An alternative has been developed to direct measurement for determining the density of the atmosphere of the Saturn moon Titan as a function of altitude. The basic idea is to deduce the density versus altitude from telemetric data indicative of the effects of aerodynamic torques on the attitude of the Cassini Saturn orbiter spacecraft as it flies past Titan at various altitudes. The Cassini onboard attitudecontrol software includes a component that can estimate three external per-axis torques exerted on the spacecraft. These estimates are available via telemetry.
The atmospheric torque vector is the product of (1) a drag coefficient (which is known from ground-based experiment and analysis), (2) the Titan atmospheric density that one seeks to determine, (3) the square of the Titan-relative spacecraft speed (which is known from navigation monitoring), and (4) the projected area of the spacecraft and the offset distance between the center of pressure and center of mass, both of which are known functions of the attitude of the spacecraft relative to the known velocity through the atmosphere. Hence, the atmospheric density is the only unknown and can be determined from the other quantities, which are known.
This work was done by Allan Lee, Jay Brown, Antonette Feldman, Scott Peer, and Eric Wang of Caltech for NASA’s Jet Propulsion Laboratory.
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Alternative Determination of Density of the Titan Atmosphere
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Overview
The document discusses a methodology developed for determining the density of Titan's atmosphere, which is crucial for both scientific research and engineering applications related to spacecraft operations. Titan, Saturn's largest moon, possesses a dense atmosphere primarily composed of nitrogen (N2), methane (CH4), and argon (Ar), making it unique among moons in the Solar System. The atmospheric density is of significant interest to planetary scientists, as it influences spacecraft behavior during flybys.
The methodology outlined in the document leverages data from the Cassini spacecraft's attitude control flight software, which estimates the external torques acting on the spacecraft. These torques are measured in three axes and are essential for calculating the atmospheric density. The process involves several key components: the drag coefficient, Titan's atmospheric density, the spacecraft's speed relative to Titan, the projected area of the spacecraft, and the offset distance between the center of pressure and the center of mass. The drag coefficient is derived from ground-based experiments, while the spacecraft's speed is provided by the navigation team. The projected area and offset distance vary based on the spacecraft's orientation relative to the incoming atmospheric molecules.
The document highlights that the methodology was first applied during the Titan-A flyby on October 26, 2004, and has since been used to analyze data from multiple Titan flybys. The results of this approach revealed potential tumbling risks for the spacecraft during low-altitude flybys, prompting mission planners to raise the altitudes of 19 out of 21 planned flybys to ensure the spacecraft's safety.
The methodology represents a novel approach to atmospheric density estimation, complementing existing instruments like the Ion and Neutral Mass Spectrometer (INMS) onboard Cassini, which also aims to analyze Titan's atmospheric constituents. The document emphasizes the importance of accurate density estimates for mission planning, as unexpected high-density profiles could jeopardize spacecraft stability during flybys.
In summary, this document presents a significant advancement in understanding Titan's atmosphere and ensuring the safety of spacecraft operations, showcasing the integration of engineering and scientific efforts in space exploration.
