Laboratory Investigation of Satellite Gas-Surface Interactions for Accurate Construction of Atmospheric Models

Accurate estimates of neutral mass densities obtained from satellite drag measurements are vital to NASA science objectives, including the construction of atmospheric models, the understanding of thermospheric dynamics, and the investigation of long-term behavior in the thermosphere. The interpretation of these measurements depends strongly on the assumptions made about atomic or molecular interactions with satellite surfaces which vary with changes in the thermosphere resulting from space-weather (geomagnetic storms) as well long-term (multi-decadal) variations. Such assumptions have been known to introduce errors into the construction of atmospheric models by modifying the aerodynamic coefficients (Ca) that determine scaling factors between observed drag and atmospheric density. We will investigate the interaction of atmospheric gases with spacecraft surfaces under conditions consistent with atmospheric pressures and composition above ~500 km altitude. The main objective of our study is to determine the fundamental gas-surface interaction (GSI) parameters needed to specify spacecraft Ca and atmospheric densities at those altitudes. The largely He atmosphere is expected to result in low surface adsorption probabilities. Characteristics of lower thermal accommodation, such as narrow scattering lobes and minimal energy transfer, should be prevalent in the GSI above ~500 km and the ambient environment is consistent with that of a hypersonic beam facility. However, there is presently no data that provides any quantitative support to the extent of gas-surface accommodation in the He-dominated upper-thermosphere and its dependence on surface roughness or surface cleanliness and GSI models disagree on this parameter by as much as 70% above 500 km. The investigation is divided into the following primary science questions: (1) What are the scattering dynamics of atomic oxygen and He on several representative spacecraft surfaces? (2) How are scattering dynamics affected by surface roughness, and by the presence of adsorbate on the surface? (3) Does aerodynamic data above the O/He transition indicate low, intermediate, or high levels of adsorption? The team will address the science questions using hyperthermal beam-surface scattering techniques developed by T. Minton at Montana State University to measure the scattering dynamics of various incident species scattering from surfaces with different adsorbate coverages and roughness's. Context for the resulting data will be obtained by comparing the aerodynamics predicted by laboratory results with the observed aerodynamics above the He/O transition (~500 km). Specifically, we will observe the ratio of drag measured by a long satellite like GRACE (or GRACE follow on) to the drag measured by a compact spherical satellite and compare it to a ratio modeled using test-particle model techniques and calibrated with the laboratory results. The science enabled by this investigation is relevant to recommendations in the National Research Council Decadal Strategy for Solar and Space Physics including (a) the support of system-science through the development of "computational models that extract the essential physics from measurements made across multiple observing platforms" and can "specify the extended space environment in order to protect society and advance growing aspirations for the use of space", (b) response of the IT system to "magnetospheric forcing over global, regional and local scales" and (c) long-term changes in the planetary environment (thermosphere). A recent NASA LWS institute focused on satellite drag which depends on the accurate determination of aerodynamic coefficients. The proposal team has expertise in hypersonic molecular-beam experiments which simulate the space environment, satellite gas-surface interactions, and the modeling and observations of satellite aerodynamics.

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