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Article

Will Grundy

Pluto orbits the Sun at a mean distance of 39.5 AU (astronomical units; 1 AU is the mean distance between the Earth and the Sun), with an orbital period of 248 Earth years. Its orbit is just eccentric enough to cross that of Neptune. They never collide thanks to a 2:3 mean-motion resonance: Pluto completes two orbits of the Sun for every three by Neptune. The Pluto system consists of Pluto and its large satellite Charon, plus four small satellites: Styx, Nix, Kerberos, and Hydra. Pluto and Charon are spherical bodies, with diameters of 2,377 and 1,212 km, respectively. They are tidally locked to one another such that each spins about its axis with the same 6.39-day period as their mutual orbit about their common barycenter. Pluto’s surface is dominated by frozen volatiles nitrogen, methane, and carbon monoxide. Their vapor pressure supports an atmosphere with multiple layers of photochemical hazes. Pluto’s equator is marked by a belt of dark red maculae, where the photochemical haze has accumulated over time. Some regions are ancient and cratered, while others are geologically active via processes including sublimation and condensation, glaciation, and eruption of material from the subsurface. The surfaces of the satellites are dominated by water ice. Charon has dark red polar stains produced from chemistry fed by Pluto’s escaping atmosphere. The existence of a planet beyond Neptune had been postulated by Percival Lowell and William Pickering in the early 20th century to account for supposed clustering in comet aphelia and perturbations of the orbit of Uranus. Both lines of evidence turned out to be spurious, but they motivated a series of searches that culminated in Clyde Tombaugh’s discovery of Pluto in 1930 at the observatory Lowell had founded in Arizona. Over subsequent decades, basic facts about Pluto were hard-won through application of technological advances in astronomical instrumentation. During the progression from photographic plates through photoelectric photometers to digital array detectors, space-based telescopes, and ultimately, direct exploration by robotic spacecraft, each revealed more about Pluto. A key breakthrough came in 1978 with the discovery of Charon by Christy and Harrington. Charon’s orbit revealed the mass of the system. Observations of stellar occultations constrained the sizes of Pluto and Charon and enabled the detection of Pluto’s atmosphere in 1988. Spectroscopic instruments revealed Pluto’s volatile ices. In a series of mutual events from 1985 through 1990, Pluto and Charon alternated in passing in front of the other as seen from Earth. Observations of these events provided additional constraints on their sizes and albedo patterns and revealed their distinct compositions. The Hubble Space Telescope’s vantage above Earth’s atmosphere enabled further mapping of Pluto’s albedo patterns and the discovery of the small satellites. NASA’s New Horizons spacecraft flew through the system in 2015. Its instruments mapped the diversity and compositions of geological features on Pluto and Charon and provided detailed information on Pluto’s atmosphere and its interaction with the solar wind.

Article

The public impact of planetary science, or, alternatively, the public value of planetary science, is poorly understood, as little research has been published on the subject. Public impact may be linked to scientific impact, but it is not the same as public impact. Nor is it the same as public benefit or public understanding. No clear, agreed-upon definition of “public impact” exists, and certainly no definition of “the public impact of planetary science” exists. It is a matter of judgment as to whether global spending on planetary science has yielded positive public impacts, let alone impacts that are worth the investment. More research on the public impact of planetary science is needed. However, the study of public impact is a social scientific enterprise, and space agencies, space research institutes, and aerospace companies historically have invested very little in social scientific research. Without further study of the subject, the public impact of planetary science will remain poorly understood.

Article

The records of planetary observations in Japan in the 7th century ad are treated separately from other records because they are written in the Nihongi. It is known that Japanese observational astronomy was recorded in the 7th century ad, but astronomy in Japan did not evolve straightforward in that century. There are thirty-one records that exist from that time, including four records on the Moon and planets. Correspondingly, a new interpretation of Japanese ancient history has been proposed. For the 8th, 9th, and 10th centuries, records have been compiled on the relative motion of the Moon and the planets, the motion of planets in the constellations, and stars seen in the daytime, as stated in Japanese recorded history. These records are written in Chinese, as in the case of the Nihongi, but have been translated into English. The orbits of the Moon and planets have been calculated using the NASA Jet Propulsion Laboratory (JPL) development ephemeris (DE) in order to confirm the validity of the records. The numbers of records and observations are not the same because one record may contain multiple observations. The accuracy of individual observations is discussed.

Article

Anja Nakarada Pečujlić

The adoption and entering into force of the 1975 Convention on Registration of Objects Launched into Outer Space (also known as the Registration Convention) was another achievement in expanding and strengthening the corpus iuris spatialis. It was the fourth treaty negotiated by the member states of the United Nations Committee on the Peaceful Uses of Outer Space (UN COPUOS) and it represents a lex specialis to the Outer Space Treaty (OST), elaborating further Articles V, VIII, and XI of the OST. Article V OST deals with safe and prompt return of astronauts in case of distress or emergency landing to the state of registry of their space vehicle, which is then further defined in the Registration Convention. Article VIII OST only implied registration and provided for the consequences thereof, namely in respect of exercising jurisdiction and control over a registered space object. However, the Registration Convention specified the ensuing obligations and regulated the necessary practical steps of space objects registration. The Registration Convention also complements and strengthens Article XI OST, which stipulates an obligation of state parties to inform the secretary-general of the nature, conduct, locations, and results of their space activities in order to promote international cooperation. The prevailing purposes of the Registration Convention is the clarification of “jurisdiction and control” as a comprehensive concept mentioned in Article VIII OST. In addition to its overriding objective, the Registration Convention also contributes to the promotion and the exploration and use of outer space for peaceful purposes. Establishing and maintaining a public register reduces the possibility of the existence of unidentified space objects and thereby lowers the risk of putting, for example, weapons of mass destruction secretly into orbit. Notwithstanding these important objectives, the negotiation history of the Convention and its lower number of ratification compared to the previous three space treaties testify to the numerous challenges that surround registration. The mandatory marking of space objects was one of the most heated points of debate between member states during the drafting of the Convention in the 1970s. Member states had conflicting views, depending on whether they were launching states or potential victims of launch failures. Additionally, questions on whether there should be one central or several registers and whether the type of information to be registered should be obligatory or optional were also pivotal in the discussion. It took five years of negotiation for member states to reach compromises and to adopt the Registration Convention, containing 12 articles. The articles covered issues ranging from registration procedure and different registries to amendments and withdrawal from the Convention. In addition, the following novelties were introduced: a new definition on “state of registry” was included; the “Moscow formula” was abandoned as the depositary was moved to the UN; and the “in five years review” clause found in Article X signified that the drafters were anticipating that technological developments could have such an impact on the Convention’s provisions that shorter time span between reviews were required than in previous space treaties. Despite the Convention’s novelties and its objective to protect the attribution of jurisdiction and control on the basis of a registry, as well as to ensure the rights provided in the Liability Convention and the Rescue and Return Agreement by offering means to identify space objects, the articles dealing with joint launch registration and registration by Intergovernmental Organizations (IGOs) are seen as weakening jurisdiction and control concept. Due to the fact that jurisdiction and control stay only with the state of registry, the other launching states may only conclude appropriate agreements to retain any of these rights. Thus, international responsibility and liability remain with all the launching states, but jurisdiction and control only with the state of registry. Furthermore, in the case of an IGO, the IGO does not have the sovereign authority to exercise jurisdiction and control, thereby raising the question who could do so instead of or on behalf of an IGO. In this regard, the Convention leaves important areas unregulated. In the following years, there were proposals to expand the Registration Convention to encompass other subject matters such as financial interests of assets in outer space; however, up until today, these issues remain regulated only by the UNIDROIT Space Assets Protocol.

Article

Bernhard Schmidt-Tedd and Alexander Soucek

Space objects are subject to registration in order to allocate “jurisdiction and control” over those objects in the sovereign-free environment of outer space. This approach is similar to the registration of ships on the high seas and for aircraft in international airspace. Registration is one of the basic principles of space law, starting with the first space-related UN General Assembly (GA) Resolution 1721 B (XVI) of December 20, 1961, followed by UN GA Resolution 1962 (XVIII) of December 1963 then formulated in Article VIII of the Outer Space Treaty of 1967, and later specified in the Registration Convention of 1975. Registration of space objects has arguably grown into a principle of customary international law, relevant for each spacefaring state. Registration occurs at the national and international level in a two-step process. To enter and object into the UN Register of Space Objects, the state establishes a national registry for its space objects and notifies the UN Secretary General of all registered objects. The UN Register is handled by the UN Office for Outer Space Affaires (UNOOSA), which has created a searchable database as an open source of information for space objects worldwide. Registration is linked to the so-called launching state of the relevant space object. There may be more than one launching state for the specific launch event, but only one state can register a specific space object. The state of registry has jurisdiction and control over the space object and therefore no double-registration is admissible. Registration practice has evolved in response to technical developments and legal challenges. After the privatization of major international satellite organizations, a number of nonregistrations had to be addressed. The result was the UN GA Registration Practice Resolution of 2007 as elaborated by the legal subcommittee of the UN Committee for the Peaceful Use of Outer Space. The complexity of space activities and concepts such as megaconstellations present new challenges for the registration system. For example, the Registration Practice Resolution recommends that in cases of joint launches each space object should be registered separately. Registration of space objects is a legal instrument relevant for state responsibility and liability, but it is not an adequate instrument for space traffic management. The orbit-related information of the registration system is useful for identification purposes but not for real-time positioning information. Orbital data to allow positioning, tracking, and collision warnings need to respond to various requirements of accuracy.

Article

Earth’s moon, hereafter referred to as “the Moon,” has been an object of intense study since before the time of the Apollo and Luna missions to the lunar surface and associated sample returns. As a differentiated rocky body and as Earth’s companion in the solar system, much study has been given to aspects such as the Moon’s surface characteristics, composition, interior, geologic history, origin, and what it records about the early history of the Earth-Moon system and the evolution of differentiated rocky bodies in the solar system. Much of the Apollo and post-Apollo knowledge came from surface geologic exploration, remote sensing, and extensive studies of the lunar samples. After a hiatus of nearly two decades following the end of Apollo and Luna missions, a new era of lunar exploration began with a series of orbital missions, including missions designed to prepare the way for longer duration human use and further exploration of the Moon. Participation in these missions has become international. The more recent missions have provided global context and have investigated composition, mineralogy, topography, gravity, tectonics, thermal evolution of the interior, thermal and radiation environments at the surface, exosphere composition and phenomena, and characteristics of the poles with their permanently shaded cold-trap environments. New samples were recognized as a class of achondrite meteorites, shown through geochemical and mineralogical similarities to have originated on the Moon. New sample-based studies with ever-improving analytical techniques and approaches have also led to significant discoveries such as the determination of volatile contents, including intrinsic H contents of lunar minerals and glasses. The Moon preserves a record of the impact history of the solar system, and new developments in timing of events, sample based and model based, are leading to a new reckoning of planetary chronology and the events that occurred in the early solar system. The new data provide the grist to test models of formation of the Moon and its early differentiation, and its thermal and volcanic evolution. Thought to have been born of a giant impact into early Earth, new data are providing key constraints on timing and process. The new data are also being used to test hypotheses and work out details such as for the magma ocean concept, the possible existence of an early magnetic field generated by a core dynamo, the effects of intense asteroidal and cometary bombardment during the first 500 million–600 million years, sequestration of volatile compounds at the poles, volcanism through time, including new information about the youngest volcanism on the Moon, and the formation and degradation processes of impact craters, so well preserved on the Moon. The Moon is a natural laboratory and cornerstone for understanding many processes operating in the space environment of the Earth and Moon, now and in the past, and of the geologic processes that have affected the planets through time. The Moon is a destination for further human exploration and activity, including use of valuable resources in space. It behooves humanity to learn as much about Earth’s nearest neighbor in space as possible.

Article

Katharina Lodders

Solar elemental abundances, or solar system elemental abundances, refer to the complement of chemical elements in the entire Solar System. The Sun contains more than 99% of the mass in the solar system and therefore the composition of the Sun is a good proxy for the composition of the overall solar system. The solar system composition can be taken as the overall composition of the molecular cloud within the interstellar medium from which the solar system formed 4.567 billion years ago. Active research areas in astronomy and cosmochemistry model collapse of a molecular cloud of solar composition into a star with a planetary system and the physical and chemical fractionation of the elements during planetary formation and differentiation. The solar system composition is the initial composition from which all solar system objects (the Sun, terrestrial planets, gas giant planets, planetary satellites and moons, asteroids, Kuiper-belt objects, and comets) were derived. Other dwarf stars (with hydrostatic hydrogen-burning in their cores) like the Sun (type G2V dwarf star) within the solar neighborhood have compositions similar to the Sun and the solar system composition. In general, differential comparisons of stellar compositions provide insights about stellar evolution as functions of stellar mass and age and ongoing nucleosynthesis but also about galactic chemical evolution when elemental compositions of stellar populations across the Milky Way Galaxy is considered. Comparisons to solar composition can reveal element destruction (e.g., Li) in the Sun and in other dwarf stars. The comparisons also show element production of, for example, C, N, O, and the heavy elements made by the s-process in low to intermediate mass stars (3–7 solar masses) after these evolved from their dwarf-star stage into red giant stars (where hydrogen and helium burning can occur in shells around their cores). The solar system abundances are and have been a critical test composition for nucleosynthesis models and models of galactic chemical evolution, which aim ultimately to track the production of the elements heavier than hydrogen and helium in the generation of stars that came forth after the Big Bang 13.4 billion years ago.

Article

Edik Dubinin, Janet G. Luhmann, and James A. Slavin

Knowledge about the solar wind interactions of Venus, Mars, and Mercury is rapidly expanding. While the Earth is also a terrestrial planet, it has been studied much more extensively and in far greater detail than its companions. As a result we direct the reader to specific references on that subject for obtaining an accurate comparative picture. Due to the strength of the Earth’s intrinsic dipole field, a relatively large volume is carved out in interplanetary space around the planet and its atmosphere. This “magnetosphere” is regarded as a shield from external effects, but in actuality much energy and momentum are channeled into it, especially at high latitudes, where the frequent interconnection between the Earth’s magnetic field and the interplanetary field allows some access by solar wind particles and electric fields to the upper atmosphere and ionosphere. Moreover, reconnection between oppositely directed magnetic fields occurs in Earth’s extended magnetotail—producing a host of other phenomena including injection of a ring current of energized internal plasma from the magnetotail into the inner magnetosphere—creating magnetic storms and enhancements in auroral activity and related ionospheric outflows. There are also permanent, though variable, trapped radiation belts that strengthen and decay with the rest of magnetospheric activity—depositing additional energy into the upper atmosphere over a wider latitude range. Virtually every aspect of the Earth’s solar wind interaction, highly tied to its strong intrinsic dipole field, has its own dedicated textbook chapters and review papers. Although Mercury, Venus, Earth, and Mars belong to the same class of rocky terrestrial planets, their interaction with solar wind is very different. Earth and Mercury have the intrinsic, mainly dipole magnetic field, which protects them from direct exposure by solar wind. In contrast, Venus and Mars have no such shield and solar wind directly impacts their atmospheres/ionospheres. In the first case, intrinsic magnetospheric cavities with a long tail are found. In the second case, magnetospheres are also formed but are generated by the electric currents induced in the conductive ionospheres. The interaction of solar wind with terrestrial planets also varies due to changes caused by different distances to the Sun and large variations in solar irradiance and solar wind parameters. Other important planetary differences like local strong crustal magnetization on Mars and almost total absence of the ionosphere on Mercury create new essential features to the interaction pattern. Solar wind might be also a feasible driver for planetary atmospheric losses of volatiles, which could historically affect the habitability of the terrestrial planets.

Article

Angel Abbud-Madrid

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article. The space and multitude of celestial bodies surrounding Earth hold a vast wealth of resources for a variety of space and terrestrial applications. The unlimited solar energy, vacuum, and low gravity in space, as well as the minerals, metals, water, atmospheric gases, and volatile elements on the Moon, asteroids, comets, and the inner and outer planets of the Solar System and their moons, constitute potential valuable resources for robotic and human space missions and for future use in our own planet. In the short term, these resources could be transformed into useful materials at the site where they are found to extend mission duration and to reduce the costly dependence from materials sent from Earth. Making propellants and human consumables from local resources can significantly reduce mission mass and cost, enabling longer stays and fueling transportation systems for use within and beyond the planetary surface. Use of finely grained soils and rocks can serve for habitat construction, radiation protection, solar cell fabrication, and food growth. The same material could also be used to develop repair and replacement capabilities using advanced manufacturing technologies. Following similar mining practices utilized for centuries on Earth, identifying, extracting, and utilizing extraterrestrial resources will enable further space exploration, while increasing commercial activities beyond our planet. In the long term, planetary resources and solar energy could also be brought to Earth if obtaining these resources locally prove to be no longer economically or environmentally acceptable. Throughout human history, resources have been the driving force for the exploration and settling of our planet. Similarly, extraterrestrial resources will make space the next destination in the quest for further exploration and expansion of our species. However, just like on Earth, not all challenges are scientific and technological. As private companies start working toward exploiting the resources from asteroids, the Moon, and Mars, an international legal framework is also needed to regulate commercial exploration and the use of space and planetary resources for the benefit of all humanity. These resources hold the secret to unleash an unprecedented wave of exploration and of economic prosperity by utilizing the full potential and value of space. It is up to us humans here on planet Earth to find the best way to use these extraterrestrial resources effectively and responsibly to make this promise a reality.

Article

Shortly after the launch of the first manmade satellite in 1957, the United Nations (UN) took the lead in formulating international rules governing space activities. The five international conventions (the 1967 Outer Space Treaty, the 1968 Rescue Agreement, the 1972 Liability Convention, the 1975 Registration Convention, and the 1979 Moon Agreement) within the UN framework constitute the nucleus of space law, which laid a solid legal foundation securing the smooth development of space activities in the next few decades. Outer space was soon found to be a place with abundant opportunities for commercialization. Telecommunications services proved to be the first successful space commercial application, to be followed by remote sensing and global navigation services. In the last decade, the rapid development of space technologies has brought space tourism and space mining to the forefront of space commercialization. With more and more commercial activities taking place on a daily basis from the 1980s, the existing space law faces severe challenges. The five conventions, enacted in a time when space was monopolized by two superpowers, failed to take into account the commercial aspect of space activities. While there is an urgent need for new rules to deal with the ongoing trend of space commercialization, international society faces difficulties in adopting new rules due to diversified concerns over national interests and adjusts the legislative strategies by enacting soft laws. In view of the difficulty in adopting legally binding rules at the international level, states are encouraged to enact their own national space legislation providing sufficient guidance for their domestic space commercial activities. In the foreseeable future, it is expected that the development of soft laws and national space legislation will be the mainstream regulatory activities in the space field, especially for commercial space activities.

Article

Rajeswari Pillai Rajagopalan

Outer space is once again facing renewed competition. Unlike in the earlier decades of space exploration when there were two or three spacefaring powers, by the turn of the 21st century, there are more than 60 players making the outer space environment crowded and congested. Space is no more a domain restricted to state players. Even though it is mostly a western phenomenon, the reality of commercial players as a major actor is creating new dynamics. The changing power transitions are making outer space contested and competitive. Meanwhile, safe and secure access to outer space is being challenged by a number of old and new threats including space debris, militarization of space, radio frequency interference, and potential arms race in space. While a few foundational treaties and legal instruments exist in order to regulate outer space activities, they have become far too expansive to be useful in restricting the current trend that could make outer space inaccessible in the longer term. The need for new rules of the road in the form of norms of responsible behavior, transparency and confidence building measures (TCBMs) such as a code of conduct, a group of governmental experts (GGE), and legal mechanisms, is absolutely essential to have safe, secure, and uninterrupted access to outer space. Current efforts to develop these measures have been fraught with challenges, ranging from agreement on identifying the problems to ideating possible solutions. This is a reflection of the shifting balance of power equations on the one hand, and the proliferation of technology to a large number of players on the other, which makes the decision-making process a lot problematic. In fact, it is the crisis in decision making and the lack of consensus among major space powers that is impeding the process of developing an effective outer space regime.

Article

Fabio Tronchetti

China has made remarkable achievements in the space sector and has become one of the most relevant players in the outer space domain. Highlights of this process have been the deployment in orbit of the first Chinese space station, Tiangong-1, on September 29, 2011, and the landing of the Yutu rover on the lunar surface on December 14, 2013. While technological developments have occurred at such a rapid pace, the same cannot be said of the regulatory framework governing Chinese space activities, which still lays at its infant stage. Indeed, unlike other major spacefaring countries, China lacks a comprehensive and uniform national space legislation; as of now, China has enacted two low-level administrative regulations addressing the issues of launching and registration of space objects. With the growth of the Chinese space program, such a lack of structured national space law is beginning to show its limits and to raise concerns about its negative impact on business opportunities and the ability of China to fully comply with international obligations. One should keep in mind that international space treaties (China is part to four international space law treaties) are not self-executing, thus requiring states to adopt domestic measures to ensure their effective implementation. Importantly, Chinese authorities appear to be aware of these issues; as stated by the Secretary-General of the Chinese National Space Administration (CNSA) in 2014, national space law has been listed in the national legislation plan, and the CNSA is directly engaged in such a process. However, questions remain as to how this drafting process will be conducted and what legal form and content the law will have. For example, China could either decide to proceed with a gradual approach, consisting in the adoption of laws addressing selected issues to be eventually assembled into one single law or to directly move to the adoption of one comprehensive law. In any case, if enacted, a Chinese national space law would represent an important step in the advancement of the Chinese space program and in the progress of international space law as such.

Article

Martha Mejía-Kaiser

International space law is a branch of public international law. Norms of treaty law and customary law provide a foundation for the behavior of the subjects of international law performing space activities. Five multilateral space treaties are in effect, which are complemented by important recommendations of international organizations such as United Nations (UN) General Assembly Resolutions and International Telecommunication Union (ITU) Regulations. The Inter-Agency Space Debris Mitigation Coordination Committee (IADC), a non-governmental body composed of several space agencies (for instance, the European Space Agency, the United States National Aeronautics and Space Administration, the Japanese Aerospace Exploration Agency, the Russian Federal Space Agency), issued its Space Debris Mitigation Guidelines in 2002. The IADC defines “space debris” as “all man-made space objects including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional” (IADC, 2002, Revision 1, 2007, 3.1. Space Debris). Although the term “space debris” was not included in any space treaty, the drafters of the space treaties considered space objects as “hazardous” because “component parts of a space object as well as its launch vehicles and parts thereof” detach in course of normal launching operations, because space objects can fragment during an attempted launch, and because space objects that re-enter Earth’s atmosphere and survive friction have the potential to cause damage. In addition, radioactive and chemical substances on board space objects may represent a hazard to populations and the environment on the Earth. Besides the threats to aircraft in flight and to persons and property on the surface of the Earth, space debris in orbit is increasing alarmingly and poses a threat to manned space missions and non-manned space objects. While the Convention on International Liability for Damages Caused by Space Objects (Liability Convention, 1972) considers the threats of space objects during launch, in outer space, and when entering the Earth’s atmosphere, there have been efforts to minimize the generation of space debris in orbit, outside the framework of the space treaties. The IADC Space Debris Mitigation Guidelines are a comprehensive list of recommendations to launching states, owners, and operators of space objects. They are increasingly recognized by states through the creation of codes of conduct, national legislation, recommendations of international organizations, and state practice. Furthermore, non-governmental institutions, like the International Organization for Standardization, are providing more detailed technical instructions for the implementation of the Space Debris Mitigation Guidelines, which are a breakthrough for the application of the guidelines by states of different economic and technical standing. Even though states are reluctant to accept new obligations through treaties, recommendations and state practice are becoming powerful instruments to avert the dangers of hazardous space debris that may create damage on the Earth or in orbit. Space debris also is becoming one of the drivers for the initiatives of the United Nations on the long-term sustainability of outer space activities to promote the existing mitigation guidelines and to formulate new guidelines for clearing outer space of debris.

Article

Sa'id Mosteshar

Although legal principles to govern space were discussed as early as the mid-1950s, they were not formalized until the Outer Space Treaty (OST) 1967 was adopted and came into force. The OST establishes a number of principles affecting the placement of weapons in outer space. In particular it provides that “the Moon and other celestial bodies shall be used exclusively for peaceful purposes” and prohibits the testing of any types of weapons on such bodies. More generally the OST forbids the placement of nuclear weapons or other weapons of mass destruction in outer space. In addition there are a number of disarmament treaties and agreements emanating from the United Nations Office for Disarmament Affairs and the Conference on Disarmament that are relevant to weapons in space. Although the disarmament provisions and international humanitarian laws place some restrictions on the use or manner of use of space weapons, none prohibit space weaponization. The absence of such prohibition is not due to many attempts over the years to prevent an arms race in space. Notable among these are Prevention of an Arms Race in Space Draft Treaty and the Prevention of the Placement of Weapons in Space Draft Treaty. In considering the laws affecting space weapons a fundamental question that arises is what constitutes a weapon and does its placement in space breach the requirement that outer space be used exclusively for peaceful purposes? As an example, does a satellite used to control and direct an armed drone breach the peaceful use provision of the OST? There may be risks that without international norms governments and substate groups may acquire and use armed drones in ways that threaten regional stability, laws of war, and the role of domestic rule of law in decisions to use force. Given their orbital velocity, any object in space could be a weapon with capability to destroy a satellite or other space object. There is also a growing population of dual-use satellites with military as well as civilian applications. These present great difficulty in arriving at a workable definition of a space weapon in the formulation of a generally acceptable treaty. In addition, there are divergent views of the meaning of peaceful use. Some, in particular the United States, consider the meaning to be “nonaggressive” rather than “nonmilitary.”

Article

P.J. Blount

The use and exploration of space by humans is historically implicated with international and national security. Space exploration itself was sparked, in part, by the race to develop intercontinental ballistic missiles (ICBM), and the strategic uses of space enable the global projection of force by major military powers. The recognition of space as a strategic domain spurred states to develop the initial laws and policies that govern space activities to reduce the likelihood of conflict. Space security, therefore, is a foundational concept to space law. Since the beginning of the Space Age, the concept of security has morphed into a multivariate term, and contemporary space security concerns more than just securing states from the dangers of ICBMs. The prevalence of space technologies across society means that security issues connected to the space domain touch on a range of legal regimes. Specifically, space security law involves components of international peace and security, national security, human security, and the security of the space environment itself.

Article

A magma ocean is a global layer of partially or fully molten rocks. Significant melting of terrestrial planets likely occurs due to heat release during planetary accretion, such as decay heat of short-lived radionuclides, impact energy released by continuous planetesimal accretion, and energetic impacts among planetary-sized bodies (giant impacts). Over a magma ocean, all water, which is released upon impact or degassed from the interior, exists as superheated vapor, forming a water-dominated, steam atmosphere. A magma ocean extending to the surface is expected to interact with the overlying steam atmosphere through material and heat exchange. Impact degassing of water starts when the size of a planetary body becomes larger than Earth’s moon or Mars. The degassed water could build up and form a steam atmosphere on protoplanets growing by planetesimal accretion. The atmosphere has a role in preventing accretion energy supplied by planetesimals from escaping, leading to the formation of a magma ocean. Once a magma ocean forms, part of the steam atmosphere would start to dissolve into the surface magma due to the high solubility of water into silicate melt. Theoretical studies indicated that as long as the magma ocean is present, a negative feedback loop can operate to regulate the amount of the steam atmosphere and to stabilize the surface temperature so that a radiative energy balance is achieved. Protoplanets can also accrete the surrounding H 2 -rich disk gas. Water could be produced by oxidation of H 2 by ferrous iron in the magma. The atmosphere and water on protoplanets could be a mixture of outgassed and disk-gas components. Planets formed by giant impact would experience a global melting on a short timescale. A steam atmosphere could grow by later outgassing from the interior. Its thermal blanketing and greenhouse effects are of great importance in controlling the cooling rate of the magma ocean. Due to the presence of a runaway greenhouse threshold, the crystallization timescale and water budget of terrestrial planets can depend on the orbital distance from the host star. The terrestrial planets in our solar system essentially have no direct record of their earliest history, whereas observations of young terrestrial exoplanets may provide us some insight into what early terrestrial planets and their atmosphere are like. Evolution of protoplanets in the framework of pebble accretion remains unexplored.

Article

The subject of astronomy in folk tradition, or folk astronomy, requires some explication. It is, for instance, not the same as ethnoastronomy, which primarily studies the astronomical ideas of contemporary societies. However, the subject overlaps with archaeoastronomy when defined widely as the interdisciplinary study of prehistoric, ancient, and traditional astronomies worldwide within their cultural context that includes both written and archaeological records. The most useful definition of “astronomy in folk tradition” might be “astronomy of the people or of the common man,” or even “lay astronomy,” left to us through tradition, where the term “astronomy” may, for further clarity, be replaced by “ideas and observations of the sky.” In any case, it is worth keeping in mind that the content of folk astronomy of one society may overlap with the content of established astronomy of another society at another time and place. Scientific ideas or theories have their roots in the past, even before the advent of any “experts.” Folk astronomy of the past is often less accessible for historical studies than mainstream astronomy, especially in a society leaving few records or artifacts. Revealing sources may, however, be found by looking beyond the conventional. For instance, various sources on mythology and religion may give information on the astronomical and cosmological ideas of previous societies. Purportedly fictional literature, like the works of Dante and Chaucer, may also yield information of this kind, although they were not explicitly composed for that purpose. But there are also writers who have deliberately written on the astronomical ideas of their society at their time, although their works were outside of the best known corpus and sometimes intended for common people. Two Old Norse examples are the 13th-century Norwegian King’s Mirror and the Icelandic 12th- to 14th-century material edited in the volume of Alfræði íslenzk II. Among other things, these sources treat phenomena that are not observable outside the subarctic region. A third example is the 14th–15th century North European Seebuch with practical information for seamen, partly linked to astronomy. In any case, two types of folk astronomy can be distinguished: (a) practical astronomy that people use as a tool in daily life, for example, to determine the time of day or year, or for travel and navigation; (b) ideas related to cosmology or cosmogony, religion, or supernatural beliefs, which would neither imply practical uses nor consequences.

Article

M.A. Ivanov and J.W. Head

This chapter reviews the conditions under which the basic landforms of Venus formed, interprets their nature, and analyzes their local, regional, and global age relationships. The strong greenhouse effect on Venus causes hyper-dry, almost stagnant near-surface environments. These conditions preclude water-driven, and suppress wind-related, geological processes; thus, the common Earth-like water-generated geological record of sedimentary materials does not currently form on Venus. Three geological processes are important on the planet: volcanism, tectonics, and impact cratering. The small number of impact craters on Venus (~1,000) indicates that their contribution to resurfacing is minor. Volcanism and tectonics are the principal geological processes operating on Venus during its observable geologic history. Landforms of the volcanic and tectonic nature have specific morphologies, which indicate different modes of formation, and their relationships permit one to establish their relative ages. Analysis of these relationships at the global scale reveals that three distinct regimes of resurfacing comprise the observable geologic history of Venus: (1) the global tectonic regime, (2) the global volcanic regime, and (3) the network rifting-volcanism regime. During the earlier global tectonic regime, tectonic resurfacing dominated. Tectonic deformation at this time caused formation of strongly tectonized terrains such as tessera, and deformational belts. Exposures of these units comprise ~20% of the surface of Venus. The apparent beginning of the global tectonic regime is related to the formation of tessera, which is among the oldest units on Venus. The age relationships among the tessera structures indicate that this terrain is the result of crustal shortening. During the global volcanic regime, volcanism overwhelmed tectonic activity and caused formation of vast volcanic plains that compose ~60% of the surface of Venus. The plains show a clear stratigraphic sequence from older shield plains to younger regional plains. The distinctly different morphologies of the plains indicate different volcanic formation styles ranging from eruption through broadly distributed local sources of shield plains to the volcanic flooding of regional plains. The density of impact craters on units of the tectonic and volcanic regimes suggests that these regimes characterized about the first one-third of the visible geologic history of Venus. During this time, ~80%–85% of the surface of the planet was renovated. The network rifting-volcanism regime characterized the last two-thirds of the visible geologic history of Venus. The major components of the regime include broadly synchronous lobate plains and rift zones. Although the network rifting-volcanism regime characterized ~2/3 of the visible geologic history of Venus, only 15%–20% of the surface was resurfaced during this time. This means that the level of endogenous activity during this time has dropped by about an order of magnitude compared with the earlier regimes.

Article

Paul K. Byrne

Mercury, like its inner Solar System planetary neighbors Venus, Mars, and the Moon, shows no evidence of having ever undergone plate tectonics. Nonetheless, the innermost planet boasts a long record of tectonic deformation. The most prominent manifestation of this history is a population of large scarps that occurs throughout the planet’s cratered terrains; some of these scarps rise kilometers above the surrounding landscape. Mercury’s smooth plains, the majority of which are volcanic and occupy over a quarter of the planet, abound with low-relief ridges. The scarps and ridges are underlain by thrust faults and point to a tectonic history dominated by crustal shortening. At least some of the shortening strain recorded by the ridges may reflect subsidence of the lavas in which they formed, but the widespread distribution of scarps attests to a planetwide process of global contraction, wherein Mercury experienced a reduction in volume as its interior cooled through time. The onset of this phenomenon placed the lithosphere into a net state of horizontal compression, and accounts for why Mercury hosts only a few instances of extensional structures. These landforms, shallow troughs that form complex networks, occur almost wholly in volcanically flooded impact craters and basins and developed as those lavas cooled and thermally contracted. Tellingly, widespread volcanism on Mercury ended at around the same time the population of scarps began to form. Explosive volcanism endured beyond this point, but almost exclusively at sites of lithospheric weakness, where large faults penetrate deep into the interior. These observations are consistent with decades-old predictions that global contraction would shut off major volcanic activity, and illustrate how closely Mercury’s tectonic and volcanic histories are intertwined. The tectonic character of Mercury is thus one of sustained crustal shortening with only localized extension, which started almost four billion years ago and extends into the geologically recent past. This character somewhat resembles that of the Moon, but differs substantially from those of Earth, Venus, or Mars. Mercury may represent how small rocky planets tectonically evolve and could provide a basis for understanding the geological properties of similarly small worlds in orbit around other stars.

Article

The formation and evolution of our solar system (and planetary systems around other stars) are among the most challenging and intriguing fields of modern science. As the product of a long history of cosmic matter evolution, this important branch of astrophysics is referred to as stellar-planetary cosmogony. Interdisciplinary by way of its content, it is based on fundamental theoretical concepts and available observational data on the processes of star formation. Modern observational data on stellar evolution, disc formation, and the discovery of extrasolar planets, as well as mechanical and cosmochemical properties of the solar system, place important constraints on the different scenarios developed, each supporting the basic cosmogony concept (as rooted in the Kant-Laplace hypothesis). Basically, the sequence of events includes fragmentation of an original interstellar molecular cloud, emergence of a primordial nebula, and accretion of a protoplanetary gas-dust disk around a parent star, followed by disk instability and break-up into primary solid bodies (planetesimals) and their collisional interactions, eventually forming a planet. Recent decades have seen major advances in the field, due to in-depth theoretical and experimental studies. Such advances have clarified a new scenario, which largely supports simultaneous stellar-planetary formation. Here, the collapse of a protosolar nebula’s inner core gives rise to fusion ignition and star birth with an accretion disc left behind: its continuing evolution resulting ultimately in protoplanets and planetary formation. Astronomical observations have allowed us to resolve in great detail the turbulent structure of gas-dust disks and their dynamics in regard to solar system origin. Indeed radio isotope dating of chondrite meteorite samples has charted the age and the chronology of key processes in the formation of the solar system. Significant progress also has been made in the theoretical study and computer modeling of protoplanetary accretion disk thermal regimes; evaporation/condensation of primordial particles depending on their radial distance, mechanisms of clustering, collisions, and dynamics. However, these breakthroughs are yet insufficient to resolve many problems intrinsically related to planetary cosmogony. Significant new questions also have been posed, which require answers. Of great importance are questions on how contemporary natural conditions appeared on solar system planets: specifically, why the three neighbor inner planets—Earth, Venus, and Mars—reveal different evolutionary paths.