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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

In the Middle Ages and Renaissance, alchemy and astrology shared a common language in the meanings and characters attributed to the celestial bodies, which provided a cosmic framework for understanding all terrestrial affairs. Astrology is the name given to a series of diverse practices based in the idea that the stars, planets, and other celestial phenomena possess significance and meaning for events on the Earth. In practical terms, astrology’s function was to predict the future, manage the present, and understand the past. In the Middle Ages and Renaissance, these three functions were not seen as separate, because time existed as a single entity—past, present, and future coexisted in the mind of God. Astrology assumed a link between Earth and sky in which all existence, spiritual, psychological, and physical, is interconnected. Time and space existed in a mutually dependent continuum, in which both were infused with the same qualities, represented in terms of astrological language by zodiac signs and planets. The practice of alchemy applied such assumptions to the material world, attempting to convert one metal into another, typically lead into gold. As all things were interconnected, including soul and body, the physical practice of alchemy was intimately connected with spiritual practice intended to purify the soul in preparation for its meeting with the divine. The planets had roles in both astrology and alchemy. A planet, from the Greek planētai, meaning wanderer, is a star, a point, or source of light in the sky that constantly alters its position. This is in contrast to the so-called fixed stars, which appear to keep exactly the same position relative to each other, at least over historic periods of time. Technical astrology largely developed in the Hellenistic, Greek-speaking world from the early third century BCE onward. With the collapse of the Roman Empire and classical learning in most of Western Europe from the 5th century, the complex astrology of the Classical world largely disappeared. It returned to Western Europe in two phases. The first phase, mainly in the 12th century, included the translation into Latin of major texts from Arabic, including those originally composed in Greek, and was accompanied by Aristotelian works that justified the use of astrology in naturalistic terms, thereby negating Christian disapproval. Alchemy was introduced into Western Europe at the same time. The second phase began mainly in the 15th century, during the Renaissance, as part of a wider revival of Classical thought encouraged by the translation of Platonic, neo-Platonic, and Hermetic works from Greek directly into Latin. By the end of the 17th century, the practice of both astrology and alchemy had largely disappeared. Astrology was confined to popular almanacs and alchemy was replaced by modern chemistry.

Article

Astrology was a central feature of Greek and Roman culture. A knowledge of astrology’s claims, practices, and world view is essential for a full understanding of religion, politics, and science in the Greek and Roman worlds. Astrology is the name given to a series of diverse practices based in the idea that the stars, planets, and other celestial phenomena possess significance and meaning for events on Earth. It assumes a link between Earth and sky in which all existence, spiritual, psychological, and physical, is interconnected. Most premodern cultures practice a form of astrology. A particularly complex variety of it evolved in Mesopotamia in the first and second millennia bce from where it was imported into the Hellenistic world from the early 4th century bce onward. There it became attached to three philosophical schools, those pioneered by Plato, Aristotle and the Stoics, all of which shared the assumption that the cosmos is a single, living, integrated whole. Hellenistic astrology also drew on Egyptian temple culture, especially the belief that the soul could ascend to the stars. By the 1st century ce, the belief in the close link between humanity and the stars had become democratized and diversified into a series of practices and schools of thought which ranged across Greek and Roman culture. It was practiced at the imperial court and in the street. It could be used to predict individual destiny, avert undesirable events, and arrange auspicious moments to launch new enterprises. It could advise on financial fortunes or the condition of one’s soul. It was conceived of as natural science and justified by physical influences or considered to be divination, concerned with communication with the gods and goddesses. In some versions, the planets were neither influences nor causes of events on Earth, but timing devices, which indicated the ebb and flow of human affairs, like the hands on a modern clock. Astrology had a radical view of time in which the future already existed, at least in potential, and the astrologer’s task was to intercede in time, altering the future to human advantage. In this sense astrology was a form of “participation mystique” in which time and space were conceived of as a single entity and individual and social benefits were to be derived from engaging with it. There was no one single version of astrology and there were disputes about what it was and what it could do, for example, whether it could make precise predictions about individual affairs or merely general statements. From the early 4th century it went into a progressive decline, facing challenges from Christianity and the fragmentation of classical culture, especially in Western Europe. It survived in Persia, exerted a powerful influence on Indian astrology, and was transmitted to the Islamic world, from where it was reimported into the Latin West in the 12th century.

Article

The polar regions of Mars contain layered ice deposits that are rich in detail of past periods of accumulation and erosion. These north and south polar layered deposits (NPLD and SPLD, respectively) contain primarily water–ice and ~5% and ~10% dust derived from the atmosphere, respectively. In addition, the SPLD has two known CO2 deposits—one thin unit at the surface and one buried, much thicker unit. Together, they comprise less than 1% of the SPLD volume. Mars also experiences seasonal deposits of CO2 that form in winter and sublimate in spring and early summer. These seasonal caps are visible from Earth and have been studied for centuries. Zooming in, exposed layers at the PLDs reveal histories of climate change that resulted when orbital parameters such as obliquity, eccentricity, and argument of perihelion changed over tens of thousands to millions of years. Simpler environmental conditions at the NPLD, especially related to seasonal and aeolian processes, make interpreting the history of that polar cap much easier than the SPLD. The history of Mars polar science is linked by numerous incremental advancements and unexpected discoveries related to the observed geology of both poles, the interpreted and modeled climatic conditions that gave rise to the PLDs, and the atmospheric conditions that modify the surface.

Article

James D. Burke and Erik M. Conway

The Jet Propulsion Laboratory (JPL) of the California Institute of Technology had its origins in a student project to develop rocket propulsion in the late 1930s. It attracted funding from the U.S. Army just prior to U.S. entry into World War II and became an Army missile research facility in 1943. Because of its origins as a contractor-operated Army research facility, JPL is the National Aeronautics and Space Administration’s (NASA) only contractor-operated field center. It remains a unit of the California Institute of Technology. In the decades since its founding, the laboratory, first under U.S. Army direction and then as a NASA field center, has grown and evolved into an internationally recognized institution generally seen as a leader in solar system exploration but whose portfolio includes substantial Earth remote sensing. JPL’s history includes episodes where the course of the laboratory’s development took turning points into new directions. After developing short-range ballistic missiles for the Army, the laboratory embarked on a new career in lunar and planetary exploration through the early 1970s and abandoned its original purpose as a propulsion technology laboratory. It developed the telecommunications infrastructure for planetary exploration too. It diversified into Earth science and astrophysics in the late 1970s and, due to a downturn in funding for planetary exploration, returned to significant amounts of defense work in the 1980s. The end of the Cold War between 1989 and 1991 resulted in a declining NASA budget, but support for planetary exploration actually improved within NASA management—as long as that exploration could be done more cheaply. This resulted in what is known as the “Faster Better Cheaper” period in NASA history. For JPL, this ended in 2000, succeeded by a return to more rigorous technical standards and increased costs.

Article

Bryan J. Holler

The International Astronomical Union (IAU) officially recognizes five objects as dwarf planets: Ceres in the main asteroid belt between Mars and Jupiter, and Pluto, Eris, Haumea, and Makemake in the trans-Neptunian region beyond the orbit of Neptune. However, the definition used by the IAU applies to many other trans-Neptunian objects (TNOs) and can be summarized as follows: Any non-satellite large enough to be rounded by its own gravity. Practically speaking, this means any non-satellite with a diameter larger than 400 km. In the trans-Neptunian region, there are more than 150 objects that satisfy this definition, based on published results and diameter estimates. The dynamical structure of the trans-Neptunian region records the history of the migration of the giant planets in the early days of the solar system. The semi-major axes, eccentricities, and orbital inclinations of TNOs across various dynamical classes provide constraints on different aspects of planetary migration. For many TNOs, the orbital parameters are all that is known about them, due to their large distances, small sizes, and low albedos. The TNO dwarf planets are a different story. These objects are large enough to be studied in more detail from ground- and space-based observatories. Imaging observations can be used to detect satellites and measure surface colors, while spectroscopy can be used to constrain surface composition. In this way, TNO dwarf planets not only help provide context for the dynamical evolution of the outer solar system, but also reveal the composition of the primordial solar nebula as well as the physical and chemical processes at work at very cold temperatures. The largest TNO dwarf planets, those officially recognized by the IAU, plus others like Sedna, Quaoar, and Gonggong, are large enough to support volatile ices on their surfaces in the present day. These ices are able to exist as solids and gases on some TNOs, due to their sizes and surface temperatures (similar to water on Earth) and include N2 (nitrogen), CH4 (methane), and CO (carbon monoxide). A global atmosphere composed of these three species has been detected around Pluto, the largest TNO dwarf planet, with the possibility of local atmospheres or global atmospheres at perihelion for Eris and Makemake. The presence of non-volatile species, such as H2O (water), NH3 (ammonia), and complex hydrocarbons, provides valuable information on objects that may be too small to retain volatile ices over the age of the solar system. In particular, large quantities of H2O mixed with NH3 point to ancient cryovolcanism caused by internal differentiation of ice from rock. Complex hydrocarbons, formed through radiation processing of surface ices, such as CH4, record the radiation histories of these objects and provide clues to their primordial surface compositions. The dynamical, physical, and chemical diversity of the more than 150 TNO dwarf planets are key to understanding the formation of the solar system and its subsequent evolution to its current state. Most of our knowledge comes from a small handful of objects, but we are continually expanding our horizons as additional objects are studied in more detail.

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

Jay M. Pasachoff and Roberta J.M. Olson

Since the landmark lunar landing of Apollo 11 on July 20, 1969, NASA’s Lunar Reconnaissance Orbiter (launched in 2009), and the Japanese Aerospace Exploration Agency’s Kaguya spacecraft (2007–2009), among other efforts, have now mapped the Moon’s surface. Before those technological advances and since the beginning of recorded time, people and civilizations have been entranced by Earth’s only natural satellite, which is the second-brightest celestial object visible in the sky from the surface of the planet. Selected examples, among thousands, show how the history of the Moon has been regarded, illustrated, and mapped in visual culture in the Western world. Early examples include representations of a formulaic crescent Moon in Babylonian times; later this dominant stylized depiction of the Moon gave way to more naturalistic images based on observation, culminating in Leonardo da Vinci’s manuscript drawings, which study the lunar structure and cratered surface, and Galileo Galilei’s first telescopic images of the Moon recorded in wash drawings and woodcuts for his book Sidereus Nuncius. Both the artistic and scientific visual acuity that made this evolution possible belonged to the burgeoning empiricism of the 14th through the 17th centuries, which eventually yielded modern observational astronomy and impacted lunar iconography. The subsequent dramatic mapping of the Moon’s surface and the naming of its features became a preoccupation of many astronomers and some artists, who assisted scientists in illustrating their work. With the seeming physical mapping of the Earth-facing side of the Moon well underway in the late 18th and early 19th centuries, the function of Earth’s satellite as a Romantic symbol gained force in the all the arts but most dramatically in the works of landscape painters in Germany (e.g., Caspar David Friedrich and Carl Gustav Carus) and in England (e.g., Samuel Palmer). At the same time, William Blake, who was obsessed with astronomical imagery, used the Moon for expressive purposes, which reached a fever pitch later in the century in the work of Vincent Van Gogh. Along with the increasing accuracy of the Moon’s portrayal through both artists’ and scientists’ representations, the dramatic history of its mapping from Earth crescendoed with the development of photography and William Cranch Bond’s first successful daguerreotype of the Moon in 1851. Further exploration of the Moon, including its far side, has gravitated to aerospace engineers in cooperation with physicists, astronomers, mathematicians, and Apollo astronauts. Nevertheless, the Moon has remained an enduring object of fascination for artists—among the many, Surrealist Joan Miró, Veja Celmins, and Andy Warhol.

Article

Space activities can bring tremendous benefits to global development and humanity. For the safety, security, and long-term sustainability of outer space, activities and developments in the exploration and use of outer space must therefore be guided by the effective formulation, implementation, and enforcement of law and governance. Concerted and quality space law education and capacity-building efforts are necessary for the cultivation of competent professionals, scholars, and next-generation experts who are cognizant of the emerging issues and challenges posed by the proliferation of space activities and actors in the global commons of outer space. In order to fully grasp space law, it is important to possess a basic understanding of space technology, space applications, and the space environment in which the exploration and use of outer space take place. Not only should space law professionals and scholars be trained in law and have a deep understanding of especially public international law, but the approach to space law education and capacity-building must also be uniquely holistic and interdisciplinary. Hence, education and capacity-building can stimulate international development and cooperation in space activities and contribute to building expertise and capacity in countries with emerging space capabilities.