Cost-Optimal Laser-Accelerated Lightsails

Abstract: Laser-accelerated lightsails enable new types of missions that are very different from the Breakthrough Starshot mission to the Centauri system that aims to send 1 gram of payload at 0.2 c. The present work widens the mission design space to 0.1 mg to 100 kt payload and 0.0001-0.99 c cruise velocity. Drawing up to 5 GW directly from the grid (to augment power drawn from local energy storage) turns out to be the key to making small missions affordable: It collapses the accelerating laser's capital cost by up to 5 orders of magnitude, enabling new possibilities such as a 10 kg Solar system cubesat that accelerates to 0.001 c (63 au/yr) using a 77 m sail and \$610M laser, costing \$58M worth of energy per mission. Trajectory equations describing lightsail acceleration are derived in closed form and used instead of numerical integration. Consequently, analyses have progressed from single point designs to whole performance maps comprised of thousands of cost-optimized point designs. The performance maps reveal qualitatively different regimes characterized by the particular constraint that drives cost, and these driving constraints change depending on mission payload mass and cruise velocity. The performance maps also reveal a family of cost-optimal missions that accelerate at Earth gravity: The heaviest such mission is a 7.4 km diameter 100 kt vessel (equivalent to 225 International Space Stations) that is accelerated for 21 days to achieve 0.07 c, reaching the Centauri system within a human lifetime. While unthinkable at this time, the required 380 PW peak radiated power (twice terrestrial insolation) might be generated by space solar power or fusion within a few centuries. Regardless, it is now possible to contemplate such missions using laser-accelerated lightsails.