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

(reference NPO48915) is currently available for download from the TSP library.

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