Measurements made by the Voyager 2 spacecraft during its flyby of Uranus in 1986 found warm stratospheric and hot thermospheric temperatures that cannot be explained by solar heating alone. This contributes to what has become known as the “giant planet energy crisis”: there is a fundamental lack of understanding of the energy balance of the giant planets in the solar system. Uranus has the coldest stratospheric temperatures, the hottest thermospheric temperatures at some altitudes, and yet the weakest internal heat flux of all four giant planets. Moreover, the Voyager 2 temperature profiles are at odds with the many contemporaneous Earth-based stellar occultation observations. This unresolved tension impedes efforts to compare atmospheres in the solar system to one another and to exoplanet atmospheres. In this dissertation, I present an investigation into the upper atmospheric temperatures of Uranus using archival Earth-based stellar occultation observations. I begin with an overview of planetary atmospheres and remote-sensing measurements of Uranus in Chapter 1. In Chapter 2, I derive and explain how stellar occultations are predicted, observed, processed, and analyzed, emphasizing my contributions. Chapter 3 describes how I validated these techniques on an archival Mars occultation. In Chapter 4, I present the results of comparing the Voyager 2 measurements to 26 archival Earth-based stellar occultations by Uranus. In Chapter 5, I present new temperature profiles from reprocessing these 26 occultations and a new one-dimensional atmospheric model based thereon. Chapter 6 outlines a low-Earth orbit mission concept to observe many new stellar occultations. Chapter 7 contains conclusions and summaries. My primary finding is that the lower thermosphere of Uranus is much cooler than reported by Voyager 2. I find that the mesopause is likely higher in altitude than previously believed and the stratosphere of Uranus has a nearly isothermal section, in alignment with the other giant planets. My new atmospheric model suggests Uranus has a significant source of heat dissipation in the stratosphere, which might be supplied by gravity waves. This work can contribute to planning for any Uranus Orbiter and Probe mission by helping to revise Uranus’ representative temperature profile and improving our understanding of Uranus’ energy balance.

An ultraviolet instrument compatible with a CubeSat form factor is currently being developed at Virginia Tech for the purpose of measuring nitric oxide in the polar night through the stellar occultation technique. This instrument will allow the investigation of how the Sun and Earth systems are related via energetic particle precipitation in the auroral regions. The work performed in this thesis supports the instrument design and requirements development by modelling the stellar occultation geometry to identify orbit parameters and target stars that could yield nitric oxide measurements during the polar winter at consistent latitudes, to best observe the build-up and fall-off of nitric oxide. The orbit study was accomplished through the development of an open-source tool in MATLAB, the Stellar Occultation Mission Planner. The results of this analysis were used to model the instrument performance and identify the required narrowband filter parameters to meet science requirements. Additional studies were performed to explore system performance for a future flight opportunity. / Master of Science / A small, light weight instrument is being designed at Virginia Tech to allow for nitric oxide in the atmosphere to be measured during the long polar nights that occur during winter in the Arctic and Antarctic regions. This instrument will allow scientists to explore how the Sun and Earth interact through space weather at high latitudes. This will be accomplished by using star light to probe the atmosphere while the instrument is onboard a small spacecraft called a CubeSat. The work performed in this thesis simulated the spacecraft orbit to determine which stars yielded the best measurements over the course of the polar night. Using these results, the instrument performance was simulated to inform the design of a filter for the instrument. Additional studies were performed to support the design of a future mission to fly the instrument in space.

This thesis serves to provide a conceptual mission design for a Titan-orbiting stellar occultation mission. Titan has a significant atmosphere much like Earth's. An improved understanding of Titan's atmosphere could provide valuable information about the evolution of Earth's climate. Titan's atmosphere is known to be in a state of superrotation, wherein the atmosphere rotates significantly faster than the surface beneath. The details of the creation and sustainment of this extreme state on Titan in terms of angular momentum exchange remain unknown despite current theories and models. These unknowns, alongside inconsistencies between current models with observations from the Cassini mission, call for an urgent need for Titan atmospheric observations able to resolve atmospheric waves. The science objectives driving the mission design include maximizing the number of measurements, the latitude versus longitude coverage, the latitude versus local solar time coverage, and the mission duration. These measurement needs can be met by a Titan orbiter utilizing a refractive stellar occultation technique. Refractive stellar occultation observes starlight bending through an atmosphere as stars set behind a body. The observed bending profile can be inverted to infer density, temperature, and pressure profiles. This research uses Systems Tool Kit (STK) as a simulation tool to predict measurement coverage for various orbits. The orbital radius was determined to be the driving independent variable which set all other design variables, including the orbital plane which was uniquely selected for a given orbital radius to maximize the number of occultations. The results of this study show that a lower orbital radius is desired as this produces the best combination of measurement number and distribution. This orbital plane should be closely aligned with the Milky Way galactic plane to see the most stars occult. For the lowest sustainable orbital altitude, Low Titan Orbit (LTO) at 1200 km, the orbital plane should be nearly polar to maximize the number of occultations and latitude coverage. The optimal orbit selection (defined by orbital elements a = 3775 km, e = 0, i = 85 degrees, Ω = 87 degrees, ω = 0 degrees, and ν = 0 degrees) for a single satellite can produce nearly 400 stellar occultation opportunities per orbit and provide full latitude versus longitude coverage. A single satellite shows gaps in latitude versus local solar time coverage at mid-latitudes normal to the satellite ground track which may inhibit the diagnosis of the angular momentum flux associated with thermal tides. If necessary, a second satellite in an orbit orthogonal to the first is suggested to close coverage gaps to provide full local time coverage over a Titan day. The optimal orbit selection of this second satellite (defined by orbital elements a = 3775 km, e = 0, i = 5.3 degrees, Ω = 5.9 degrees, ω = 0 degrees, and ν = 0 degrees) provides an additional 343 occultation opportunities per orbit and increases latitude versus local solar time coverage by a factor of 1.5. The understanding of Titan's Earth-like atmosphere could provide insight into climate evolution here on Earth. This concept proposes a novel approach to improving this understanding. / Master of Science / This thesis serves to provide a conceptual mission design for a Titan-orbiting stellar occultation mission. Titan, one of Saturn's 82 moons, has a significant atmosphere much like Earth's. An improved understanding of Titan's atmosphere could provide valuable information about the evolution of Earth's climate. Titan's atmosphere is known to be in a state of superrotation, wherein the atmosphere rotates significantly faster than the surface beneath. The details of the creation and sustainment of this extreme state on Titan remain unknown despite current theories and models. These unknowns, alongside inconsistencies between current models with observations from the Cassini mission, call for an urgent need for Titan atmospheric observation. The science objectives driving the mission design include maximizing the number of measurements, the latitude versus longitude coverage, the latitude versus local solar time coverage (on a 24-hour scale), and the mission duration. These measurement needs can be met by a Titan orbiter utilizing a refractive stellar occultation technique. Refractive stellar occultation observes starlight bending through an atmosphere as stars set behind a body. The observed bending profile can be inverted to infer density, temperature, and pressure profiles. This research uses a simulation tool to predict measurement coverage for various orbits. The radius of the orbit was determined to be the driving independent variable which set all other design variables, including the orbital plane which was uniquely selected for a given orbital radius to maximize the number of occultations. The results of this study show that a lower orbital radius is desired as this produces the best combination of measurement number and distribution. This orbital plane should be closely aligned with the Milky Way galactic plane to see the most stars occult. For the lowest sustainable orbital altitude, Low Titan Orbit (LTO) at 1200 km, the orbital plane should be nearly polar to maximize the number of occultations and latitude coverage. The optimal orbit selection for a single satellite can produce nearly 400 stellar occultation opportunities per orbit and provide full latitude versus longitude coverage. A single satellite shows gaps in latitude versus local solar time coverage at mid-latitudes normal to the satellite ground track which may inhibit the diagnosis of atmospheric waves tied to Titan's night and day cycle. If necessary, a second satellite in an orbit orthogonal to the first is suggested to close coverage gaps to provide full local time coverage over a Titan day. The optimal orbit selection of this second satellite provides an additional 343 occultation opportunities per orbit and increases latitude versus local solar time coverage by a factor of 1.5. The understanding of Titan's Earth-like atmosphere could provide insight into climate evolution here on Earth. This concept proposes a novel approach to improving this understanding.

The objective of this thesis was to select a star for observation and determine the error in the retrieval technique for a rocket experiment to measure lower thermospheric Nitric Oxide in the polar night using stellar occultation technique. These objectives are accomplished by planning the geometry, determining the requirements for observations, window for launch and discussing the retrieval technique. The planning is carried out using an approximated (no drag) and simulated rocket trajectory (provided by NSROC: NASA Rocket Operations Contract). The simulation for the retrievals is done using data from Student Nitric Oxide Explorer. Stars were taken from a catalogue called TD1. Launch times were obtained from the geometry planned resulting from selecting a zenith angle after choosing a maximum occultation height and determining rocket apogee. Window for observing Spica was found to be 20 minutes. The retrieval technique and simulations showed that column densities and volume densities should be retrievable to less than 5% and 20% respectively observing occultation heights 90-120km. The study suggests that choosing a star positioned north w.r.t the observation location gives us more poleward latitudes and larger launch window. Future research can be carried out applying the stellar occultation and retrieval technique to a satellite. / Master of Science