© Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 1 Space Object Ontology Robert John Rovetto New York, United States of America ontologos@yahoo.com 2007 University of Maryland alumnus 2011 University at Buffalo, SUNY alumnus February 2016 Keywords. Ontology, computational ontology, formal ontology, philosophical ontology, applied ontology, space object, astronautical ontology, space ontology, space domain ontology, astrodynamics, space situational awareness, astroinformatics Abstract. This paper develops the ontology of space objects for theoretical and computational ontology applied to the space (astronautical/astronomical) domain. It follows "An ontological architecture for Orbital Debris Data" (Rovetto, 2015) and "Preliminaries of a Space Situational Awareness Ontology" (Rovetto, Kelso, 2016). Important considerations for developing a space object ontology, or more broadly, a space domain ontology are presented. The main category term 'Space Object' is analyzed from a philosophical perspective. The ontological commitments of legal definitions for artificial space objects are also discussed. Space object taxonomies are offered and space object terms are defined. 1. Introduction This paper develops the ontology of space objects for a space object ontology, or more generally a space domain ontology. It follows ideas from [1] and [2]. The category term 'space object' is analyzed from a philosophical perspective, and strategies for space object taxonomies are offered. Given the inherent generality of the term, it is refined for a more thorough taxonomy. Legal definitions are presented in order to further demonstrate the intricacies of defining the general term, and to draw out important ontological commitments. We will see that these commitments are important to a formal ontological definition of more specific types of space object. Applications of this inquiry are found in the disciplines of formal ontology, astroinformatics, information fusion, and artificial intelligence, where data annotation, data-sharing, semantic interoperability, and knowledge representation are goals. The phrases 'space domain ontology' and 'space ontology' are general phrases for any astronautical or astronomical ontologies. A space object ontology is a type of space domain ontology that focuses on space objects. The space domain is the domain of study (or reality) that consists of the space about the Earth and its phenomena. However, to ontologically represent these objects fully we must also represent the environment they inhabit and dynamically interact in. I begin the paper by summarizing ontology and the domain to be ontologically represented. I then perform a high-level analysis of the category term 'space object'. Moving on to specific legal definitions, some common properties and identity conditions for space artifacts are explicated. Explaining the ontological import of legal definitions demonstrates the link between the space policy, space object, and ontology communities. Following this, I offer some space object taxonomies and qualitative definitions. A space object ontology file has been created and is displayed in section 8. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 2 Ontology (philosophy) category terms are camel-cased, often in bold as well. Those also in italics are relation terms. Words or phrases only in italics or bold indicate emphasis or key phrases. 2. Ontology Ontology is the philosophical study of reality, or a particular universe of discourse (a domain). An ontology is a general theory of a domain, and consists of categories, relationships and their definitions. It represents and describes actual and potential domain objects and their interrelations.1 Ontology has since found application in computational disciplines such as computer science, artificial intelligence, and informatics. The result is computational ontology or ontology engineering [3], collectively known as applied ontology2. This applied research field uses distinctions and methodologies from philosophy and formal ontology. Formal ontology, a branch of analytic metaphysics, represents an ontology with logics, philosophical methodologies, and many domain-neutral concepts. Philosophical concepts and distinctions such as unity, identity, whole-part, persistence, existence, generality-specificity (e.g. universal-particular) are defined with first-order, modal, and higher-order logics to aid in explicitly characterizing the entities in a given domain. Probabilistic and other non-classical approaches are also employed. By representing the ontological theory in a computable format, we produce computational ontologies, which are computational artifacts that present an ontology of a given application or domain. Domain ontologies express the knowledge of the entities in a scientific (or other) domain, represent those entities, whereas application ontologies have a specific application as a goal and may use domain ontologies. Foundational ontologies provide a theory of the most general categories and distinctions to subsume the above lower-level ontologies. Applied ontologies consist of a taxonomy whose terms have a formal semantics, the totality of which is computable by way of a knowledge representation or ontology language, such as Common Logic [4].3 Some goals of this interdisciplinary field are (Table 1): Concept(ual) analysis and clarification Terminology & Taxonomy development Data Annotation Data-sharing Automated reasoning Knowledge-discovery Knowledge representation Semantic interoperability Database interopability Table 1: Goals of Philosophical and Computational Ontology Ontology engineering is the design and engineering process by which computational ontologies are produced and employed. The ontology development process may or may not use specific scientific disciplines to inform its production, but it is necessary if scientific accuracy is desired (and for space ontologies, it should be). The next section summarizes the domain to be ontologically characterized. 11 Potential or possible entities are of critical importance because (i) we must not necessarily stand fast to one theory of time (whether presentist, 4D or otherwise), and (ii) modern science, including astrodynamics, is very much about predicting future behavior of objects and events. Thus, modality is a vital aspect for space ontology 2 http://www.iospress.nl/journal/applied-ontology/ 3 Note that although Description Logic and OWL are common, this paper does not recommend their use because of their greatly limited expressivity. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 3 3. The Domain: Objects in the Space Environment The domain to be ontologically characterized is that of space objects, broadly construed as objects in astronomical, but at least orbital, space environments. To formally represent these entities we must know: (I) what space objects there are (and can be), (II) their properties, and (III) how they are related to one another and to other entities, (IV) in what way they interact, and (V) what patterns exist or can develop therein. Given I-V, we therefore begin with some ontological (philosophical) assumptions: the distinctions between properties, property-bearers, their interrelations, and processual entities. We ask whether there is a scientifically accurate manner to categorize space objects. Short of this, and in any case, we seek a classification of space objects that will improve peaceful space domain awareness, help solve space domain problems, and help ensure future space travel. We seek to identify (or create) a list of category terms (relation, relata, and otherwise) that will compose a space object or space domain taxonomy. Philosophy and formal ontology *will facilitate this effort. By applying philosophical and computational ontology to this domain we yield a space domain ontology, whether a space object ontology, a space process ontology, space object behavior ontology, or otherwise. Figure 1 visually represents the objects in the broad space domain. Although, the space situational awareness and astrodynamics communities primarily consider artificial space objects, the analysis must consider all objects that would fall under the general category space object. Figure 1: Space domain Objects These space objects (phenomena) not only inhabit, but form part of that space environment. Relative to our terrestrial home, this includes the space surrounding Earth. The boundaries or divisions of this space © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 4 can be specified according to contemporary delineations such as Low, Mid, and High Earth Orbit, or according to natural physical thresholds, such as distances from the sun where water ice forms. However we are to ontologically characterize the space environment, it and the objects thereof are to be asserted in mutual causal interaction, in accordance with the dynamics expressed by scientific physical principles. That is, the ontology should reflect the mutual causality and interrelational aspect of reality. The environment, e.g., space-time, and material objects are not isolable. As such, a contemporary physics understanding of space and time (not necessarily a 4D or block universe view, mind you) such that the universe is dynamic and changing is here assumed. This is in opposition to the simplistic Newtonian (and earlier but related Aristotelian) container or "billiard-ball" view of space and time. The latter not only does not do sufficient justice to the dynamic and processual nature of reality, but it reflects an uncreative and limited world view and mindset. Although it may be a useful fiction or heuristic, from scientific and philosophical perspectives Newtonian and Aristotelian conceptions are quite lacking, both in philosophy and science. Existing ontologies (foundational or otherwise) that adopt such Aristotelian thinking should therefore be avoided. To create a veridical space ontology, we must be as scientifically accurate as possible, while accepting that the ontology is subject to revision as our scientific knowledge changes. As such the ontology must have the open world assumption as a principle. The next section discusses the methodology and resources for space object ontology development. 4. Methodology and Resources To create a space domain ontology, like any other, we approach the task in the following manner. Note that software development methodologies can guide ontology development, but any well-built computational ontology should include concept(ual) analysis.  Identify: goals, the domain, domain-problems to solve, requirements, and communities of interest  Domain research: domain reference documents, domain-experts  Terminology & Taxonomy development: list key terms, organize and define the terms in a taxonomy (a step towards an ontology), identify subsuming category terms  Philosophical and Concep(tual) analysis of key concepts and terms  Formalization of domain knowledge and term definitions using most expressible logics  Computational Ontology, Ontology Engineering o Translate natural language definitions and formal logic definitions to Knowledge representation and ontology languages. The latter should be as expressive as the logics used o Taxonomy/Ontology editor applications  Informatics and Data-science Drawing on table 1, domain-specific goals of the space object ontology are: space domain explication; space object data annotation, space object data sharing; space object catalog interoperability; and space object knowledge representation and discovery. The communities of interest (COI) are those of the space community, broadly construed to include: astronautics, astrodynamics, space situational awareness (SSA) (or space domain awareness), astronomy, and astroinformatics [5]. The SSA community will find this ontology application useful because in creating a space object © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 5 ontology, we also create a space object taxonomy. SSA communities maintain space object catalogs, and a standardized computable and semantically-rich taxonomy (i.e. an ontology) serves to annotate the data elements in catalogs. This will ideally facilitate interoperability among catalogs or SSA databases [1]. To create a scientifically accurate taxonomy of space objects we need to have sufficient scientific knowledge of said objects. This includes general knowledge of their physical and dynamic properties (including orbital elements), material composition, and the causal interrelationships at work among them and their environment. It will also include more specific knowledge of the space system architectures and functionalities. In pursuit of this domain research, consult SSA data (remote sensing data), experiential data4 of the orbital and near-earth object population, and domain professionals. Philosophical concepts will help generally categorize and formally describe this knowledge and the corresponding real-world entities. Members of the space community are to provide domain expertise, including verifying the accuracy of the knowledge expressed by the logical formalizations in the ontologies. Domain-specialists, e.g. astrodynamicists, are essential parts of any ontology development process that seeks to produce veridical domain ontologies. In totto, this partially constitutes the ontology development methodology. 4.1 Reference Documents The following non-exhaustive list of domain-specific resources is recommended. They serve as domain knowledge reference sources for the development of a space object taxonomy and the ontology of which it is a proper part. These reference documents help identify the common domain-specific terms and definitions to include in the ontology. To foster automated inference, the definitions may need to be adjusted, however.  Dictionary of Technical Terms for Aerospace Use (1965) by Allen 5  ESA Science Glossary 6  Canadian Space Agency Astronautics Vocabulary 7  NASA thesaurus 8  U.S. Department of Defense Dictionary of Military and Associated Terms (2015) Existing space and physics-related ontology resources that may also be used, incorporated into, or extended by, a space object ontology are these. Class terms from existing ontologies can be imported into other space domain ontologies and vice versa.  Ontology of Astronomical Object Types. 9  Astronomy/Science Ontology 10 4 Experiential data may include experiences with space objects and data from physical experiments. 5 http://er.jsc.nasa.gov/seh/menu.html 6 http://sci2.esa.int/glossary/ 7 http://www.asc-csa.gc.ca/eng/resources/vocabulary.asp 8 http://www.sti.nasa.gov/thesvol1.pdf , http://www.sti.nasa.gov/thesvol2.pdf 9 International Virtual Observatory Alliance (IVOA). Ontology of Astronomical Object Types. URL= http://www.ivoa.net/documents/Notes/AstrObjectOntology/ © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 6  NASA Sweet Ontologies [11]  Quantities, Units, Dimensions and Data Types Ontologies [12]  "A Space Surveillance Ontology Captured in an XML Schema" [13] (not an ontology in the modern sense) A survey and assessment of existing ontologies is critical to assess their quality, compatibility, and degree of domain coverage. For example, the NASA Sweet Ontologies has many terms relevant for a variety of scientific disciplines, but there are a number of undefined categories, some of which have questionable hierarchical placement. Discussion with developers is highly recommended to grasp the conceptualization in mind. The first resource in the list-the Astronomical Types Ontology (IVOA)- appears to also have a thorough taxonomy, one that accurately captures the various sorts of natural astronomical bodies in the universe. Good ontology development practice also calls for defining all terms save any primitives (undefined terms), which should nevertheless have some explicit clarifying remarks. Ontology users must be clear on the intended meaning of terms. For a given space domain ontology (existing or potential), at least two approaches are conceivable. One is to assert a set of more general class terms to subsume those in the existing (or developing) resource. Foundational (top-level) ontologies, such as the Descriptive Ontology for Linguistics often serve this function, and is a second approach. That is, to subsume the domain-specific taxonomies, upper-level (more general) and top-level ontologies, such as DOLCE may be used. The toplevel provides a common highly-general taxonomy for distinct ontologies to subsume their terms. Each top-level, however, takes one or more philosophical and ontological perspectives, and assumes certain metaphysical ideas. Care should therefore be taken if choosing an existing upper ontology; they should be assessed. User-base and politics must not be the deciding factor in adopting a particular upper or top-level. Rather, at least these two considerations determine which upper-ontology to use (if any). First, the degree to which the upper-ontology facilitates achieving the specified goals and real-world domain problems. Second, the degree to which the upper-level correctly (veridically, truthfully, faithfully) expresses and subsumes the domain science, knowledge and concepts. That being said, a toplevel ontology is arguably not strictly necessary. A second approach, at the more specific (lower) level of our space domain ontology, is to assert class terms with similar meaning (as synonyms) as those in existing ontology resources, and interconnect the distinct ontologies. These approaches aims to create a link between disparate databases. Moving on, the next section presents an analysis of the category of Space Object. 10 "Astronomy/Science Ontology", Department of Astronomy, University of Maryland, USA. URL= http://www.astro.umd.edu/~eshaya/astro-onto/ , http://www.astro.umd.edu/~eshaya/astro-onto/ontologies/astronomy.html 11 National Aeronautical and Space Administration (NASA) Semantic Web for Earth and Environmental Terminology (SWEET) Ontologies, Jet Propulsion Laboratory, California Institute of Technology. URL= http://sweet.jpl.nasa.gov/ 12 Quantities, Units, Dimensions and Data Types Ontologies (QUDT) URL= http://www.qudt.org/ 13 "A Space Surveillance Ontology Captured in an XML Schema", October 2000, Mary K. Pulvermacher, Daniel L. Brandsma, John R. Wilson, MITRE, Center for Air Force C2 Systems, Bedford, Massachusetts © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 7 5. The Category of Space Object 'Space object' is a term the space community has made frequent use of since approximately 1966 [6, p.6]. In what follows I will analyze its meaning and it referents from an ontological (philosophical) perspective. As part of the conceptual analysis for this domain, we must ask these questions prior to defining 'space object'.  What are our intuitive and prima facie conceptualizations/meanings of 'space object'?  What is the intended meaning of 'space object'?  Does the intuitive meaning match the intended meaning?  What do we mean by 'space'?  What do we mean by 'object'? The second question cuts to the chase, but all are necessary to establish consistency and clarity of meaning. Once the sense of the term is established, it will later be formally represented. In answering these questions we should also answer:  What are the referents (if any) of 'space object'? (What is its extension or denotation?) By answering the last question, we then proceed to identify the defining properties of the referents. This helps in distinguishing the various sorts of space object. Like other highly general terms, 'space object' is has an inherently ambiguity. Its meaning is grasped in context, but more often than not it refers to objects in orbit or somewhere in space beyond Earths atmosphere. As used in some space communities the term is typically for artificial space objects in the orbital and near-Earth environments. What more, each specific space community may have its own sense that constrains the sorts of object being referred to (Table 2). Space Community Scope of Space Object Astronomy All natural celestial bodies and phenomena in the universe: galaxies, stars, planets, moons, asteroids, magnetic fields, gravity waves, etc. Astrodynamics Artificial space objects in orbital space: telecommunications satellites, spacecraft, etc. Astrodynamic physical principles (e.g., laws of nature) 14 Space Situational Awareness Space Domain Awareness Those space objects of astrodynamics, plus near-earth and deepspace regions, and arguably also natural celestial bodies. Astroinformatics Data elements referring to actual space objects, or the space objects themselves Table 2: Discipline-specific scope of 'space object' It is doubtful that the SSA and astrodynamics community (the primary COIs for this paper) are interested in atomic nuclei, gravity waves, and Galaxies being classified as Space Objects. By contrast, the astronomy community would. The former disciplines have a smaller scope of objects (spacecraft, artificial satellites, etc.) than the latter, a scope that more accurately forms a subset of the broad category of Space Object. In other words, the extension of 'space object' for the astrodynamics community is smaller than that of astronomy. 14 These laws should be represented in a space object ontology. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 8 For astrodynamics and SSA communities, the ontology in question is more precisely an: orbital object ontology, of which an orbital debris ontology [1] may be a part. A space situational awareness ontology [2] would certainly have a scope large enough to encompass such an ontology as well. As outlined in [1], create modular domain-specific space ontologies. Although a particular community may use the term to denote only certain sorts of objects, the term is in fact more general. It is certainly wider in extension if we assume that its meaning comes from the meaning of its constituent terms. That is, part of the generality may come from that of 'space' and 'object', terms that reflect domain-neutral concepts and ontological categories from the history of philosophical and scientific thought. Space (and/or space-time) is oft-considered fundamental in philosophy (e.g., metaphysics, ontology) as well as physics. It is commonly treated as a foundational concept in psychology and some philosophical ontologies, but has a rich theoretical history, being a subject of much intellectual debate. From a linguistic perspective, the word 'space' has multiple meanings, but our intended meaning is that of astronomical space. Our domain objects are those of the space environment in which Earth resides and beyond Earth's atmosphere.15 In itself, this is a rather wide scope: all objects in the entire spatiotemporal universe are included unless we assert constraints, such as a narrower meaning for 'object' or 'space object'. The category of Object, likewise is inherently broad. Independent of context, the word 'object' is also ambiguous and flexible in meaning. As an ontological category, Object is often considered domain-neutral16. Just like the term 'entity' (which is here used as the most general term), we may use 'object' to describe: astronomical objects, biological objects, social objects, historical artifacts, etc. It will therefore be a matter of explicitly constraining the scope of the term. Note that some of the objects of study in Astronomy beg the question of what an object is. Entities such as magnetic fields and gravity waves require thorough research to ontologically describe because they may not be accurately characterized using traditional philosophical distinctions (e.g., object vs. process). The intended sense of 'object' vis-à-vis space objects are spatial or physical objects. These are objects that are spatio-temporal (at least spatial), and that have or can have dynamic and energetic properties. More specifically, the objects of interest for SSA communities-satellites, spacecraft, space debris, asteroids, etc.-are primarily material objects. However, the widest sense of 'space object' means a material object class is too restrictive: astronomy and astrophysics studies entities in the universe that do not have material parts, but consist of energy or fields, or some combination thereof. At one of the highest level of abstraction, the category of Endurant (or Continuant) 17 is an applicable top-level (domain-neutral) category to subsume material space artifacts such as spacecraft18 but 15 I use the word 'of' to express the causal interconnection of environment (e.g.space-time) with the objects. 16 http://plato.stanford.edu/entries/object/ 17 The philosophical category of Endurant/Continuant encompasses those property-bearing entities that continue to exist (persist) through time whether or not they undergo change. 18 An important fact to keep in mind is that as we move to high-level (more abstract) concepts and category terms, we will be moving closer to arbitrary definition and philosophical theories of the particular sort entity. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 9 depending on its definition may not be appropriate for the category of Space object in its widest sense due to exotic astronomical phenomena. The ambiguities of the term 'space object' and 'space object ontology' are apparent. There is the problem of resolving the mismatch between the implicitly broader sense of the term 'space object' with the narrower sense of the term as used in a given space discipline. The most accurate approach to address this is to avoid using the phrase/title 'space object ontology' unless 'space object' is used in its broadest sense. The alternative would be to explicitly state that the ontology uses the term in a more narrow sense, but I do not recommended because we can create more precise category terms denoting the specific types of space objects of interest. We should therefore use 'space object' in its broadest sense to avoid confusion and proceed to asset more specific categories as needed. The following property of some space objects facilitates this specialization. A single definition of 'space object' ensures semantic consistency, but note that when translating natural language definitions to artificial ones (ontology languages), we will inevitably run into decreased expressivity. It behooves us, then, to be clear about the meaning we wish to express. Given that space artifacts are often referred to by our category term, an important property to semantically capturer is artificiality. Not only do space objects such as satellites and spacecraft have physical and material properties, they have artificial properties. They are artificial, i.e., created by persons. The category of Artificial Space Object or Space Artifact, which encompasses most objects referred to by use of the term 'space object' in the SSA and astrodynamics community, is applicable. It does not include all types of space object however. For that we may apply the natural-artificial distinction as in Figure 1. The inherently broad sense of 'space object' requires addressing natural celestial bodies as well as artificial space object. The natural-artificial distinction allows us to group space objects such as asteroids and planets (natural space objects, or celestial bodies), and telecommunications satellites, orbital debris, and spacecraft (space artefacts). The following taxonomy is therefore a reasonable start. It is implicitly structured using the classsubclass (subsumption or 'is a') relation, whereby a subclass inherits the properties of its superclasses. Indentation indicates class subsumption, and forward slashes ('/') synonyms or co-extensional classes. A computable assertion, for instance, would be: Artificial Space Object is_a Space Object. Physical Object Space Object Natural Space Object Artificial Space Object / Space Artifact With the preceding in mind, the following qualitative definitions of 'space object' are offered. We move from the most general to the more specific according to the above constraints. This demonstrates the nuances in the philosophical analysis that forms part of the ontology development process. This definitional form is used: an A is a B that C's. Including the definiendum's (A) superclass (B) in the definiens fosters computability. Space, Object, space environment, inhabits, interacting in, located in, positioned in, occupies, or moves in/through are left undefined. The latter four are relations, © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 10 formally expressible and computationally implementable as n-ary predicates in a knowledge representation language. Note that for the sake of brevity, if a definition only includes 'inhabits', then it serves as a shorthand for all or some of these relations. Variations of the most general definition that takes location or environmental context as a defining aspect are 1A-C. Space Object =def. an object inhabiting, interacting in, occupying, located in, positioned, or (DEF-1A) moving through space an object inhabiting , interacting in, occupying, located in, positioned, or (DEF-1B) moving through the space environment an object inhabiting , interacting in, occupying, located in, positioned, or (DEF-1C) moving through the orbital or near-Earth space environment They vary in their description of the spatial regions, 1C being the most specific. Without further qualifications 1A and 1B encompass all objects in the universe. Everything from drifting hydrogen molecules, to GPS satellites, planets and Galaxies and perhaps objects the surface of a celestial body, can be classified as space objects.19 If the preceding taxonomy is accepted, we constrain the meaning further in the following manner. Space Object =def. a Physical Object (natural or artificial) inhabiting space (DEF-2A) a Physical Object (natural or artificial) inhabiting the space environment (DEF-2B) a Physical Object inhabiting the orbital, near-Earth or deep-space environments (DEF-2C) Only 1C and 2C constrain the position of the space object to the spatial neighborhood of Earth. However, given the generality of 'space object' it should be applicable to any planetary body. The Earth itself is a space object, after all. All but 1C and 2C are general enough and convey a less relative sense of 'space object', unless 'space environment' is defined for Earth. These definitions use the physical nature of the referents of the term 'space object' as a guide to classifying Space Object. As such they attempt a more objective definition. It is perfectly reasonable, however, that the category (or concept) of Space Object is not a natural mind-independent ontological category, but one that is attributed by persons in certain social contexts. Let us explore this alternative for classifying Space Object: Role. 19 This raises the question: are grains of sand, or cars in the street, space objects? © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 11 5.1 Space Object as a Role played by Physical Objects and Material Artefacts The category of Space Object can be categorized as a type of Role. The most easily recognizable roles are social roles held by persons, such as Medical Doctor, Professor, etc., but we may also assert roles held by material objects in space. These objects play one or more roles only in relation to our attributions and social circumstances. The role is attributed to the space object under an interpretation according to which Space Object is a socially-constructed category. Roles are existentially dependent entities that are held and played by entities, or realized in certain circumstances. That is, they require, for their existence, other existing conditions or states. The holder of the role stands in certain relationships to other entities and performs certain activities. A played, has_role, or bears relation relates the material object (material space artefact) with the socially-ascribed role. These roles are not essential properties of the material object itself. Consider a scenario in support of this approach. Is a space telescope not yet launched into orbit a Space Object? On the one hand, we have the intuition that it is a space object even when sitting on Earth. This intuition exists because the telescope was intentionally designed to operate in space. It has been produced to have a physical structure that affords its operation in the space environment. Thus, it can reasonably be classified as a Space Object. To formally represent this account of space objects, we would define the corresponding specialized category (Artificial Space Object) as an object intended to operate in space. Actually being in space is not a necessary condition, but it would be a sufficient condition to be a Space Object. On the other hand, there is a contrary intuition. Only when in orbit/space is it properly classified as a space object because being located in the space environment is a central property of the category (as definitions 1A-C emphasizes). The telescope would not be an object of interest for space sensors...yet. One formal approach addressing this intuition is to assert the telescope as a Space Object at one time but not at an earlier time. This involves a category change that some would object to, and may be computationally more complex. To avoid category change, we assert the individual telescope (SpaceTelescope1) as bearing the Space Object Role when in orbit around earth, but not when on the surface. See the following first-order logic expressions (with natural language translation). is_a(Space Telescope, Space Artefact) Space Telescopes are Space Artefacts. is_instance_of(SpaceTelsecope1, Space Telescope) SpaceTelescope1 is an instance of Space Telescope. ~[has_role(SpaceTelsecope1, Space Object Role, t1)] SpaceTelescope1 does not have the role Space Object Role at time t1. has_role(SpaceTelsecope1, Space Object Role, t2) SpaceTelescope1 has the role Space Object Role at time t2. The instantiation relation is a relation holding between categories and their instances. If taking this approach, the term Space Object Contact (asserted as a type of Role) may be more appropriate for space surveillance and SSA communities: only when the object is observable in space that it is deemed a Space Object. Two role-based definitions of space object are as follows. Space Object Role / Space Object Contact =def.  A role held by a physical object that inhabits the space environment. (DEF-3) © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 12  A role held by a physical object that has been detected in the space environment. (DEF-4) From here assert more specific space object roles, such as Space Artefact Role. Finally, the category of Payload (or Space Payload, more specifically) is arguably best asserted as a type of Role. The Hubble space telescope, for example, played the role of Payload during a past launch process until its deployment (or orbit insertion). After that point in time, it ceased to bear the Payload Role. At all times, however, it persists as an instance of the class Space-based Telescope (or Astronomical Telescope. Likewise for space objects: when in space, the particular space artefact or whatever material object, plays the Space Object Role, but when on the surface of some celestial body it does not. Regardless of the parent category of Space Object, part of distinguishing one sort of space object from another involves identifying that which distinguishes it from another sort of entity. Differentia may be aspects or properties of the space object, its involvement in (or enactment of) some process, some relationship(s) to other entities, or patterns thereof. Differentia in class definitions of the form An A is a B that C's (the genus-species-differentia form, assumed here) take the place of C. Relevant scientific reference documents help identify the characterizing attributes to distinguish types of space object. This is the subject of the next section. 6. Properties of Space Objects Properties, features, attributes, qualities, or characteristics distinguish one object from another. They may be physical, structural, social or otherwise; and we may distinguish static properties from dynamic properties as well. Properties help ontologically characterize a given object. Like Roles, they are sorts of Dependent Entity in the sense of being mutually existentially dependent on some propertybearer. A property is had or borne by a property-bearer (such as a material object). These minimally binary predicates can be asserted to relate the property-bearer and the property: for some particular GPS satellite and mass, we assert GPS-Sat1 has_property Mass1. Material objects, such as electrons and spacecraft, are examples of property-bearers: they bear properties such as mass, charge, etc. Essential properties help describe the identity of an object. An essential property is a property without which the property-bearer would not be the type of entity it is. For example, from a functional perspective, an optical space telescope is essentially a telescope whose function is to collect photons from interstellar space. The function (also a dependent entity) is an essential property of this space artefact. Table 3 presents a list of category terms appropriate to characterize space objects. Physical, Dynamic & Relational Properties Misc. Legal or Social Entity Terms Mass Length Owner Shape Width Nationality Material composition Height Manufacturer Albedo Cross-Section Operational Status Velocity, Rotation Launch Date Mission Ballistic Coefficient Reentry Date © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 13 Keplerian Orbital Elements Function State Vector Table 3: Property classes to characterize space objects Space artifacts are essentially artificial. Artifacts bear these properties: being human-made, and thus having a historical human-made origin, being intentionally designed and created, and having one or more specific function. The function exists not only because it is intentionally ascribed/attributed by persons, but also because the artefact was constructed to fulfil that function. It has the physical capability to do so, and that capability (and function) is in part due to its material structure, a structure that is the manifestation of human intentions and actions. Some space artefacts, particularly whole spacecraft and satellites, have one or more intentionally designed functions20 as essential properties of the spacecraft as a whole. A functional approach is helpful for classifying types of space artefacts: space vehicles, artificial satellites, etc. Not all artificial space objects necessarily have functions, however. Consider shards of spacecraft (a space artefact part) from orbital collisions. The shards (space debris) themselves do not have a function, but the whole of which they were once a part certainly did. In other words, since space debris may not have a function in themselves separate from the whole artefact, having a function is neither a necessary condition nor an essential property for all space objects. As we will see in the section on taxonomies, we need not explicitly use the terms 'artificial', 'artefact' and 'natural' in space object categories. Rather than using the natural-artificial distinction, we may instead distinguish space object categories by using some combination of physical parameters and other sorts of property. This includes using the properties common to artificial objects. So long as terms such as 'artificial' and 'artefact' are used, the ontology should develop or adopt a theory of artifacts (and of intentionality and function), but I will not discuss this here. In any case, although agreement as to defining properties of space object types may be an arbitrary decision, it is best achieved by consulting both a variety of scientific and philosophical viewpoints and reference documents (including existing classifications). For the general term 'space object' defining features can also be drawn from legal definitions. 7. Legal Definitions of 'space object' and their Ontological import This section discusses some legal definitions of 'space object' toward a formal ontological definition, drawing out their ontological commitments. Ontological commitments21 are that which a theory or statement explicitly or implicitly implies to exist or that could exist. Given that ontology is concerned with actual and potential existence, the inquiry is relevant. The type of space objects these definitions refer to are primarily artificial space objects tracked by the SSA community. 20 We may distinguish types of function, e.g., an engineering design function for artefacts. 21 http://plato.stanford.edu/entries/ontological-commitment/ © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 14 Christopher M. Hearsey [6] provides an informative survey of the legal definitions of 'space object'. He notes its various senses: "the treaty definition of space object represents specific meanings under different treaties", and aims to find "those common elements that give meaning to the term space object under general international law"[6, p.4]. Finding common elements is one task of ontological (philosophical) inquiry. These elements may serve to characterize artificial space objects in particular. Hearsey asks when an object becomes a space object, and what makes an object a space object. In philosophical jargon, these questions are about the identity conditions of space objects: what must be the case, what properties the object must have, and what relationships it must stand in to be classified as a space object. As noted in section 6, to answer these identity questions (and toward stable definitions22), use physical, material and functional properties to describe categories of space object. This include spatiotemporal, structural, chemical, and dynamic properties. Social properties will also characterize the objects, providing insights into the wider socio-political interrelations between space actors, their space assets in orbit, and activities they can or should engage in (e.g. space debris removal). "The term ['space object'] itself activates obligations, affects treaty rights, and other rights and duties under international law." [6, p.3] An ontology of space objects should therefore make reference to the appropriate legal and social entities, e.g., rights, duties, obligations. They can help differentiate one category from another. Various treaties have defined or used 'space object' in a like manner, the common thread being that the objects in question are primarily artifacts such as spacecraft, satellites and space debris. That being said, although "most states [at least 88] [6, p.24] promulgate the term space object in close conformity to the article I definition of the Liability and Registration Conventions" [6], making it reasonable to use it to inform (but not determine) an ontological definition, we must be aware of at least two limitations. First, "the definition of space object, as expressed in article I of the Liability and Registration Conventions, provides only a basis in which to understand the meaning and scope the terms legal effect under international law."[6, p.23] and two, it is circular. Quoting [7], [6, p.7-8] writes that two definitions of 'space object' were considered: (L1) "the object itself and its component parts, as well as the means of delivery and its component parts" and (L2) "articles on board the space object and articles detached, thrown or launched from the space object. The former was chosen, resulting in the circular definition (L3): "The term 'space object' includes component parts of a space object as well as its launch vehicle and parts thereof" [8][9]. By using 'space object' in its own definition, a common principle of formal definitions is violated: non-circularity. It is true that context and intuitions help us grasp the intended meaning. It is also true that there are different sorts of definition, and it is certainly possible that this principle only applies to formal definitions in formal sciences, but circularity is best avoided for analytical applications and where conceptual clarity and computation is desired. We will therefore proceed as if this principle is worth adhering to for the present inquiry. 22 Also note that even though we seek a physics-based ontological definition, the term 'space object' is sufficiently generic to resist such a definition. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 15 Note also that L3 excludes objects mentioned in L2 that can rightly be called space objects by the SSA community. Another limitation is that the definition does not explicitly state the type of space object in mind. We need a definition that makes explicit what is implicitly assumed. The second use of 'space object' in L3 implicitly means artificial space objects or spacecraft. Granting this, another principle is violated: the definiens should contain terms that are simpler (conceptually less complex) or broader in meaning than the definiendum. As we have established, however, 'space object' is broader than 'spacecraft' and 'artificial space object'. L1 correctly uses 'object' as the term more general than the definiendum: a Space Object is indeed a type of Object. Let us consult other definitions. [6] quoting Gorove again, we read the following qualification to the above definition: "Thus any object without which the spacecraft would be regarded incomplete, may be taken to be a component part." [6, p.8] This is ontologically significant because it references the structural and the functional composition of the artificial space object. Mereology-the theory of parts and wholes23-will be useful for ontological descriptions, as will distinguishing types of part: structural parts, functional parts, etc. It is important to describe the mereological structure of space objects, and their historical development (provinence). For example, we must know whether an orbital debris object was part of a GPS satellite, a space shuttle, or other space object. This partonomic knowledge plays a part in determining origins and thus legal considerations such as ownership and responsibility. By formally representing the parts (components and otherwise) of a space artefact, an automated reasoner will infer that an identified space object of a particular component class belongs to a particular class of, say, spacecraft. According to L1 and L3, the component parts of a space craft are space objects. Some or all of these components are proper parts in the sense of mereological parthood. Without them a spacecraft would not be whole or structurally complete. In functional terms, the function of the spacecraft is not achievable, actualizable or realizable until the spacecraft is structurally and functionally whole. That is, all its component parts are in place, and all are capable of performing their function. This forms the artefactual unity-the whole-that is the spacecraft. Cheng's description of 'space object', as quoted in [6 p.8-9], explicitly references the space environment (an essential aspect not mentioned in L3, but mentioned in the section 5 definitions): "any object launched by humans into outer space, as well as any component part thereof, together with its launch vehicle and parts thereof." A functional definition by Belgium that combines various identity conditions, including component parts and launch vehicle being space objects (the latter two not quoted here), is: "Any object launched or intended for launching on an orbit around the Earth or to a destination beyond Earth orbit; [...] Such a device shall also be considered a spacecraft even where it is operated experimentally for the purposes of its development or validation stage." [10] 23 "Mereology". http://plato.stanford.edu/entries/mereology/ © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 16 Mention of 'device' and 'spacecraft' makes the intended meaning once again clear: space artifacts. The novel conditions are intention to launch and experimental operation. We also read this scope-defining statement in a proposal by some nations: "For the purpose of [the] Convention, the term 'space object' also includes [...] all component parts on board, detached or torn from the space object." [6, p.8] This particular definition ensures the scope includes certain types of space debris, namely spacecraft fragments and separated components. [6, p9] mentions Gal's [11] definition, which adds an important relational aspect: the type of motion. "only those objects can be regarded as space objects which perform an orbiting movement round the earth or other celestial bodies, or which have been launched with that purpose," Here, actual and intended orbital motion are necessary conditions for an object to be a space object. An approximate first-order logic translation would be: ∀x [ instance_of(x, SpaceObject) ⟶ orbits(x, Earth) or orbits(x, y) & instance_of(y, Celestial Body)] For all objects x, if x is an instance of the class Space Object, then x orbits Earth or x orbits y, and y is an instance of some celestial body. Notice, that although artificial space objects are the focus, without explicitly stating the artificial nature of the object in question, this definition remains broad enough such that the Moon and other natural satellites satisfy its conditions. The ontological commitments of these definitions are to: the space object, space object parts, launch and delivery vehicle, objects onboard, orbital motion, celestial bodies, purpose, and function. In a similar vein, Hearsey [6, p.21] identifies these common elements among legal definitions of 'space object'. (1) object (2) intent to launch (3) launched (4) launch vehicle (5) payload (6) component parts and parts thereof (7) satellite I will now discuss these common elements from an ontological (philosophical) perspective. Many if not all have either corresponding or subsuming ontological categories. As such, these entities serve to identify types of space object. Object was discussed at the beginning of this investigation (section 5), but is a necessary condition: if x is a space object, then x is an object. The intent to launch, (2), is an intention, which is a mental entity or cognitive entity. It is quite possibly a collective socio-psychological entity. That is, aside from intentions held by individual human beings, we may assert collective intentionality. On this, [6, p.25] has a helpful condition that we can adopt for describing this space object property. We read: space objects are "intended to be launched into outer space by an authorizing government, as registered, or by way of a national activity". This quotation arguably ontologically commits the speaker to: © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 17  Intention(s)  At least one holder or issuer of the intention, i.e., human beings, perhaps governments;  A registered document that concretely expresses the intention, and/or  Some national activity (e.g., a process) which helps manifest or execute the intention The intention temporally precedes launching, but exists during the conceptual development and design phase of the space artifact. It continues to exist, if not through the lifetime of the space object, then at least until launching. These considerations are about the persistence of the entity in question. Having been launched (3) can be ontologically represented as a historical property or event. During its occurrence, however, it is an occurring event or process (e.g., a Launching Process). At some time t, space object x was launched, and at some later time t+n x was inserted into orbit. During the interval [t, tn] a launching process in which x was involved was occurring. Launch vehicles, payloads, component parts and their parts, and artificial satellites (4-6) are Material Artifacts and Material Objects. According to legal definitions they are artificial space objects. As [6, p.21] notes, spacecraft crew and other occupants would not be component parts.24 Likewise for fuel exhaust. Taken together, 1 through 7 more precisely characterize the category of Artificial Space Object or Space Artefact. Being artefactual, the category of Function and its subtypes are relevant. If Space Object is treated as a type of Social or Legal Entity, then it will change as legal definitions do. A social or legal entity is one whose properties are socially or legally determined, rather than physically or naturally determined. If "national legal requirements seem to limit generally the scope of the type of object a space object can be" [6, p.13] and a space object ontology uses legal definitions as an authoritative source, then it would also seem that the subclasses of Space Object would be dictated by the types of space object specified by those legal requirements. I do not recommend this strategy since it contradicts intuitions on, and ordinary usage of the category of space object. It might be relevant to a role-based representation of the category of Space Object, however. If we seek a physics-based definition of 'space object', then an ontological definition should not change when legal considerations do, but when physical science is corrected or otherwise updated. The latter approach has been assumed thus far, but the generality of the term appears to require a degree of arbitrary ascription, which gives us cause to consider the Role approach (section 5.1). Moving on, the next section offers some space object taxonomies and definitions of key terms. 8. Types of Space Object and Space Object Taxonomies 24 One may argue, however, that some sorts of space artifacts (e.g., shuttlecraft)-where human operation is required-calls for human beings to play a Role of a functional, agentive and non-integrative part of the spacecraft. Note that persons should not be represented as subtypes of the category of Object because of the implicit non-human, and often dehumanizing, association of the term 'object'. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 18 The following space object taxonomies are structured according to the class subsumption relation, also called the 'Is a' relation. For example, the class Telecommunications Satellite is a subclass of Artificial Satellite. The latter is more general (more encompassing) than the former, carrying less properties that characterize it. As mentioned in the section 7, partonomies, which are taxonomies structured with parthood relations ('is part of', 'is proper part of', etc.) along the lines of General Extensional Mereology, will be helpful for describing the composition of space artefacts. Pending further domain research, key dimensions to classify space objects are: function, physical properties, spatial location (e.g., at a particular orbital altitude), and motion (e.g., orbital motion, on an interplanetary trajectory, etc.). The most general space object classes use a locational aspect, whereas the most specific artificial space object classes are easily classified according to design or operational function. The natural-artificial distinction is an intuitive starting point to help organize a space object taxonomy (Table 4), but class terms need not explicitly include the words 'natural', 'artificial' or 'artefactual'. 'Natural space object' refers to what have historically been called celestial bodies, i.e., the objects of study in astronomy. For this communication, we may substitute 'body' by 'object' with no loss in meaning. Natural space objects Artificial / Artefactual Space Objects Asteroids Spacecraft Comets Space Telescope Moons (natural satellites) Telecommunications satellite (artificial satellite) Planets Space Debris / Orbital Debris Stars Space Station Table 4: Types of Space objects grouped according to the natural-artificial distinction Naturally formed space objects can be represented using the category of Astronomical Object, similar to the IVOA Ontology of Astronomical Object Types (its class term being 'AstrObject').25 'Astronomical Object' is here synonymous with 'Celestial body', but a subclass of Natural Space Object to allow for astronomical phenomena that are best categorized as other than a body or object, where body and object are conceived as some unitary material whole. Pending a comprehensive review of the IVOA ontology, we may reuse it in its entirety or merely import some of its classes. Table 5 presents a space object taxonomy that explicitly includes the natural-artificial distinction. The natural space object classes to the right may be structured similar to, if not reused from, the IVOA ontology. Given the broad nature of the category of Space Object, subsuming Astronomical Object under it is justifiable. Taxonomy 1 Natural Space Object Classes Space Object Natural Space Object 25 This brings up one case: some space debris are natural space objects, a scenario that can be used in favor of categorizing Space Debris as a type of Role played by space objects. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 19 Celestial Body / Astronomical Object Artificial Space Object Spacecraft / Space Vehicle Space Shuttle Space Launch Vehicle ... Artificial Satellite <Satellite classes> Space Station Space Debris <Debris classes> Celestial Body / Astronomical Object Asteroid Comet Meteor Planetary Body Planet Moon Stellar Body Star <Stellar classes> Galactic Body Galaxy <Galactic classes> Nova Supernova <Supernova classes> Table 5: Space object taxonomy 1 (left) with Natural space object classes (right) The terms " [...]'space vehicle', 'spacecraft', 'spaceship', 'satellite', and 'space station' represent the types of objects launched into outer space"[X p.7]. They refer to artificial space objects, and have a place in a space object taxonomy, but some should be synonyms, e.g. 'space vehicle' and 'spaceship'. The Space Station class is disjoint from Space Vehicle. Despite the fact that both transport persons or cargo, the former is not designed to travel vast trajectories under their own power, or to be autonomous vehicles on part with shuttlecraft or rockets. Rather, space stations are designed to occupy a position in orbit, to be a human place in space as it were. Figure 2 displays space object classes using the Protégé ontology editor. Figure 2. Space Object Ontology in Protégé ontology editor. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 20 Taxonomy 2 below differs by removing use of the term 'natural'. I also use the category of Space Artefact and generalize that of Spacecraft to be equivalent. I appeal to the general sense of 'craft' according to which a craft is any work by persons. This results in the meaning: any space (astronautical) work by persons. The category of Space Vehicle appeals to the sense of 'vehicle' as an artefactual means for transporting persons or cargo through space. Taxonomy 3 maintains the generalized Spacecraft sense, but removes Space Artefact and the terms 'natural' and 'artificial'. Instead, it implicitly adopts a strategy according to which we refer to features that characterized artifacts, e.g. being human-made, function(ality), intention, design, historical origins, etc. Taxonomy 2 Taxonomy 3 Space Object Celestial Body / Astronomical Object Space Artefact / Spacecraft Space Vehicle Space Shuttle Space Launch Vehicle Artificial Satellite <satellite classes> Space Debris <Debris classes> Space Object Celestial Body / Astronomical Object Spacecraft Space Vehicle Space Shuttle Space Launch Vehicle <Artificial satellite classes> Space Debris <Debris classes> Table 6: Space Object Taxonomies 2 and 3 At least two space object classes need further study: satellites and debris. Natural satellites are satellites formed by physical processes independent of human action, e.g., moons. Artificial satellites are spacecraft that orbit an astronomical body. They come in a variety of types, each designed to perform one or more specific functions. Since orbital motion is the property, state or process common to all satellites, we must distinguish them according to an additional criteria, such as design function. Non-exhaustive taxonomies of human-made satellites, and space debris are as follows. Artificial Satellite (Tele)Communications Satellite Navigation Satellite GPS Satellite Search & Rescue Satellite Earth Observation Satellite Weather Satellite Reconnaissance Satellite ... Space Debris Comet Fragments Spacecraft Fragments Defunct Spacecraft Mission-related Space Debris ... © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 21 Eliminating Artificial Satellite from the taxonomies, but asserting its types yields Taxonomy 3 (and 4). This strategy is supported by the fact that any object can orbit a planet, making orbital motion insufficient to thoroughly distinguish one artificial space object from another. Similarly, we may remove Space Artefact and Artificial Space Object from all taxonomies, and assert their respective types with the appropriate identity conditions: being made by person, having a design function, etc. This strategy is suggested when a philosophical account of artefactuality is either undesired or will complicate either the taxonomy or automated reasoning. Finally Taxonomy 4 removes the Space Object class, but asserts a Space Object Role (or Space Object Contact Role). Lower-level space object classes are asserted, to be formally related to Space Object Role class by an n-ary (at least ternary) has_role predicate as described in 5.2. The Spacecraft class serves to group the artificial space objects. Taxonomy 4 Celestial Body / Astronomical Object Spacecraft Space Vehicle Space Shuttle Space Launch Vehicle <Artificial satellite classes> Space Debris <Debris classes> Role Social Role Medical Doctor Role ... Material Artefact Role Space Object Role / Space Object Contact Role Table 7: Space Object Taxonomy 4 Regardless of the taxonomy, candidate superclasses of artificial space objects include: Space Artefact, Spacecraft, Material Artefact, Physical Object, Material Object, Physical Entity, and so on. The latter three may serve as superclasses for natural space objects (Astronomical Object) as well. Given the exotic and category-defying phenomena in the universe, I recommend a more general class such as Astronomical Entity, Astronomical Body, or the traditional Celestial Body. The next section proposes some definitions for the main space object classes. 9. Definitions The following are definitions of the central space object categories. Parenthetical notes indicate if the definitions correspond to the preceding taxonomies or whether they are more generic definitions. As such, I provide more than one definitional option for some terms. For any given definiendum, bullet points mark variations that may use different parent classes. '...' signify that it uses the preceding definition but with a different superclass. Forward slash ('/') separates synonyms or co-extensional © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 22 categories. To reiterate, the inhabits relation may be substituted for or supplemented with other relations, such as interacts in, is located in, is position in, moves through, etc. (to be defined elsewhere). Space Object =def. a Physical Object inhabiting the space environment (DEF-2B) Space Object Contact =def. A Physical Object detected in space by a ground or space-based sensor. Space Object Role / Space Object Contact Role =def.  A role held by a physical object that inhabits the space environment. (DEF-3)  A role held by a physical object that has been detected in the space environment. (DEF-4)  A role held by a material object that has been detected in space by a ground or space-based sensor. Supplemental Information: Detection is either a state or process (but not defined here). Natural Space Object / Celestial Body / Astronomical Object =def. a Space Object formed by physical processes according to natural physical principles (laws of nature), independent of human action. (Taxonomies 1-3) Supplemental Information: it is the space phenomena that forms the subject matter of astronomy. Examples: planets, stars. Instances: Earth, the Sun. Artificial Space Object =def.  a Space Object, its component parts, portions or parts thereof, and contained articles, created by persons; and intended, designed and physically structured to operate in the space environment (Taxonomy 1)  A material object, ... This definition encompasses whole spacecraft as well as space debris originating from space artefacts. Function is not mentioned in order to include space objects that do not have a function, but are nonetheless artificial. Including functions, we have Space Artefact, below. Space Artifact / Spacecraft =def.  A Space Object, its component parts, portions or parts thereof, and contained articles; that has been created by person; and intended, designed and physically structured to operate in the space environment, and that has one or more specific functions. (Taxonomy 2, 3)  A Material Artifact, ... Space Vehicle =def.  a Space Artefact / Spacecraft designed, and whose function is, to transport persons or cargo (payloads) through the space environment. (Taxonomy 2)  a Spacecraft designed, and whose function is, to transport persons or cargo (payloads) through the space environment. (Taxonomy 3) Spacecraft / Space Vehicle (general) =def.  a Space Artefact whose minimal (at least one) function is to transport persons or cargo in the space environment. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 23  A material object created by persons, whose minimal (at least one) function is to transport persons or cargo in the space environment Space Station =def. a Space Artefact whose minimal function is to provide persons with a long-term and relatively stable/stationary location in the space environment from which to perform various activities such as scientific research. These definitions do not include legal conditions or properties, but if desired, including conditions such as being launched by a nation-state is a matter of course. Conclusion This paper has developed the ontology of space objects, presenting a qualitative analysis of the category of Space Object and subcategories. I offered space object taxonomies, and definitions of the category. A generic space object ontology would be useful to categorize objects in space and annotate data about those objects from existing space object catalogs and space situational awareness databases. By fostering space domain awareness, data-sharing, and interoperability we improve SSA and spaceflight safety. The preceding has provided much needed theoretical contributions to developing ontologies and taxonomies for the space domain and its objects. Space objects include: artificial objects such as spacecraft, space stations; and natural space objects as studied in astronomy. Physical properties principles (laws of nature, astrodynamics laws, etc.) should be incorporated for a more quantitative ontological representation of space objects and their characterization. We must bear in mind that "the definition of space object has evolved over time with the innovation in the types of technologies that can be launched into outer space. States have sought to generalize how they define such objects or kept the terminology open-ended."[6. p.26]. As we advance the state of the art in spacecraft design, astronautics, and space science and development, the amount (and possibly sorts) of space objects will increase. Any definition should reflect this generality, not least to ensure the flexibility needed to describe and characterize lower-level domain space objects, such as GPS satellites. © Robert J. Rovetto 2016. All rights reserved. Preprint. Currently under journal review as of Feb 2016. 24 References [1] Rovetto, Robert J. An Ontological Architecture for Orbital Debris Data. Earth Science Informatics, 9(1), 67-82 (Published Aug 6 2015, submitted Dec 2014. Printed Feb 2016).Springer Berlin Heidelberg. URL= http://link.springer.com/article/10.1007/s12145-015-0233-3 DOI: 10.1007/s12145-015-0233-3. [2] Rovetto, Robert J., Kelso, T.S. Preliminaries of a Space Situational Awareness Ontology. (2016) 26th AIAA/AAS Space Flight Mechanics meeting, Napa, California, Feb 17 2016. [3] Staab S, Studer R (Ed.)(2009) Handbook on Ontologies, International Handbooks on Information Systems, Springer 2nd ed. [4] Common Logic International Organization for Standards. URL= http://www.iso.org/iso/catalogue_detail.htm?csnumber=39175 [5] Borne K.D. (2010) Astroinformatics: data-oriented astronomy research and education. Earth Science Informatics, Volume 3, Issue 1, pp 5-17. URL= http://link.springer.com/article/10.1007%2Fs12145-010-0055-2 (2009) Astroinformatics: A 21st Century Approach to Astronomy Astro2010 State of the Profession Position Paper. URL= http://arxiv.org/pdf/0909.3892v1.pdf [6] Hearsey, Christopher M. Comparative Study of the Definition of Space Object in National Space Laws and its Legal Effect under International Law, May 2 2012 Univ of Miss school of law. URL= http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2072514 [7] GLENN H. REYNOLDS & ROBERT P. MERGES, OUTER SPACE: PROBLEMS OF LAW AND POLICY 210 (Westview Press 1997) (1989). [8] Article 1 of Convention on Registration of Objects Launched into Outer Space The General Assembly, http://www.unoosa.org/pdf/gares/ARES_29_3235E.pdf, http://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introregistration-convention.html [9] "Art 1(d) Convention on International Liability for Damage Caused by Space Objects", http://www.unoosa.org/pdf/gares/ARES_26_2777E.pdf, http://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introliability-convention.html [10] National legislation and practice relating to the definition and delimitation of outer space Note by the Secretariat-Addendum (28 January 2013), UN General Assembly http://www.unoosa.org/pdf/reports/ac105/AC105_865Add12E.pdf, p.2 [11] Gyula Gal, Thirty Years of Functionalism, 40 PROC. COLL. L. OUTER SPACE 125 (1997).