Challenges of Deflecting an Asteroid or Comet Nucleus with a Nuclear Burst

The paper begins by highlighting the unique preventability of asteroid/comet impacts compared to other natural disasters. While other disasters necessitate evacuation and post-impact damage control, asteroid impacts can potentially be mitigated with sufficient warning. Near-Earth Object (NEO) surveys aim to catalog at least 90% of NEOs larger than ~140 meters within 15 years. The central question addressed is how to effectively deflect a detected impactor. The authors briefly review various deflection methods, detailing their advantages and disadvantages before delving into a more in-depth analysis of using nuclear munitions in a standoff approach. The devastation potential of asteroid impacts is emphasized by comparing the energy released by a 100-meter object to that of the Mike nuclear test and the Tunguska event. The paper underscores the urgency and importance of developing effective deflection strategies.

The paper reviews the historical context of the awareness of asteroid impacts, from early records to the Alvarez hypothesis linking an asteroid impact to the Cretaceous-Tertiary extinction. The evolution of NEO surveys and their goals (initially 90% of NEOs >1km, later 90% of PHOs >140m) are also discussed. The Torino scale, designed to communicate asteroid impact risks to the public, is mentioned. The paper acknowledges the limitations of current NEO surveys, particularly regarding smaller objects (<140m) which are difficult to detect with sufficient advance warning. It highlights the lack of detailed knowledge about the internal structure of asteroids, which affects their response to deflection attempts. Existing literature regarding different deflection methods (e.g., pulsed lasers, gravity tractors, kinetic impactors) is also briefly reviewed before focusing on the nuclear deflection method.

The core methodology of the paper involves using numerical simulations to model the effects of nuclear bursts on asteroids. The authors utilize the RAGE (Radiation-Hydrodynamics code) with radiation transport to simulate the response of simplified PHOs to variations in device yield, composition, and porosity. The simulations model energy deposition (x-rays and neutrons) and hydrodynamic response. Neutron energy deposition is calculated using the MCNP Monte-Carlo transport code. The authors justify the use of separate codes for energy deposition and hydrodynamic response due to the differences in timescales involved. The paper describes the parameters of the simulations: a 100-meter diameter spherical target of uniform composition (basalt, water ice, iron, and graphite), with burst yields of 10, 100, and 1000 kilotons (kt), and standoff distances of 20 and 70 meters. The simulation results provide estimates of ablated material and imparted deflection velocity. Later simulations include a non-spherical model, using the shape of asteroid 25143 Itokawa, coupled with MCNP simulations for neutron irradiation. The RAGE simulations were conducted for various materials and burst energies, analyzing center of mass velocities and expansion of ablated material. Results of these simulations are then used to determine the efficiency of various nuclear burst scenarios.

The simulations reveal that a 100 kt burst produces a greater center-of-mass velocity than a 10 kt burst, and lower initial density also increases velocity. Increasing the standoff distance (from 20m to 70m) significantly reduces the effectiveness of the burst (by a factor of 5-10). The simulations demonstrate the importance of burst proximity to the target's surface, with closer bursts yielding greater deflection velocities. However, a 1000 kt burst shows significant disruption of the target, rendering it unsuitable for deflection. The authors find that a 100 kt burst 20 meters from the surface imparts a center-of-mass velocity of approximately 190 cm/s. This velocity is sufficient for a deflection with lead times as short as 4-5 months. Finally, simulations using the irregular shape of asteroid Itokawa demonstrate the applicability of the methodology to more realistic asteroid geometries.

The findings highlight the feasibility of using nuclear munitions for asteroid deflection, especially for shorter warning times. The study emphasizes the importance of considering various factors, including asteroid composition, standoff distance, and burst yield, in optimizing deflection strategies. The simulations, while simplified, provide crucial insights into the energy coupling between a nuclear burst and an asteroid. The differences observed between spherical and irregular models underscore the necessity of using accurate shape models for future simulations and deflection planning. The paper's methodology serves as a foundation for creating a comprehensive catalog of deflection simulations considering a broader range of parameters, assisting in decision-making during real-world scenarios.

The paper concludes that nuclear standoff bursts represent a technically feasible method for asteroid deflection. The simulations demonstrate that significant deflection velocities can be achieved, even with relatively short warning times. However, further work is required to incorporate more realistic asteroid models (considering material strength, porosity, and fractures), as well as explore a wider range of parameters. Future research should focus on building a comprehensive catalog of deflection simulations to aid decision-making in real-world threat scenarios. The paper acknowledges the political and treaty implications associated with using nuclear weapons for planetary defense.

The study uses simplified models of asteroids, assuming uniform composition and spherical shapes in initial simulations. While later simulations incorporate an irregular shape, factors like material strength, porosity, internal structure, and the potential for fragmentation are not fully accounted for in all models. The political and international relations complexities surrounding the use of nuclear weapons are acknowledged but not explicitly modeled.