GALCIT Colloquium

Objects (NEOs), such as asteroids or comets, generally pose low probability but high consequence risks as potential Earth-impact hazards. Planetary Defense (PD) is the multidisciplinary and inter-agency effort to characterize NEOs and provide modeling and simulation, hazard assessment, and strategies to mitigate or prevent potential impact events. PD is a broad field with varied disciplines and approaches to modeling and validating NEO scenarios.

Lawrence Livermore National Laboratory (LLNL) is a leader in computational modeling and highperformance computing, particularly within the scope of simulating various mitigation and impact scenarios as well as assessing the respective regional or planetary consequences. The Double Asteroid Redirection Test, DART, was the first planetary defense mission of its kind and a proof-ofconcept for the feasibility of a kinetic deflection defense mission. LLNL provided substantial impact modeling data during the development of the mission and continues to provide ongoing simulation updates reflecting the evolving dynamical changes experienced by the double body system post impact. The advent of the DART mission underscores the importance of planetary defense and the development of a wide range of mission concepts capable of preventing high consequence risks on Earth. This talk aims to introduce some of the ongoing projects being performed at LLNL's Planetary Defense group including recent innovations and relevant challenges.

We will be discussing the group's contemporary efforts in simulating nuclear energy deflection as a mitigation technique. Modeling a complete radiation transfer and hydrodynamics package is incredibly computationally expensive but can be avoided by implementing well-defined energy deposition models. The current approach is to utilize such a model for faster turnaround to determine relevant mission parameters such as yield, height of burst, and imparted delta-v. We shall also be discussing the group's efforts in modeling kinetic impacts on asteroid surfaces. Appropriate modeling techniques are required to accurately gauge the damage propagation within an asteroid and determine whether the impact will induce deflection or disruption. The laboratory's Discrete Element Method and Spheral SPH package are equipped to model high velocity impact and undergo rigorous validation and verification.

In addition to mitigation techniques, we shall also be discussing contemporary hazard assessment. Smaller sized asteroids (< 50 m) comprise most of the NEO population and tend to fully break up in the atmosphere, leading to airburst. The planetary defense group is currently developing a pipeline capable of modeling asteroid atmospheric entry and propagating the simulated energy deposit into a solver that can output the ground overpressure to assist in regional consequence assessments. Due to the unavailability of large datasets of airburst events, such a model must be evaluated against independent, laboratory replicable experiments. Finally, in the high statistical probability that an NEO impacts the ocean, the group has been modeling the resultant tsunami wave propagation from such an event. An in-house Eulerian solver, ALE3D, is used to model the impact and assess primary and secondary hazard effects which may include tsunami waves driven by pressure or gravity and climate effects as vaporized matter enters the atmosphere.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.