Laser propulsion Science and Technology  
[PERSON] 1,2(c)  
Footnote 26: [[https://www.mdpi](https://www.mdpi)]([https://www.mdpi](https://www.mdpi))the users of orbital space is critical to enable such solutions to be fielded. The ongoing growth in the number of orbital debris objects does not waver or diminish based on international politics. The aerospace community is faced with major challenges in this arena, such as the launch of satellite mega-constellations that have the potential to dramatically increase the amount of debris within future decades even if used responsibly [9]. The field of Laser Orbital Debris Removal is in the process of maturing as a technology that may be useful for countering some orbital debris--some materials, of some sizes, in some situations. It is likely that a broader spectrum of technologies is necessary to fully address the debris problem; for instance, lasers are a poor choice for the removal of the greatest amounts of debris compared to proven alternatives such as grappling arms, attachment of conventional thrusters, or even tethers or drag sails. However, lasers may be an imminently reasonable solution to remove \(\sim\)10 cm debris particles, or sweep smaller debris from orbital space, assuming certain engineering obstacles and political reluctance can be overcome, and the cost-benefit ratio is realistic [10]. There is also the important question of the form in which a laser would be best deployed to support laser propulsion. Proposed implementations include space-based chaser satellites, an orbital laser platform, a ground-based system, onboard laser thrusters, and more. Laser maintenance in most high-energy terrestrial laboratories is already challenging, and locating the laboratory in space or on a satellite exposed to radiation and atomic oxygen is essentially guaranteed to exacerbate most issues and potentially spawn new ones. Conversely, the size of and challenges associated with a ground-based beam director, to say nothing of the difficulties of propagating a powerful beam through the atmosphere, strain modern engineering limits to achieve a design capable of practical application. Shared control and cooperative sites distributed among partner spacefaring countries may be necessary to make any such laser propulsion system widely powerful enough, precise enough, and palatable enough to field. Niche applications of laser propulsion have been proposed and sometimes promise to deliver unique aerospace solutions. I would like to highlight a few of those technological applications not yet covered in this Special Issue that I feel are highly worthy of mention for their promise. One of the more extreme ideas is for relativistic launch to \"nearby\" star systems, with the idea of sending a hardened chipsat across the void, propelled by a powerful laser array. This application has acquired the moniker 'Breakthrough Starshot' [11]. The engineering, physical, and optical challenges in this endeavor are truly monumental and essentially consist of devoting the entire output of a typical regional electrical power station into a distributed system of many lasers, all focused and phase-locked through the atmosphere onto a single tiny object, all maintained long enough to accelerate it to a fraction of light speed, and without destroying its circuitry or structure via extreme acceleration. The pursuit of this goal is likely to mature various interesting adjacent technologies; however, it is still an open question whether this proposal is achievable in a realistic time span. It may be an aspirational goal, given current human scientific and engineering capabilities. Another niche application is the cooperative tractor beam, achieved using confined laser ablation to produce thrust. This technology targets a macroscopic structured object that is pulled toward the laser system. This application has been studied in different forms by various researchers including, in alphabetical order, [PERSON] et al. [12], [PERSON] [13], [PERSON] [14], and [PERSON] et al. [15].