Abstract. Asteroids and comets that cross Earth's orbit pose a credible risk of impact, with potentially severe disturbances to Earth and society. We propose an orbital planetary defense system capable of heating the surface of potentially hazardous objects to the vaporization point as a feasible approach to impact risk mitigation. We call the system DE-STAR, for Directed Energy System for Targeting of Asteroids and exploRation. The DE-STAR is a modular-phased array of kilowatt class lasers powered by photovoltaic's. Modular design allows for incremental development, minimizing risk, and allowing for technological codevelopment. An orbiting structure would be developed in stages. The main objective of the DE-STAR is to use focused directed energy to raise the surface spot temperature to ∼3000 K, sufficient to vaporize all known substances. Ejection of evaporated material creates a large reaction force that would alter an asteroid's orbit. The baseline system is a DE-STAR 3 or 4 (1-to 10-km array) depending on the degree of protection desired. A DE-STAR 4 allows initial engagement beyond 1 AU with a spot temperature sufficient to completely evaporate up to 500-m diameter asteroids in 1 year. Small objects can be diverted with a DE-STAR 2 (100 m) while space debris is vaporized with a DE-STAR 1 (10 m).

This paper presents the motivation behind and design of a directed energy planetary defense system that utilizes laser ablation of an asteroid to impart a deflecting force on the target. The proposed system is called DE-STARLITE for Directed Energy System for Targeting of Asteroids and ExploRation-LITE as it is a small, stand-on unit of a larger standoff DE-STAR system. Pursuant to the stand-on design, ion engines will propel the spacecraft from low-Earth orbit (LEO) to the near-Earth asteroid (NEA). During laser ablation, the asteroid itself becomes the "propellant"; thus a very modest spacecraft can deflect an asteroid much larger than would be possible with a system of similar mission mass using ion beam deflection (IBD) or a gravity tractor. DE-STARLITE is capable of deflecting an Apophis-class (325 m diameter) asteroid with a 15-year targeting time. The mission fits within the rough mission parameters of the Asteroid Redirect Mission (ARM) program in terms of mass and size and has much greater capability for planetary defense than current proposals and is readily scalable to the threat. It can deflect all known threats with sufficient warning.

Current strategies for diverting threatening asteroids require dedicated operations for every individual object. We propose a stand-off, Earth-orbiting system capable of vaporizing the surface of asteroids as a futuristic but feasible approach to impact risk mitigation. We call the system DE-STAR (Directed Energy System for Targeting of Asteroids and exploRation). DE-STAR is a modular phased array of laser amplifiers, powered by solar photovoltaic panels. Lowcost development of test systems is possible with existing technology. Larger arrays could be tested in sub-orbital demonstrations, leading eventually to an orbiting system. Design requirements are established by seeking to vaporize the surface of an asteroid, with ejected material creating a reaction force to alter the asteroid's orbit. A proposed system goal would be to raise the surface spot temperature to >3,000K, evaporating all known substances. Engagement distance required for successful diversion depends on the asteroid's mass, composition and approach velocity. Distance to focus and desired surface spot temperature then determine laser array size. Volatile-laden objects (such as comets) ~100m wide and approaching at 5km/s could be diverted by initiating engagement at ~0.05AU, requiring a laser array of ~100m side length. Phased array configuration allows multiple beams, so a single DE-STAR of sufficient size would be capable of targeting several threats simultaneously. An orbiting DE-STAR could serve diverse scientific objectives, such as propulsion of kinetic asteroid interceptors or other interplanetary spacecraft. Vaporization of debris in Earth orbit could be accomplished with a ~10m array. Beyond the primary task of Earth defense, numerous functions are envisioned.

We present results of optical simulations for a laser phased array directed energy system. The laser array consists of individual optical elements in a square or hexagonal array. In a multi-element array, the far-field beam pattern depends on both mechanical pointing stability and on phase relationships between individual elements. The simulation incorporates realistic pointing and phase errors. Pointing error components include systematic offsets to simulate manufacturing and assembly variations. Pointing also includes time-varying errors that simulate structural vibrations, informed from random vibration analysis of the mechanical design. Phase errors include systematic offsets, and timevarying errors due to both mechanical vibration and temperature variation in the fibers. The optical simulation is used to determine beam pattern and pointing jitter over a range of composite error inputs. Results are also presented for a 1 m aperture array with 10 kW total power, designed as a stand-off system on a dedicated asteroid diversion/capture mission that seeks to evaporate the surface of the target at a distance of beyond 10 km. Phase stability across the array of λ/10 is shown to provide beam control that is sufficient to vaporize the surface of a target at 10 km. The model is also a useful tool for characterizing performance for phase controller design in relation to beam formation and pointing.

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