For decades, placing solar panels in space and beaming power down to Earth sounded like science fiction. What was once a futuristic idea is now something several countries are actively building towards. Japan is at the forefront of this race. Japan Space Systems has spent decades on space-based solar power (SBSP) with backing from the Ministry of Economy, Trade and Industry, and has already shown it can form and steer a power beam over a long distance in an aircraft-based test [1]. Its next step is OHISAMA, which is a Japanese word that translates to “the sun”; On-orbit experiment of HIgh-precision beam control using small Satellite for microwave power transmission: OHISAMA [2]. Essentially, it is a government-backed SBSP project designed to demonstrate how a small satellite can harvest solar energy in orbit and beam it directly to a ground-based receiver [3]. The appeal is that SBSP addresses the main weakness of solar power on Earth. Ground-based panels stop producing at night and lose output in bad weather, while panels in orbit could collect sunlight continuously [4]. If it works, SBSP would extend solar power from a daytime, weather-dependent resource into a steady, any-time-of-day source of clean energy.
This, however, raises an unresolved legal question. When a satellite sends a beam of energy down from space to a receiving station on the ground, how should that beam be classified under international law? Is it trespassing, like a foreign plane flying over a country without permission? Could it be classified as a weapon, since a beam is powerful enough to deliver usable electricity and, in theory, powerful enough to be aimed at something else? Or is it more like a radio or TV signal, just one that carries power instead of data? Or is it something genuinely new, a category international law has never had to accommodate because nothing before it has been a continuous, functional link between a zone no state can own and a zone a state owns completely?
The most defensible framing is the most cautious one: SBSP beams should not be forced into a legal category built for something else. Under current international law, SBSP beams are not a sovereignty violation or a weapon, and they are not simply a radio signal that happens to carry power. Telecommunications and space-liability laws already provide partial, though imperfect, oversight for these beams, but the beams do not directly fall into a known category. The real work ahead is closing definitional gaps in those regimes rather than inventing sovereignty doctrine from scratch.
The core concept behind SBSP was initially proposed by the aerospace engineer Peter Glaser in 1968. He proposed placing larger solar collector arrays in orbit, where sunlight is undiluted by the atmosphere and interrupted by night. These arrays would convert the captured sunlight into electricity. The system would then convert the captured sunlight into electricity. These microwaves would be beamed down to a receiving antenna called a rectenna. Then, the rectenna would convert the microwaves back into usable grid energy [5]. The appeal is quite straightforward. Orbital solar collection is not subject to weather, seasons, or the day-night cycle that limits terrestrial solar power. Thus, a single orbital array can in theory deliver continuous baseload power.
What the Beam Is, and Why Now
The beam, mechanically, can be classified as electromagnetic energy. After solar panels generate electricity, solid-state amplifiers convert it into microwaves, which are beamed through the atmosphere to the rectenna [6]. That makes a power beam closer to a radio or television signal than to an aircraft. It has no mass and occupies no volume the way a plane or satellite does. Its safety depends on power density, the power per square meter of the beam’s cross-section.
According to Vedda and Jones, the maximum allowable microwave power level has been set to 10 watts per square meter in most countries based on Institute of Electrical and Electronics Engineers and American National Standards Institute exposure criteria. Proponents argue that spreading the beam over rectenna kilometers keeps the density at or below this limit wherever people or aircraft could be exposed. The 10 watts per square meter ceiling is about 1% of the intensity of direct sunlight. Proposed designs typically taper the beam, with the highest density at the center of the rectenna and the lowest at its edge, so the limit is applied at and beyond the perimeter, with access to the interior restricted. Designs also include active safeguards such as retrodirective pointing, in which the satellite transmits only toward a ground-based pilot signal and the beam defocuses if that signal is lost [7]. Ultimately, staying under the limit is a design tradeoff. Lower density requires a larger receiving area. But if a system holds to that limit, a person or aircraft passing through the beam would not face a harmful exposure level. Still, this must be demonstrated and regulated rather than assumed, because higher-density beams can cause serious harm.
Several states are converging on the same technology. For instance, in the United States, Caltech’s Space Solar Power Demonstration (SSPD-1) achieved the first wireless power transmission in space and detectable beaming toward Earth in 2023 [8]. China has announced that a satellite launched in 2028 would test wireless power transmission at 10 kilowatts [9]. Europe’s Solaris program, the United Kingdom's CASSIOPeiA architecture, and Japan’s OHISAMA demonstration run in parallel [10]. The existing regimes were drafted for other technologies, and early demonstrations will start to settle how their definitions are read. A race compounds the gaps between the existing law and definitions. Here, “a race” means a competition to be the first to demonstrate power beaming at an operationally meaningful scale, not simply to reach orbit first. Thus, it is important to take initiative and prepare early, if possible. This is because each program has an incentive to move ahead on its own reading of the ambiguous terms rather than wait for a coordinated interpretation.
Characterizing the Beam: An Intrusion or a Weapon?
The beam can be characterized as an intrusion, weapon, or something else entirely. The first possibility is that the beam is similar to a radio wave. Satellites already send electromagnetic signals across many countries’ territories every day through broadcasts. No state asks another for permission daily. Instead, the International Telecommunication Union (ITU) allocates frequencies, registers orbital slots, coordinates internationally, and sets rules against harmful interference. Therefore, a beam from space to a ground station is the same type of activity. So it could slot into a system that already exists, one that states are already accepting.
The second possibility is that the beam is a weapon. A gigawatt-scale microwave beam is physically similar to a directed-energy weapon, and a beam that can be aimed at a rectenna could in theory be retargeted. The question is whether that possibility alone triggers treaty restrictions. It does not, and the argument rests on a common misreading of the Outer Space Treaty (OST). Article IV of the OST bans weapons of mass destruction (WMD) in orbit generally, but its stricter “peaceful purposes only” language applies specifically to the Moon and other celestial bodies, not to Earth orbit [11]. Thus, a satellite beaming power from orbit is likely not covered by the stricter clause at all. Even if this were not the case, beaming power from a satellite would still count as a peaceful purpose, and it was not sent into orbit as a weapon. Only the narrower WMD ban could apply, and a power beam is not a WMD. This is because although the beam can exceed the maximum allowable microwave power level of 10 watts per square meter in theory, that is not the intended purpose of the beam. A WMD is intended to be used for widespread destruction or severe injury on a massive scale. Yet, the purpose of SBSP is for the collection of renewable energy. The capability to redirect a beam is not the same as classifying it as a weapon under the treaty text. Further, scholars have argued that Article IV is outdated for today’s military use of space questions, which itself indicates that the treaty was not created to address this problem [12].
State practice points the same way. Under Article 31(3)(b) of the Vienna Convention of the Law of Treaties, the subsequent practice of the parties depends on how a treaty is read [13]. The United States, China, Japan, the United Kingdom, and the European states pursuing SBSP are all parties to the OST, and each is treating power beaming as a civil energy program, rather than a prohibited weapons deployment. Still, parallel national programs do not settle the question alone. They show that states have not objected, not that they have examined and endorsed this reading. No authoritative body has yet done so.
Neither possibility currently succeeds as settled law. Intrusion fails because the beam is not the kind of object the overflight regime governs. The weapons possibility fails because the treaty text does not reach it. That does not leave the beam ungoverned, however. It means the two most intuitive categories are simply not correct solutions. They only partially reach SBSP. Yet, each possibility identifies a real concern. The intrusion possibility, that a state’s control over its territory should not depend on what a foreign satellite chooses to project, and the weapons possibility, that a beam is powerful enough to deliver electricity, could in principle be misused. However, each applies a rule built for something else, so the concern is right, but the legal fit is wrong. The question that remains is which existing regime fits the beam best.
The Regulatory Floor That Actually Exists
The first set of rules applicable to the states comes from the OST itself, governing 118 countries, and it applies regardless of how the beam is classified. Article VI makes states internationally responsible for national activities in outer space, whether carried out by government agencies or non-governmental entities, and requires that the appropriate state authorize and continuously supervise private activity [14]. A private company operating an SBSP satellite is therefore never outside the treaty framework, and its state answers for what it does, including any unauthorized modification of the system. Article VII makes the launching state internationally liable for damage its space objects cause [15]. Article IX requires parties to conduct all their activities in outer space “with due regard to the corresponding interests of all other States Parties to the Treaty.” A state that has every reason to believe its planned activity would cause potentially harmful interference “shall undertake appropriate international consultations before proceeding with any such activity or experiment" [16]. That consultation duty is exactly the mechanism the beam as an intrusion was reaching for. It addresses transited airspace and spillover without requiring a sovereignty-based ban. It does so by turning a question of consent into a question of process. The intrusion theory would have given every transited state a veto over the beam, which is unworkable for a system whose path crosses many territories. This would amount to a right to refuse that the treaty does not otherwise grant. These obligations reach even states that never joined the 1972 Convention on International Liability for Damage Caused by Space Objects (Liability Convention). Its core liability principles are widely regarded as customary international law (CIL), in the same way that non-parties to the UN Convention on the Law of the Sea (UNCLOS), such as the United States, treat its navigation provisions as binding [17]. Such states therefore have little room to disregard them.
Telecommunications law provides a second, distinct body of rules that applies alongside the OST. A power beam is physically a radio transmission, and any transmission using radio frequencies, including a microwave power beam, is already covered by the ITU's Radio Regulations [18]. The ITU defines “harmful interference” in binding treaty language as interference that endangers a radionavigation or other safety service, or that seriously degrades, obstructs, or repeatedly interrupts other radio services operating under the regulations [19]. The definition applies to all transmitting stations, whatever their purpose, so it does not matter that this transmission carries power rather than data. Membership in the ITU is close to universal, and its instruments are treaties, so these obligations bind states’ programs on the same terms as any other. This is the clearest and most enforceable existing rule that already touches SBSP beams. However, one prominent issue remains. The regulations define interference by reference to radio services, and an opponent could argue that power beaming falls outside them, so the rule reaches SBSP by extension, not by clear text. The interference duty is written for stations carrying on radiocommunication, meaning the transmission of information by radio waves, and “harmful interference” is defined by its effect on such services. A power beam transmits energy rather than information, so there are two separate questions: whether the beam counts as a radio service at all, and whether the interference duty applies to it if it does not. Applying the duty to SBSP therefore requires reading it by analogy. It also says nothing about a beam’s effect on people, property, or sovereignty beneath it. The ITU framework therefore leaves a gap. It protects radio services from interference, but it does not address physical harm to people, property, or aircraft beneath the beam. Compensating for that kind of harm falls to the liability regime.
The third layer is the 1972 Liability Convention, which applies here because the beam is generated and transmitted from a space object. According to Article II of the Liability Convention, the launching state is absolutely liable for damage its space object causes on Earth’s surface or to aircraft in flight, without any need to prove fault [20]. The drafters had falling debris in mind, so whether a beam malfunction or energy causing damage instead of a physical object counts as damage caused by a space object is open to interpretation. The Convention has been invoked only once, in the Cosmos 954 incident [21]. But nothing in the treaty reads “damage” to cover harm from an orbital system’s debris while excluding harm from its energy output, and absolute liability, designed for risks whose fault is hard for a victim to prove, fits a beam failure naturally.
The Convention’s reach supports this reading. Roughly a hundred states had ratified it as of 2021, and four intergovernmental organizations, including the European Space Agency, have accepted its obligations [22]. Even for states outside of the Convention, Article VII of the OST makes a launching state internationally liable for damage its space objects cause [23]. This is the strongest available accountability mechanism for a worst-case beam failure, even though it has never been tested.
A fourth body of law operates only once the beam reaches the ground, and it is the one least prepared for SBSP. Terrestrial energy law regulates grid connection, safety standards, and market integration, but it was built around electricity generated within a national territory. Therefore, it does not address electricity produced by orbital platforms [24]. The resulting questions are fundamental. It is unclear who counts as an energy producer, since space agencies lack a mandate to sell electricity and some national agencies are barred from commercial markets [25]. It is unclear whether energy collected in orbit qualifies as “renewable” under definitions like the European Unions’, which were written with terrestrial sources in mind [26]. Because one platform could redirect its beam to several receiving stations in several countries, the electricity could be classed as imported, as domestic, or as a new category requiring its own definition [27]. The same uncertainty reaches the applicable law when a beam malfunctions, which could be the transmitting entity’s, the receiving state’s, or that of the state registry [28]. These gaps matter alongside the treaty obligations above because liability under space law does not settle which national regulator licenses the receiving station, certifies its safety, or decides whether it counts as critical national infrastructure.
Further, in Section 11 of the Artemis Accords from 2020, the deconfliction provision, signatories agree to give each other advance notice of space activities and to set up informal safety zones so that operations do not interfere with one another [29]. This closely matches what SBSP will need: a mechanism to coordinate where and when high-power beams operate relative to other states’ territory and activities. It also echoes the binding duty in Article IX of the OST to consult before proceeding with an activity that could cause harmful interference [30]. Yet, the Accords use soft language, “intend to," instead of "shall," and set no fixed size, duration, or process for safety zones [31]. Therefore, the Accords cannot be the primary instrument for SBSP because they do not set a clear, binding timetable for states. This is a promising direction, not an enforceable rule. The law is developing in real time with nothing really settled yet. This matters because the first SBSP demonstrations will not wait for a binding framework, and in the interim the practice of the earliest operations will shape what later entrants treat as normal. A non-binding instrument can guide that process, but it cannot stand in for the enforceable definitions and obligations that the gaps identified above require.
Relevant Legal Precedent
The argument rests on a distinction. An outright prohibition is one kind of legal claim. A source of responsibility once harm occurs is another. The beam does not and should not face the first, but it can face the second. Two foundational cases, neither arising in space, draw that distinction sharply.
The Trail Smelter arbitration of 1938 and 1941 supplies the responsibility side. It established that no state may use its own territory in a way that causes serious harm to another. In this case, fumes drifted across the U.S.-Canada border from a smelter in British Columbia [32]. This fits perfectly: if a beam drifts off target, degrades and widens its footprint, or causes damage across a border, Trail Smelter gives the harmed state a basis for holding the operating state responsible. No SBSP-specific treaty is needed based on this precedent. This also succeeds where the intrusion theory failed. That theory treated the beam’s mere transit through airspace as a wrong in itself. Trail Smelter instead locates the wrong in the harm the beam actually causes. That fits what a beam is: not a trespasser crossing a line, but a physical effect that can be measured and traced back to its source. Since the principle is within the framework of CIL, it applies even to states that never joined the OST and Liability Convention. This closes one of the gaps the treaty-based regimes leave open.
The Lotus case, decided in 1927, pulls in the opposite direction. In this case, a French mail steamer, Lotus, and a Turkish coal-carrying ship collided. The Turkish ship sank, and this resulted in eight Turkish deaths. When Lotus arrived in Constantinople, Turkish authorities arrested the French officer on watch and tried him for manslaughter. The legal question was whether or not this was permissible as the incident happened in international waters. The court ruled that Turkey did not violate international law by prosecuting the French officer. This established the Lotus Principle: holding that what is not explicitly prohibited by international law is permitted [33]. No treaty specifically forbids beaming power through another state’s airspace, so Lotus could support the view that SBSP operators may proceed freely until a rule says otherwise. This is the same reasoning that would treat the lack of an SBSP-specific treaty as proof of a legal gap.
These two cases highlight the core conflict surrounding the strategic approach to SBSP. Trail Smelter says cross-border harm creates responsibility even without a specific rule. Lotus says the absence of a specific rule is itself permission. Yet, Lotus only grants permission where no rule reaches the conduct at all, and here several already do. The ITU’s interference rules, the Liability Convention's absolute liability standard, and the OST’s own consultation and responsibility duties are exactly the specific rules Lotus has in mind. They already exist, which is why the “already governed” reading beats the “ungoverned” one.
The Core Question Answered and What Is To Come
A beam of power from orbit looked like it could belong to four distinct legal categories: a sovereignty violation, a weapon, a telecommunications analogy, or something with no clean category yet. The analysis above supports the conclusion that the beam is neither a sovereignty violation nor a weapon. Yet, it is not ungoverned either. The first two categories misdescribe it, and the third partially captures it.
Law that reaches SBSP falls into three tiers, ranked by how much work each can do today. At the top are existing rules that already bind the beam, in the middle sits an emerging framework that signals direction without binding anyone, and at the bottom are two classification theories that fail but clear the ground for the rest. The strongest tier consists of rules built for something else that happened to reach the beam anyway: the ITU’s harmful interference standard, the Liability Convention’s absolute liability, and consultation. These bind now. They apply no matter how the beam is classified, and they need no new treaty to take effect. This tier is the direct evidence that SBSP is already, imperfectly, governed.
The weakest tier is the two theories that were tested and rejected: sovereignty intrusion and weaponization. Both failed for the same reason: each tried to fit the beam into a category built for something else, whether a physical vehicle or a device built or converted to destroy. Even though both theories were rejected, that does not signal that the beam is unregulated. It clears the ground so the stronger tier can do the work. That is the proposition’s second claim: the real task is closing gaps in the regimes that already apply, not inventing a new sovereignty doctrine from no basis.
Between these two tiers sits the Artemis Accords’ deconfliction framework. It shows where the law is heading, but it cannot yet bind anyone. Section 11 committed signatories to notify one another of their operations and coordinate to avoid harmful interference, and it provides for “safety zones” that expressly do not amount to sovereignty claims [34]. These commitments refine the due-regard and consultation duties already in Article IX of the OST [35]. The Accords are political commitments, not treaty obligations. The Accords matter less as a source of obligation than as evidence of how the Article IX duties are being refined.
That accountability is a temporary solution, not a long-term one. Each of these regimes was built for a narrower problem: interference between radio users, damage from falling debris, and resource extraction. Stretching them to cover a continuous, cross-border energy transmission leaves the real gaps. There is no settled boundary between airspace and outer space, no binding process for prior notification or consent, no way to verify that a system has not been converted from civilian to hostile use, and no clear answer on whether “damage” under the Liability Convention includes harm from an energy beam.
A fair objection is that if existing law already binds the beam partially, no new treaty or language is needed. From this point of view, the ITU’s interference rules, the Liability Convention’s absolute liability, Article IX consultation, and the customary rule from Trail Smelter already give states what they need. Negotiating a dedicated instrument would be slow, politically uncertain, and risky if it reopened principles that states consider settled. Yet, this objection understates the cost of waiting. Rules that reach the beam only by extension will be interpreted for the first time only after failure or dispute. This will only happen when states have the strongest incentive to read them in their own favor. A dedicated instrument may be slower to negotiate than an interpretation is to assert. But it is the only way to settle these questions by agreement rather than by who acts first. This is why these gaps need to be closed now, while the first systems are still demonstrations.
Closing those gaps, rather than litigating sovereignty or weapons law from scratch, is the work that the field actually needs. The most defensible path is a dedicated international instrument that bridges space law and energy law. It should be built for what SBSP actually is, rather than for the debris, broadcasting, and resource-extracting problems the current patchwork was designed to solve. Based on these regulatory gaps, the following suggested treaty language illustrates how a future international agreement could address SBSP without replacing the existing legal framework.
The priority would be to define the technology without turning it into a sovereignty claim. Suggested treaty text could define an “SBSP System” as “a space object whose function is to convert solar energy and transmit it as a directed beam to a receiving station,” while defining a “Power Beam” as “electromagnetic energy transmitted for the purpose of delivering usable power. It could then state that a “Power Beam is not, by reason of transit alone, a violation of sovereignty,” making notice, consultation, and responsibility the consequences of transit rather than automatic sovereignty violation.
Such an approach could take the useful part of the sovereignty theory without adopting its conclusion. A transited state would have a meaningful role in addressing safety and interference concerns, but not an automatic veto over the beam simply because it crosses its territory.
The same principle applies to weaponization. Rather than classifying SBSP systems as weapons because they could theoretically be retargeted, suggested treaty language could require “transparency by design.” Each system could include a mechanism preventing transmission unless an authorized receiving station is actively locked on, a fixed power ceiling, and records of beam targeting and output. This would address the possibility of dual use through technical safeguards rather than attempting to prohibit the underlying technology.
Liability presents the remaining major gap. The Liability Convention provides an important foundation, but its application to deliberate or accidental energy transmission is not settled. A future agreement could therefore clarify that “Beam Damage” is treated as damage caused by a space object and that the launching State is absolutely liable for such damage occurring on Earth’s surface or to an aircraft in flight. This would extend an existing liability structure to a new form of potential harm rather than creating an entirely separate regime.
Thus, closing the gaps, rather than developing new sovereignty or weapons doctrines through litigation, becomes the work the field actually could utilize. A future agreement could turn the existing patchwork into a more coherent framework. It can do so by defining technology, establishing safeguards against misuse, and clarifying liability without treating SBSP as a sovereign intrusion or a weapon.