1,720,995 research outputs found
Assessing the Feasibility of Ion Beam-Based Asteroid Deflection for Planetary Defense
No full textThe potential threat of asteroid impacts on Earth has prompted significant research into methods for asteroid deflection such as kinetic impactor, gravity tractor, nuclear options and ion beam deflection (iBeam). Among these methods, iBeam is a promising technique as it can achieve significant deflection without the risk of unintended asteroid disruption. In this paper, we analyze the feasibility of iBeam missions for deflecting potentially hazardous asteroids. We first present the architecture for iBeam missions, including the design of the spacecraft and subsystem models, multiple ion-beam plasma plume models, and a dynamics model for the spacecraft-asteroid system. Utilizing these models, we implement closed-loop control for maintaining iBeam spacecraft position relative to the asteroid with a proportional-integralderivative controller and manage attitude using a nonlinear proportional-derivative controller. We then perform Monte Carlo (MC) simulations of complete iBeam missions over various asteroid characteristics to study iBeam efficiency. Through the MC simulations, we study the effect of asteroid diameter (50-100 m), density (2-8 g cm-3), spin axis (principal axis vs minor axis), and shape (spherical vs irregular) on the deflection. Results indicate that under the assumed capabilities of the iBeam spacecraft, successful deflection of 50 m diameter spherical asteroids is achievable within 6 months for densities under 4 g/cm3 or within 5 years for densities below 8 g/cm3. Similarly, for 150 m diameter spherical asteroids, successful deflection within 5 years is possible if densities are under 2 g/cm3. Additionally, the study reveals a marginal influence of spin axis and shape on deflection. Compared to a reference case of a spherical asteroid, a maximum change of 5.6% is observed for irregular-shaped asteroids rotating around their minimum moment of inertia axis pointed towards the spacecraft
Astrodynamics-Informed Kinodynamic Motion Planning for Relative Spacecraft Motion
Full text linkThe work presented in this dissertation takes inspiration from robotic sampling-based motion planning ideas and presents an astrodynamics-informed kinodynamic motion planning (AIKMP) algorithm which is a single-step sampling-based kinodynamic approach to orbital motion planning problems (as opposed to existing two-step sampling-based motion planning approaches in literature). The AIKMP algorithm can quickly find solutions to spacecraft relative transfer problems in a very cluttered environment and it iteratively improves on its computed transfer solution and thus holds the potential of computing near-optimal transfers given a sufficient number of algorithm iterations – without needing an initial guess of the solution. This algorithm introduces an astrodynamics-informed pruning module that allows the motion planner to maintain and store a sparse set of nodes improving its overall computation efficiency by ≈ 98% and storage efficiency by ≈ 80%. This work also presents a novel extension of the linearized Lambert solution (LLS) called the closed-loop linearized Lambert guidance solution that allows a spacecraft to apply correction burns during the transfer to improve the targeting accuracy in relative guidance problems in the presence of perturbations like Drag, J2, and Solar Radiation Pressure (SRP). This novel closed-loop guidance law is applied to Spacecraft Formation Flying (SFF) problem, and by presenting theoretical developments backed with multiple simulation results it is shown that the algorithm allows for stringent targeting accuracy with fuel efficiency in different SFF problems. It is also demonstrated that closed-loop LLS guidance can be used to enable a spacecraft to safely follow a reference trajectory generated by an orbital motion planner like the AIKMP algorithm at a cost of ≈ 3% fuel increase as compared to open-loop guidance that is unable to provide safety guarantees in a cluttered and perturbed environment. Lastly, this work presents an extension of the popular E/I vector separation method derived for the case of drifting relative motion to infuse passive collision avoidance capabilities in sampling-based orbital motion planners. The resulting motion planner performs tree extension in a more informed way such that collision avoidance constraints are satisfied by each node and edge in the sampling-based tree. This way, every transfer solution computed from the tree is guaranteed to be collision-free for the entire duration of the transfer in the presence of multiple static (static relative to a moving reference frame) and moving obstacles – a capability that is crucial for SFF applications. Overall, the AIKMP algorithm describes the sufficient number of steps that can be adapted from a sampling-based robotic motion planner with required modifications to achieve fuel-efficient and collision-free sampling-based motion planning in astrodynamics applications. A notable aspect of this proposed motion planning framework lies in its modularity, as individual modules of the algorithm can be adapted to any existing sampling-based motion planner (tree-based planners or graph-based planners) to improve their applicability to orbital motion planning.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Astrodynamics-Informed Kinodynamic Motion Planning for Relative Spacecraft Motion
Full text linkThe work presented in this dissertation takes inspiration from robotic sampling-based motion planning ideas and presents an astrodynamics-informed kinodynamic motion planning (AIKMP) algorithm which is a single-step sampling-based kinodynamic approach to orbital motion planning problems (as opposed to existing two-step sampling-based motion planning approaches in literature). The AIKMP algorithm can quickly find solutions to spacecraft relative transfer problems in a very cluttered environment and it iteratively improves on its computed transfer solution and thus holds the potential of computing near-optimal transfers given a sufficient number of algorithm iterations – without needing an initial guess of the solution. This algorithm introduces an astrodynamics-informed pruning module that allows the motion planner to maintain and store a sparse set of nodes improving its overall computation efficiency by ≈ 98% and storage efficiency by ≈ 80%. This work also presents a novel extension of the linearized Lambert solution (LLS) called the closed-loop linearized Lambert guidance solution that allows a spacecraft to apply correction burns during the transfer to improve the targeting accuracy in relative guidance problems in the presence of perturbations like Drag, J2, and Solar Radiation Pressure (SRP). This novel closed-loop guidance law is applied to Spacecraft Formation Flying (SFF) problem, and by presenting theoretical developments backed with multiple simulation results it is shown that the algorithm allows for stringent targeting accuracy with fuel efficiency in different SFF problems. It is also demonstrated that closed-loop LLS guidance can be used to enable a spacecraft to safely follow a reference trajectory generated by an orbital motion planner like the AIKMP algorithm at a cost of ≈ 3% fuel increase as compared to open-loop guidance that is unable to provide safety guarantees in a cluttered and perturbed environment. Lastly, this work presents an extension of the popular E/I vector separation method derived for the case of drifting relative motion to infuse passive collision avoidance capabilities in sampling-based orbital motion planners. The resulting motion planner performs tree extension in a more informed way such that collision avoidance constraints are satisfied by each node and edge in the sampling-based tree. This way, every transfer solution computed from the tree is guaranteed to be collision-free for the entire duration of the transfer in the presence of multiple static (static relative to a moving reference frame) and moving obstacles – a capability that is crucial for SFF applications. Overall, the AIKMP algorithm describes the sufficient number of steps that can be adapted from a sampling-based robotic motion planner with required modifications to achieve fuel-efficient and collision-free sampling-based motion planning in astrodynamics applications. A notable aspect of this proposed motion planning framework lies in its modularity, as individual modules of the algorithm can be adapted to any existing sampling-based motion planner (tree-based planners or graph-based planners) to improve their applicability to orbital motion planning.</p
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Robust Onboard Vision-Based Navigation and Characterization at Small Celestial Bodies
Full text linkNavigating near small celestial bodies—such as asteroids or planetary moons—is a complex yet crucial task for various space missions. Upon arrival, knowledge about these bodies is usually limited, requiring extensive characterization phases. Accurate estimates of the body's rotational motion, shape, and dynamical environment are essential for safe and effective navigation. Optical navigation is typically used; this involves acquiring images of the surface to determine the relative motion between the spacecraft and the small body. However, the complex topography and reflectance of small bodies can create challenging lighting conditions that complicate the measurement extraction and association processes. Traditionally, this issue has been addressed through landmark-based navigation from the ground. This method involves reconstructing a detailed surface model and predicting the appearance of surface landmarks to extract line-of-sight observations. However, this method is complex and time-consuming, which limits the scalability and capabilities of such missions. In this work, we introduce vision-based algorithms to automate several tasks in navigation and characterization. We propose new techniques for pole estimation, spacecraft navigation, and surface-hazard detection. The proposed navigation algorithm utilizes visual point clouds. This novel approach in space-based optical navigation leverages the whole 3D structure extracted from imagery to determine the camera pose. Conversely, traditional landmarks-based techniques require detection and tracking of individual, predefined landmarks across multiple observations. We derive uncertainty-quantification models based on the point-cloud distribution and show strong agreement with the estimates. These techniques leverage the geometric principles and are designed to be robust against the typical challenges of lighting and viewpoint changes in small-body imaging. The advancements in this work indicate that future missions to small celestial bodies could benefit from the increased automation and onboard execution of perception algorithms.</p
Robust Onboard Vision-Based Navigation and Characterization at Small Celestial Bodies
Full text linkNavigating near small celestial bodies—such as asteroids or planetary moons—is a complex yet crucial task for various space missions. Upon arrival, knowledge about these bodies is usually limited, requiring extensive characterization phases. Accurate estimates of the body's rotational motion, shape, and dynamical environment are essential for safe and effective navigation. Optical navigation is typically used; this involves acquiring images of the surface to determine the relative motion between the spacecraft and the small body. However, the complex topography and reflectance of small bodies can create challenging lighting conditions that complicate the measurement extraction and association processes. Traditionally, this issue has been addressed through landmark-based navigation from the ground. This method involves reconstructing a detailed surface model and predicting the appearance of surface landmarks to extract line-of-sight observations. However, this method is complex and time-consuming, which limits the scalability and capabilities of such missions. In this work, we introduce vision-based algorithms to automate several tasks in navigation and characterization. We propose new techniques for pole estimation, spacecraft navigation, and surface-hazard detection. The proposed navigation algorithm utilizes visual point clouds. This novel approach in space-based optical navigation leverages the whole 3D structure extracted from imagery to determine the camera pose. Conversely, traditional landmarks-based techniques require detection and tracking of individual, predefined landmarks across multiple observations. We derive uncertainty-quantification models based on the point-cloud distribution and show strong agreement with the estimates. These techniques leverage the geometric principles and are designed to be robust against the typical challenges of lighting and viewpoint changes in small-body imaging. The advancements in this work indicate that future missions to small celestial bodies could benefit from the increased automation and onboard execution of perception algorithms.</p
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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