At first glance, the modern space race has little in common with the traditional space age. Instead of being limited to a few superpowers, a growing number of nations and private actors are involved. Science and exploration remain important motivating forces, but stakeholders are increasingly setting their sights on commercial activities like resource extraction. Nonetheless, the multilateral treaty framework governing activities in outer space has not evolved – and isn’t likely to do so. Instead, space actors may become increasingly reliant on customary international law (“CIL”) for legal standards governing activity beyond the Earth’s atmosphere. However, defining and evaluating existing CIL presents its own challenges.
The rights and obligations determined by CIL differ in several ways from those formally codified in written treaties. Generally, CIL exists where a general and consistent state practice is accepted as law. While a treaty only binds states that are a party, CIL binds all states, unless they consistently object. In some instances, the mere prospect that an agreement could become CIL may inhibit a state from signing on. CIL is a powerful source of law, but defining or elaborating CIL is an imprecise exercise.
The International Court of Justice itself has rarely stated its methodology for determining the existence, content, and scope of the rules of CIL it applies. While there is a general consensus that CIL is determined via inductive reasoning , there are situations (like space law) where a practice is too new, creating a need for deductive reasoning. Inductive reasoning involves inferring a general rule from a pattern of state practice and legal obligation, while deductive reasoning extracts a specific rule from existing and generally accepted principle. Early space exploration demonstrates examples of both inductive and deductive approaches to evaluating CIL.
The U.N. General Assembly passed its first resolution on space activities in 1961. The resolution espoused foundational principles on space exploration: that outer space is open and free for exploration and use by all states; state sovereignty does not extend to outer space, outer space is not subject to national appropriation; and states retain control and jurisdiction over objects launched into outer space. These principles were codified in Articles I and II of the OST and are also accepted as CIL.
Examples of the “deductive” approach to CIL abound in the early years of space exploration. The launch of Sputnik created “instant” CIL –states immediately acquiesced (by not objecting) to the principle that outer space was beyond territorial sovereignty. Other early examples include the recognition of the boundary between air space and outer space at 100 km above sea level and the acceptance of the right to carry out remote sensing of natural resources without the consent of the targeted state.
In many other areas of space law, CIL is contested or unclear. Given the small sample size of state actors, evaluating state practice is difficult. For example, both Soviet probes and the U.S. Challenger flew over adjacent states in the 1960s. Those states did not protest, suggesting that CIL may provide a right of free passage of space objects on their way into or out of orbit. However, some argue that this right is limited to the airspace of states adjacent to major powers and that the practice is not sufficiently general to be CIL. In the case of anti-satellite weapons (“ASAT”), the practice of a comparably limited number of states was determined not to constitute CIL. Just a few states have the capability to test or use ASAT weapons, and of those only three states that actually possess those weapons have found it unnecessary to deploy them – and there’s no indication that the restraint is a product of legal obligation.
The idea that accepted custom can evolve over time further complicates evaluations of CIL. Article II of the OST banned the national appropriation of space resources, an ideal that was widely regarded to be CIL at the time. As time passed, state practice, domestic legislation, and legal scholarship demonstrate a shift in CIL that now considers the extraction of space materials a permissible “use” under Article II. The CIL regarding the non-appropriation principle still bans the private ownership of real property, but this too could evolve.
As commercial space becomes a reality and new actors grapple with the legal voids in the current system, questions about the nature and scope of CIL will become more important. States may increasingly rely on applying settled CIL principles lifted from environmental law, chemical weapons law, and other areas. Non-state actors may take on an unprecedented role in the development of CIL. Or, the criteria for defining and elaborating CIL in space may continue to expand and evolve.
 See Steven J. Markovich, Andrew Chatzky, and Anshu Siripurapu, Space Exploration and U.S. Competitiveness, Council on Foreign Relations, (June 10, 2020), https://www.cfr.org/backgrounder/space-exploration-and-us-competitiveness.
 Drafted and signed in the context of the Cold War, five treaties make up the basis for the international law of outer space. See Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, Jan. 27, 18 U.S.T. 2410, 610 U.N.T.S. 205 [hereinafter Outer Space Treaty]; Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space, Apr. 22, 1968, 19 U.S.T. 7570, 672 U.N.T.S. 119; Convention on International Liability for Damage Caused by Space Objects, Mar. 29, 1972, 24 U.S.T. 2389, 961 U.N.T.S. 187; Convention on the Registration of Objects Launched into Outer Space, Jan. 14, 1975, 28 U.S.T. 695, 1023 U.N.T.S. 15; Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, Dec. 5, 1979, 1363 U.N.T.S. 3 [hereinafter Moon Agreement].
 See Vladlen S. Vereshchetin & Gennady M. Danilenko, Custom as a Source of International Law of Outer Space, 13 J. Space L. 22, 23 (1985)(citing the growing lack of consensus, technological challenges, and other limitations on the utility of treaties).
 Ram S. Jakhu & Steven Freeland, The Relationship between the Outer Space Treaty and Customary International Law, 59 Proc. Int’l Inst. Space L. 183, 186 (2016).
 Id. at 23
 See Michael Listner, Customary International Law: A Troublesome Question for the Code of Conduct?, The Space Review (Apr. 28, 2014), https://www.thespacereview.com/article/2500/1.
 Jakhu, supra note 5, at 189.
 Stefan Talmon, Determining Customary International Law: The ICJ’s Methodology between Induction, Deduction and Assertion, 26 Eur. J. of Int’l L. 418, 418 (2015).
 Id. at 420.
 UN G.A. Res 1721 (XVI) (Dec. 20, 1961).
 Jakhu, supra note 5, at 191.
 Id., at 190.
 Vereshchetin, supra note 4 at 28,
 David A. Koplow, Asat-isfaction: Customary International Law and the Regulation of Anti-Satellite Weapons, 30 MICH. J. INT’l L. 1187 (2009).
 Fabio Tronchetti, The Non-Appropriation Principle Under Attack: Using Article II of the Outer Space Treaty in Its Defense, 50 Proc. l. outer space 526, 530 (2007).
 Abigail D. Pershing, Note, Interpreting the Outer Space Treaty’s Non-Appropriation Principle: Customary International Law from 1967 to Today, 44 Yale J. of Int’l . 149, 162-167 (2019).
 David A. Koplow, Asat-isfaction: Customary International Law and the Regulation of Anti-satellite Weapons, 30 Mich. J. Int’l L. 1187, 1247-1258 (2009).
 Valentina Vecchio, Customary International Law in the Outer Space Treaty: Space Law as Laboratory for the Evolution of Public International Law, 66 Zlw 491, 501 (2017).
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