Prospects For Interstellar Travel Without Breaking The Laws Of Physics

If humanity can refrain from driving itself to extinction, it could send small probes to the nearest star in the foreseeable future, and could send a small village of people there on a one way trip that could be completed in a human lifetime within a few centuries. These are the boundaries of "hard" science fiction.

The key concept is to use lasers from near Earth to supply the space craft with power so that the space craft can have less fuel per payload on board, which in turn means it takes less energy to get the lighter craft relative to payload size up to marginally relativistic speeds.

Breakthrough Starshot is an initiative to explore the Centauri system using laser-accelerated sailcraft. Earlier work produced a point design for a 0.2 c mission carrying 1 g of payload. 

The present work widens the design space to missions having 0.1 mg to 100 kt payload and 0.0001-0.99 c (6-60,000 au/yr) cruise velocity. Also, the beam director may now draw up to 5 GW of power directly from the grid to augment the power drawn from its energy storage system. Augmenting stored energy with grid power shrinks beam director capital cost by 1-5 orders of magnitude. 

The wider design space encompasses new possibilities: 

A 0.1 mg microbiome accelerated to 0.01 c in only 2 min by a beam director that expends $6k worth of energy. 

A 10 kg Solar system cubesat accelerated to 0.001 c (60 au/yr) by a $600M beam director that expends $60M worth of energy per mission. 

A progression from cost-optimized point designs to whole performance maps has been made possible by replacing numerical trajectory integration with closed-form equations. Consequently, the system model now computes 1-2 orders of magnitude more point designs per unit time than before. Resulting maps reveal several different solution regimes that are characterized by their performance-limiting constraints. 

The performance maps also reveal a family of missions that accelerate at Earth gravity. The heaviest such mission is a 2 km diameter 100 kt vessel (equivalent to 225 International Space Stations) that is accelerated for 23 days to achieve 0.07 c, reaching the Centauri system within a human lifetime. While unthinkable at this time, the required 340 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 a mission as a laser-accelerated sailcraft.

Kevin L. G. Parkin, "Cost-Optimal System Performance Maps for Laser-Accelerated Sailcraft" arXiv:2205.13138 (May 26, 2022).