Measuring Earth's Energy Imbalance via Radiation Pressure Accelerations Experienced in Orbit: Initial Simulations for "Space Balls"

The direct measurement of Earth's radiative Energy Imbalance (EEI) from space is a challenge for state-of-the-art radiometric observing systems. Current spaceborne radiometers measure the individual shortwave (Solar incoming and Earth reflected solar radiation) and longwave (Earth emitted thermal radiation) components of Earth's energy balance with unprecedented stability, but with calibration errors that are too large to determine the absolute magnitude of global mean EEI or net radiative flux, respectively, as the components' residual. Best estimates of multi-year (2005-2020) EEI are derived from temporal changes in planetary heat content, predominantly ocean heat content, and amount to similar to 0.9 Wm(-2). To monitor EEI directly from space, we propose an independent approach based on accelerometry that measures non-gravitational radial accelerations induced by radiation pressure. To provide requirements for a near-spherical "Space Balls" spacecraft and mission design, we develop a simulation environment using JPL's Mission Analysis, Operations, and Navigation Toolkit Environment (MONTE) software libraries and present-day radiative fluxes from the Clouds and Earth's Radiant Energy System (CERES). At its current initial stage, the toolset allows us to simulate accelerations acting on a spherical spacecraft due to solar radiation pressure, Earth's reflected shortwave (albedo) and emitted longwave radiation, as well as due to aerodynamic force. Induced accelerations as well as their sensitivity to mean orbit altitude and spacecraft absorptivity agree well with back-of the-envelope calculations and previous simulations that assess the role of radiation pressure accelerations for orbital drift. Future investigations will expand the MONTE-based simulation environment with additional shape and confounding force models. Preliminary simulations with an integrated spacecraft dynamics model suggest that the main confounding accelerations for a non-perfect, faceted sphere are related to Yarkovsky, aerodynamic force and relativistic effects, which will have to be mitigated to facilitate a high-accuracy EEI measurement from space.