object’—collectively indicate that the protection conferred by these treaties covers all persons participating in spaceflight, regardless of status. This interpretation is also consistent with a broader duty to rescue human beings in distress, as found in treaties on aviation and shipping as well as in customary international law (as discussed above).
6. Space Mining
45 Article II Outer Space Treaty’s prohibition on ‘national appropriation’ is at the centre of a debate about space resource utilization (‘space mining’) and whether property rights may be acquired over resources on the Moon, asteroids, or other celestial bodies. It is the position of the United States that this prohibition applies only when resources are ‘in place’ and once extracted, they may be lawfully purchased and sold. This is a controversial position, opposed by numerous States including Russia, China, and the Group of 77 (G77), which currently comprises 134 developing States (Developing Countries).
46 One can interpret the Outer Space Treaty as simply not addressing space mining, with the issue having been left deliberately unsettled. The negotiation of the 1979 Moon Agreement, which centrally addresses space mining, suggests a collective understanding that a gap remained to be filled. But while the United States participated in the Moon Agreement negotiations, it has in subsequent years gone out of its way to dismiss this treaty, likely because it includes the principle of the common heritage of mankind and provides a mechanism for initiating multilateral negotiations on an international governance regime for space mining. As of May 2023, the Moon Agreement has only 18 States parties, with this expected to drop to 17 States parties in January 2024, as Saudi Arabia formally deposited a one year notice of treaty withdrawal in January 2023 (Treaties, Withdrawal).
47 The US position is that the Outer Space Treaty, by not explicitly prohibiting space mining, thereby allows it. This is an application of the ‘Lotus principle’ (Lotus, The). The United States has sought to strengthen its position through subsequent practice, which can be a relevant factor in treaty interpretation. It has enacted national legislation permitting space mining, encouraged other States to do likewise, and negotiated a series of bilateral agreements, called the Artemis Accords, which include the statement ‘extraction of space resources does not inherently constitute national appropriation under Article II of the Outer Space Treaty’ (Section 10). But these Artemis Accords also state that they ‘represent a political commitment to the principles described herein’ (Section 1) which indicates that they are non-binding agreements and therefore of less significance with regard to subsequent practice for the purposes of interpreting the prohibition against national appropriation. As of May 2023, 23 States have been persuaded to sign the Artemis Accords with the United States.
48 In 2020, then-NASA Administrator Jim Bridenstine announced that NASA was seeking to purchase lunar regolith (the loose material that forms the surface of the Moon) from private companies to further contribute to subsequent practice in support of the US interpretation of Art. II Outer Space Treaty. Contracts with four companies were signed, requiring regolith to be collected and NASA provided with both imagery of the material and data regarding its location. NASA would then purchase the regolith through an ‘in-place ownership transfer’, without any commitment or even a plan to pick it up in the future.
49 In 2021, with support from States on both sides of the issue, the Legal Subcommittee of COPUOS created a working group on ‘space resource activities’. At the end of its five-year mandate, we can hope for greater clarity on the interpretation of Art. II as well as proposals for new, truly multilateral guidelines or rules that govern space mining.
7. Planetary Defence
50 Planetary defence involves the detection, characterization, risk assessment, and if required, deflection or destruction of ‘Near-Earth Objects’ (‘NEOs’), namely asteroids and comets, that may potentially strike Earth. Depending on their size and impact location, threats posed by NEOs can be negligible to existential. In 2020, an Ad-Hoc Working Group on Legal Issues established by the Space Mission Planning Advisory Group (‘SMPAG’) released a detailed report examining the international law considerations arising from a NEO threat. SMPAG currently comprises representatives of 18 Space agencies, including those of the United States, Russia, China, and Europe.
51 If a NEO is identified to be on an impact trajectory and is of a size that will limit damage to one or a small number of States, they clearly have the right to attempt a deflection mission. No rule of international law prohibits a unilateral or collective deflection mission directed against a NEO. However, damage caused to any third State from such a mission (for instance, if the altered trajectory results in an impact upon a State that was not initially threatened) would engage issues of State responsibility and liability. This remains the case even if the deflection was undertaken by a space company since States are responsible for the activities of national entities under Art. VI Outer Space Treaty.
52 Surprisingly, the Ad-Hoc Working Group concluded that space-capable States have no obligation to come to the assistance of States directly threatened by a NEO impact but lacking the ability to mount a deflection mission. The Working Group did not however consider general principles of law, which are recognized as a source of international law in Art. 38 (1) (c) ICJ Statute, nor, specifically, the ICJ finding in the Corfu Channel Case that ‘elementary considerations of humanity’ constitute ‘general and well-recognized principles’ of law (at 22). When this ruling is combined with an assessment of the duty to rescue human beings in distress that is found in numerous treaties on aviation and shipping, one could just as plausibly conclude that there is a duty to assist in the face of an impending NEO impact. This, though, would need to be balanced, in a fact-specific determination, against the risks and expense required of any intervening State. In other words, the duty to assist another State within the context of a NEO threat is not absolute and involves questions of proportionality.
53 A nuclear explosive device (‘NED’) is one option for deflecting a NEO threatening Earth (Nuclear Weapons and Warfare), and if the threat was discovered late, might be the only option. The use of a NED in space for any reason would violate the Partial Test Ban Treaty and customary international law. However, it could still be authorized by a resolution of the United Nations Security Council through its peremptory power under Chapter VII UN Charter over conflicting rules of international law (United Nations, Security Council). In the absence of this, a State could still conceivably employ a NED against a NEO and claim circumstances precluding wrongfulness, most likely necessity (Necessity, State of).
8. Inhabitation, Self-Determination, and Deep Space Travel
54 With two space stations currently in low Earth orbit, the inhabitation of space is underway. Building upon the experience of the International Space Station and China’s Tiangong space station, we could eventually see communities on other celestial bodies such as the Moon and Mars. This next stage in the exploration and use of space will raise new legal issues, particularly if private individuals rather than governmental astronauts are involved. The absence of any known life within our Solar System should prevent a repeat of the tragedies of colonialism, although it will not prevent questions of decolonization, self-determination, sovereignty and statehood (State) from eventually arising. Some powerful actors are already thinking ahead, with the Terms of Service for becoming a customer of SpaceX’s Starlink mega-constellation containing the following provision:
For Services provided on Mars, or in transit to Mars via Starship or other spacecraft, the parties recognize Mars as a free planet and that no Earth-based government has authority or sovereignty over Martian activities. Accordingly, Disputes will be settled through self-governing principles, established in good faith, at the time of Martian settlement (11. Governing Law).
55 As noted at the beginning of this entry, space law extends to wherever human activities in space take place. This includes robotic probes, with Voyager 1 having entered interstellar space in 2012. However, it took 35 years to reach that point, and our closest neighbouring star system of Alpha Centauri is 4.35 light-years away—a voyage of about 18,000 years at comparable speeds. One can only imagine what international law will look like then, given that the modern Westphalian System of State sovereignty is only about four centuries old.
9. Planetary Protection and Extra-terrestrial Life
56 If extra-terrestrial life is ever found, the discovery could reshape humanity’s understanding of its place in the Universe. International cooperation has already helped prepare us for this. Article IX Outer Space Treaty requires that States avoid both the ‘harmful contamination’ of the Moon and celestial bodies and ‘also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter’. The international quasi-governmental Committee on Space Research (‘COSPAR’) has elaborated on this through its Planetary Protection Policy, the most recent version of which dates to 2022. Planetary protection involves guarding celestial bodies against contamination by Earth life and guarding Earth from life forms that may be potentially returned from other bodies. Although soft law, the Planetary Protection Policy is followed by space agencies and most other space actors. One prominent instance of non-observance came in 2019 when tardigrades (nearly indestructible microscopic creatures, also called ‘water bears’) were carried on the privately funded Israeli Beresheet spacecraft which crashed while attempting to land on the Moon.
57 Discovering intelligent extra-terrestrial life would raise different issues for international law, which is of course a system developed by humans for humans. In 1956, Andrew Haley coined the term ‘Metalaw’ to encapsulate the hypothesis of fundamental legal principles that have true universality, beyond that term’s ordinary meaning in international law. It is suggested that these principles might include diplomatic immunity (Immunity, Diplomatic) and pacta sunt servanda, ie, agreements must be kept, as basic requirements of peaceful interaction.