A common objective for primitive body missions (i.e., those to asteroids, comets, and small planetary moons) is to map the target body surface as completely as possible. Ideally, this map is constructed from a large collection of images containing multiple views of every point on the surface in a variety of lighting conditions and from a variety of viewpoints. For most missions to near- Earth objects (NEOs), the inherent instability of the orbital environment due to solar radiation pressure (SRP) necessitates a mapping trajectory strategy that requires deterministic maneuvers every few days for several weeks to achieve the desired imaging geometries. The recently discovered quasi-terminator orbits (QTOs) offer an alternative approach that can achieve the imaging geometries needed for mapping without any deterministic maneuvers. By eliminating the need for frequent maneuvering, QTOs can significantly reduce the complexity and intensity of global mapping operations for robotic or manned missions to NEOs.
QTOs are quasi-periodic orbits that are derived from the well-known, stable terminator orbit solutions to the SRPperturbed orbit dynamics at primitive bodies. Terminator orbits are not generally suitable for mapping because the phase angle (i.e., the Sun-body-spacecraft angle) stays nearly constant around 90°, which constrains the surface viewing and illumination geometries. However, the quasi-periodic oscillatory motion around a stable terminator orbit can be computed explicitly and amplified using tools from dynamical systems theory. The QTOs that result can achieve phase angles as low as several 10s and as high as ≈150° while retaining the stability and robustness properties of the terminator orbits. The resulting variety of relative geometries allows for the surface imaging across a range of solar incidence angles, emission angles, and emission azimuths.
The QTOs have been derived using normalized coordinates such that the solutions only depend on the relative strength of solar pressure versus gravity (“beta”). Applicability to a particular mission can quickly be assessed from published plots of orbit period, minimum radius, and maximum radius for any mission beta. Generally speaking, QTOs are most often applicable for beta values corresponding to robotic missions to NEOs or manned missions to very small NEOs.
This work was done by Stephen B. Broschart, Gregory Lantoine, and Daniel J. Grebow of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48915
This Brief includes a Technical Support Package (TSP).
Quasi-Terminator Orbits for Mapping Small Primitive Bodies
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Overview
The document titled "Quasi-Terminator Orbits for Mapping Small Primitive Bodies" is a technical support package prepared by Stephen Broschart, Gregory Lantoine, and Dan Grebow from NASA's Jet Propulsion Laboratory. It discusses the dynamics and characteristics of quasi-terminator orbits, which are essential for mapping small celestial bodies, such as asteroids and comets.
Quasi-terminator orbits are defined by a parameter known as "β," which encapsulates the dynamics of the orbit. The document emphasizes that missions with similar β values will exhibit comparable quasi-terminator orbit shapes, facilitating easier comparisons across different mission concepts. This normalization process allows for a more straightforward application of orbit solutions, regardless of the specific target or spacecraft involved.
The document outlines two primary types of quasi-terminator orbits: "sun-side" and "dark-side" orbits. Sun-side orbits are those that remain illuminated by the sun, while dark-side orbits are in shadow. Each type presents unique challenges and advantages for mapping and studying the surface characteristics of primitive bodies. The authors provide insights into how these orbits can be utilized effectively for scientific exploration and data collection.
Additionally, the document highlights the importance of understanding the unit length and unit time normalization factors, which vary depending on the target body. These factors are crucial for comparing the scale and duration of quasi-terminator orbits across different missions, enhancing the ability to plan and execute successful exploration strategies.
The technical support package is part of NASA's Commercial Technology Program, aimed at disseminating aerospace-related developments with broader technological, scientific, or commercial applications. It serves as a resource for researchers and engineers involved in space exploration, providing foundational knowledge and practical guidance for utilizing quasi-terminator orbits in future missions.
Overall, this document is a valuable contribution to the field of spaceflight mechanics, offering insights into the dynamics of quasi-terminator orbits and their application in mapping small primitive bodies. It underscores the significance of these orbits in advancing our understanding of celestial bodies and supports ongoing efforts in planetary exploration.
