A contact force model was developed for use in touch and go (TAG) surface sampling simulations on small celestial bodies such as comets and asteroids. In TAG scenarios, a spacecraft descending toward the surface of a small body comes into contact with the surface for a short duration of time, collects material samples with a sampler device, and then ascends to leave the surface. The surface contact required 6-DOF (degrees of freedom) dynamics models due to coupling of the attitude and translation dynamics during the contact.
The model described here is for contact scenarios that utilize a rotating brush wheel sampler (BWS) to collect surface material. The model includes stiffness and damping of the surface material during BWS vertical motion, lateral friction from the BWS dragging across the surface, and lateral shear from the rotating BWS scooping the surface material.
This model is useful for any mission to asteroids or comets that incorporates surface sampling operations.
This work was done by Lars James C. Blackmore, Brian P. Trease, Behçet Açıkmeşe, Milan Mandic, and John M. Carson of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47194
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Surface Contact Model for Comets and Asteroids
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Overview
The document is an internal report from NASA's Jet Propulsion Laboratory (JPL) titled "Comet GN&C Research Report: Surface Contact Model for Comets and Asteroids," dated September 25, 2009. It focuses on the development of models for surface contact forces essential for spacecraft performing touch-and-go (TAG) sampling operations on small celestial bodies, such as comets and asteroids.
The report emphasizes the need for six degree-of-freedom (6-DOF) models to accurately simulate the dynamics involved during contact scenarios. These models account for the coupling of spacecraft attitude and translation dynamics, which are critical when a spacecraft descends, contacts the surface, collects samples, and ascends back into space.
The surface contact forces are categorized into vertical and lateral components. The vertical forces are modeled using a spring and damper system, where the downward force exerted by the spacecraft on the surface is countered by a restorative force from the surface. This relationship is mathematically expressed as ( F_{\text{vertical}} = -kz - c\dot{z} ), where ( z ) represents the displacement of the spring from its relaxed state, and ( \dot{z} ) is the velocity associated with that displacement. This model helps in understanding how the spacecraft interacts with the surface during the brief contact period.
The report also outlines the significance of these models in validating design methodologies for comet sample return missions. By simulating the contact dynamics, researchers can better predict the behavior of spacecraft during TAG operations, ensuring successful sample collection and return.
In summary, this document serves as a technical foundation for understanding the complexities of surface interactions between spacecraft and small celestial bodies. It highlights the importance of accurate modeling in the planning and execution of space missions aimed at exploring and sampling comets and asteroids, ultimately contributing to our knowledge of these distant objects and the broader solar system. The research presented is part of a larger effort to enhance autonomous guidance, navigation, and control systems for future space exploration missions.
