Orbital and Flight Stability Models for Breakthrough Starshot's AO System

Abstract

Breakthrough Starshot is an international engineering project which aims to develop a spacecraft launch system capable of reaching our nearest star neighbors in under 1 human lifetime. To achieve such high-speed interstellar travel, the Starshot system plans to accelerate an ultra-lightweight nanocraft with 100 GW of radiation pressure. This laser light will come from a massive ground-based launch projector and will focus through the atmosphere onto a 4 meter wide lightsail attached to the nanocraft. With 10 minutes of continuous laser power, the force of this light can accelerate a 1g nanocraft to 20% the speed of light. Such speeds can enable travelling to our nearest star neighbor in just 20 years. For the laser to effectively focus on the nanocraft after traversing the turbulence of Earth’s atmosphere, the Starshot system will need an Adaptive Optics system to ‘pre-correct’ the beam before transmission. To measure the distorting effects of the atmosphere, an artificial beacon must be placed high in altitude near the nanocraft through its launch. Proposed is a satellite-based laser guide star (SLGS) which will be attached to the mothercraft satellite which releases the nanocraft during launch. However, the satellite’s lateral and radial motion in the sky from the nanocraft threatens to introduce angular and focal anisoplanatic errors, respectively, that would make proper adaptive correction impossible. By creating a fully parameterizable orbital simulation of the mothercraft and nanocraft’s positions relative to the launch projector, it was found that there exist orbital configurations which keep these anisoplanatic errors within reason. One such orbit has an orbital period of 4 days and an eccentricity of 0.88. Following adaptive correction, it is vital that residual wavefront error does not introduce perturbances which cause the sail to fall out of the beam. Given that the sail behaves like a particle trapped in a potential well, a statistical model was created which was used to determine the likelihood a sail would fall out of a beam. This model, coupled with a dynamic numerical simulation of a sail riding in a beam with a given distribution of residual tilt error, was used to determine that the Starshot system is expected to be highly intolerant to residual tip/tilt errors. It is recommended that future work involve identifying a method for dampening the sail’s lateral energy throughout launch.