Radiometric Design and Analysis of the Breakthrough Starshot Initiative in Launch Space

Abstract

The Breakthrough Starshot Initiative is a project that aims to send a nanocraft to the Proxima Centauri system. The nanocraft will deploy a solar sail driven by a 100 GW laser beam at 1064 nm projected from a segmented aperture of several kilometers in diameter1. Adaptive Optics (AO) will be implemented to facilitate the transfer of energy efficiently and effectively2. The AO system will need a guide beacon for wave-front measurements, and there are multiple architectures being investigated. One such architecture involves the use of reflected drive light as a beacon. As a result, it is imperative to quantify the nature of the system’s radiometric response. A model has been developed for the examination of radiometric properties pertaining to the Starshot system. The complete radiometric model uses two MATLAB models, referred to as the kinematic model and propagation model, for a robust analysis. The kinematic model predicts the nanocraft’s motion while the propagation model quantifies and characterizes the light returning from the sail. Outputs from the kinematic model include craft position, velocity, and acceleration at any time throughout the duration of the launch. Outputs from the propagation model include power and irradiance maps incident upon the sail and reflected back toward the launch projector at multiple craft positions. The greater Starshot system can be broken down into two parameter spaces: launch space and drive space. The launch space consists of the launch projector and Earth’s atmosphere while drive space consists of the sail itself. For the purposes of this study, the drive space parameters (i.e., sail shape, BRDF, absorptance/reflectance, etc.) are fixed. The exact values of these variables are chosen based on current Starshot estimates and congruent research. Instead, this study focuses on the launch space parameters. The primary parameter to consider here is the shape of the PSF of the outgoing beam from the launch projector and the implications this shape will have on wave-front sensing. The shapes chosen for simulating are consistent with those being currently investigated for optimizing sail stability: Gaussian, donut, and four-lobed. With each specified system configuration, the model outputs have been generated leading to a study of which launch space configuration (i.e., PSF shape) is optimal for wave-front sensing. Atmospheric turbulence is simulated using the propagation model and different levels of atmospheric correction are applied in an effort to base-line the requirements that will need to be imposed on the future AO system. A trade study detailing these findings is provided.