For this new Space Law article on Space Legal Issues, let’s focus on solar sails and their legal status. Solar sails (also called light sails, or photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. Based on the physics, a number of spaceflight missions to test solar propulsion and navigation, have been proposed since the 1980s. Let’s have a quick look at them and the legal status of these particular spacecraft.
A useful analogy to solar sailing may be a sailing boat; the light exerting a force on the mirrors is akin to a sail being blown by the wind. High-energy laser beams could also be used as an alternative light source to exert much greater force than would be possible using sunlight, a concept known as beam sailing. Solar sails offer the possibility of low-cost operations combined with long operating lifetimes. Since they have few moving parts and use no propellant, they can potentially be used numerous times for delivery of payloads.
Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet (or small body). A typical spacecraft going to Mars, for example, will be displaced thousands of kilometres because of solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the orientation of a spacecraft, a factor that must be included in spacecraft design.
The Russian Konstantin Tsiolkovsky first proposed using the pressure of sunlight to propel spacecraft through space and suggested “using tremendous mirrors of very thin sheets to utilise the pressure of sunlight to attain cosmic velocities”. Potential applications for solar sails range throughout the Solar System, from near the Sun, to the comet clouds beyond Neptune. The craft can make outbound voyages to deliver loads, or to take up station keeping at the destination. They can be used to haul cargo and possibly also used for human travel.
IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun), the first spacecraft to successfully demonstrate solar sail technology in interplanetary space, is a Japan Aerospace Exploration Agency (JAXA) experimental spacecraft. The spacecraft was launched on May 20, 2010, aboard an H-IIA rocket (an active expendable launch system operated by Mitsubishi Heavy Industries), together with the Akatsuki, also known as the Venus Climate Orbiter (VCO) and Planet-C, a Japanese space probe tasked to study the atmosphere of Venus. On December 8, 2010, IKAROS passed by Venus at about eighty thousand kilometres distance, completing the planned mission successfully, and entered its extended operation phase.
LightSail is a project to demonstrate controlled solar sailing within Low Earth Orbit (LEO) using a CubeSat. The project was developed by The Planetary Society, a global non-profit organisation devoted to space exploration. It consists of two spacecraft: LightSail 1 and LightSail 2. LightSail 1 was an engineering demonstration mission designed to test its new sail deployment method in outer space, while LightSail 2, launched on June 25, 2019 by SpaceX, is a fully functional spacecraft intended to demonstrate true solar sailing. As a solar sail, LightSail’s propulsion relies on solar radiation, and not the charged particles of the solar wind.
The legal status of solar sails
The term Object in reference to outer space was first used in 1961 in General Assembly Resolution 1721 (XVI) titled International cooperation in the peaceful uses of outer space to describe any object launched by States into outer space. Professor Bin Cheng, a world authority on International Air and Space Law, has noted that members of the COPUOS during negotiations over the space treaties treated spacecraft and space vehicles as synonymous terms. The Space Object can be considered as the “conventional launcher (ELV)”, the “reusable launcher (RLV)”, the “satellite”, the “orbital station”, the “probe”, the “impactor”, the “space telescope”, the “International Space Station (ISS)”… As Professors Diederiks-Verschoor and Kopal wrote in An Introduction to Space Law, the term space object “is indeed the commonly used expression, but it must always be borne in mind that its exact meaning is still not quite clear”.
An object is defined by the Oxford English Dictionary as “A material thing that can be seen and touched”. The five Onusian treaties don’t use the term satellite, instead opting for “object launched into outer space” in the 1967 Outer Space Treaty or “space object” in the 1972 Liability Convention and the 1976 Registration Convention. The 1967 Outer Space Treaty doesn’t really provide a definition for “object launched into outer space” other than an indication in Article VIII that it includes the “component parts” of the “object launched into outer space”. To add to the mix, Article V of the 1967 Outer Space Treaty uses the term “space vehicle” and the 1968 Rescue Agreement (which is essentially an elaboration of Article V of the OST) uses the term “spacecraft”. A good definition is given by Professor Hobe who write that a “space object is a human made object launched into outer space intended to be used in (as opposed to merely transit through) outer space”.
Let’s remember that “A treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose”, Article 31 of the Vienna Convention on the Law of Treaties of 1969. In addition, “Recourse may be had to supplementary means of interpretation, including the preparatory work of the treaty and the circumstances of its conclusion, in order to confirm the meaning resulting from the application of article 31, or to determine the meaning when the interpretation according to article 31: (a) leaves the meaning ambiguous or obscure; or (b) leads to a result which is manifestly absurd or unreasonable”, Article 32 of the Vienna Convention on the Law of Treaties of 1969.
Let’s recall that a space object causing damage triggers international third-party liability under the Convention on International Liability for Damage Caused by Space Objects (entered into force in September 1972). Article I (d) of which enounces that “the term space object includes component parts of a space object as well as its launch vehicle and parts thereof”. Its Article II adds 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”.
A space object requires, thanks to the Convention on Registration of Objects Launched into Outer Space (entered into force in September 1976), registration. Article II of which states that “When a space object is launched into Earth orbit or beyond, the launching State shall register the space object by means of an entry in an appropriate registry which it shall maintain. Each launching State shall inform the Secretary-General of the United Nations of the establishment of such a registry”.
Finally, the term space object effectively triggers application of much of both the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (entered into force in October 1967) and the Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space (entered into force in December 1968). Article VII of the first declares that “Each State Party to the Treaty that launches or procures the launching of an object into outer space, including the Moon and other celestial bodies, and each State Party from whose territory or facility an object is launched, is internationally liable for damage to another State Party to the Treaty or to its natural or juridical persons by such object or its component parts on the Earth, in air space or in outer space, including the Moon and other celestial bodies”.
Article 5 of the latter states that “1. Each Contracting Party which receives information or discovers that a space object or its component parts has returned to Earth in territory under its jurisdiction or on the high seas or in any other place not under the jurisdiction of any State, shall notify the launching authority and the Secretary-General of the United Nations. 2. Each Contracting Party having jurisdiction over the territory on which a space object or its component parts has been discovered shall, upon the request of the launching authority and with assistance from that authority if requested, take such steps as it finds practicable to recover the object or component parts. 3. Upon request of the launching authority, objects launched into outer space or their component parts found beyond the territorial limits of the launching authority shall be returned to or held at the disposal of representatives of the launching authority, which shall, upon request, furnish identifying data prior to their return”.
The 1967 Outer Space Treaty doesn’t really provide a definition for “object launched into outer space” other than an indication in Article VIII that it includes the “component parts” of the “object launched into outer space”. It states that “A State Party to the Treaty on whose registry an object launched into outer space is carried shall retain jurisdiction and control over such object, and over any personnel thereof, while in outer space or on a celestial body. Ownership of objects launched into outer space, including objects landed or constructed on a celestial body, and of their component parts, is not affected by their presence in outer space or on a celestial body or by their return to the Earth. Such objects or component parts found beyond the limits of the State Party to the Treaty on whose registry they are carried shall be returned to that State Party, which shall, upon request, furnish identifying data prior to their return”. We’ll conclude with the definition given by Professor Hobe who wrote that a “space object is a human made object launched into outer space intended to be used in (as opposed to merely transit through) outer space”. As a conclusion on solar sails and their legal status, these spacecraft are space objects.