arXiv subjects

Alexander N. Cohen

Publications and source records attributed to Alexander N. Cohen.

9 recordsLinked to original sources

arXiv2026

Asteroid Disruption and Deflection Simulations for Multi-Modal Planetary Defense

Planetary defense from asteroids via deflective means alone does not offer viable solutions in terminal scenarios where there is little warning time before impact. The PI method of planetary defense enables operation in terminal interdiction modes where there is little warning time prior to impact, but can also operate in the same extended time scale interdiction modes as made possible by traditional deflection techniques, which results in a versatile, multi-modal planetary defense capability. The method is also practical and cost-effective since it relies solely on launch vehicles and penetrator materials already available today, and thus presents itself as a logical and competitive option for planetary defense. As per the PI method, we investigate the effectiveness of rubble pile asteroid disruption and deflection via hypervelocity impacts with 10:1 aspect ratio cylindrical tungsten penetrators. We present the results of an ongoing simulation campaign dedicated to investigating the PI method, using the Lawrence Livermore National Laboratory (LLNL) arbitrary Lagrangian-Eulerian (ALE) hydrodynamics code ALE3D run with the High-End Computing Capability (HECC) at NASA Ames Research Center. We model heterogeneous rubble pile asteroids with a distribution of spherical boulders of varying initial yield strengths set within a weak binder material. We find that rubble pile asteroids of this type in the 20 - 100 meter-class can be effectively mitigated via 20 km/s impacts with 100 - 1000 kg penetrators via the coupling of the penetrator kinetic energy into the bulk material of the asteroid.

astro-ph.EP↗

arXiv2026

Optical and acoustic ground effects simulations from terminal defense asteroid disruption via the PI method

Our simulations suggest that PI ("Pulverize It"), a NASA Phase II NIAC study, is an effective multi-modal approach for planetary defense that can operate in extremely short interdiction modes (with intercepts as short as hours prior to atmospheric entry) as well as long interdiction time scales with months to years of warning. The basic process is complete disruption of the threat via fragmentation. In scenarios with sufficiently long warning time, the fragment cloud spreads enough to miss Earth, resulting in no ground effects. In "worst-case" scenarios, when the warning time is short, the fragments (typically <10 m in diameter) will enter Earth's atmosphere, where their energy is dissipated in a series of ground-level optical pulses and de-correlated shock waves, mitigating any significant damage. We investigate the optical and acoustic ground effects through a set of simulation codes that model the interaction of asteroid fragments with Earth's atmosphere following terminal threat interception. Even in short-warning time cases where fragments enter the atmosphere, our simulations suggest that threats mitigated by the PI method produce vastly less damage on the ground when compared to the same unfragmented case, yielding optical energy deposition below 200 kJ/m$^2$ and shock wave over-pressures under 3 kPa. Our simulations support the proposition that threats like 2023 PDC, the hypothetical 800 m diameter asteroid from the 2023 Planetary Defense Conference impact exercise, can be effectively mitigated through fragmentation. We find that a terminal defense mitigation scenario that disrupts 2023 PDC into 1 million fragments with an intercept of 60 days before ground impact results in minimal ground effects.

astro-ph.EP↗

arXiv2026

Evaluating Short-Warning Mitigation via Intentional Robust Disruption of a Hypothetical Impact of Asteroid 2023 NT1

We investigate various short-warning mitigation scenarios via fragmentation for a hypothetical impact of asteroid 2023 NT1, a Near-Earth Object (NEO) that was discovered on July 15, 2023, two days after its closest approach to Earth on July 13. The asteroid passed by Earth within ~0.25 lunar distances, with a closest approach of ~1$\times10^5$ km and velocity of 11.27 km/s. Its size remains largely uncertain, with an estimated diameter range of 26-58 m and a most probable estimate of 34 m [JPL Sentry, September 15, 2023] (weighted by the NEO size frequency distribution). If 2023 NT1 had collided with Earth, it could have caused significant local damage. Assuming a spherical asteroid with a diameter of 34 m, uniform density of 2.6 g/cm$^3$, and impact velocity of 15.59 km/s, a collision would have yielded an estimated impact energy of ~1.5 Mt, approximately three times the energy of the Chelyabinsk airburst in 2013. We analyze the effectiveness of mitigation via intentional robust disruption (IRD) for objects similar to 2023 NT1. We utilize Pulverize It (PI), a NASA Innovative Advanced Concepts (NIAC) study of planetary defense via fragmentation, to model potential mitigation scenarios through simulations of hypervelocity asteroid disruption and atmospheric ground effects in the case of a terminal defense mode. Simulations suggest that PI is an effective multi-modal approach for planetary defense that can operate in extremely short interdiction modes, in addition to long interdiction time scales with extended warning. Our simulations support the proposition that threats like 2023 NT1 can be effectively mitigated with intercepts of one day (or less) prior to impact, yielding minimal to no ground damage.

astro-ph.EP↗

arXiv2025

Multi-parameter constraints on empirical infrasound period-yield relations for bolides and implications for planetary defense

How effective are methods for estimating bolide energies from infrasound signal period-yield relationships? A single global period-energy relation can obscure significant variability introduced by parameters such as the atmospheric Doppler wind profile and the bolide's energy deposition profile as a function of altitude. Bolide speed, entry angle, burst altitude, and multi-episode fragmentation all may play a role in defining the detected period of the shockwave. By leveraging bolide light curve data from the Center for Near Earth Object Studies (CNEOS), we re-examined the period-energy relation as a function of these parameters. Through a bootstrap approach, we show that various event subsets can deviate from widely cited period-energy models and we identify which specific conditions most strongly reshape the period-energy scaling. The results define both the fidelity and reliability of period-energy relations when no additional data beyond the infrasound record is available and improve the outcome when supporting data from bolide trajectories and light curves are included. Ultimately, these findings expand the scope of earlier models, providing a nuanced and robust framework for infrasound-only yield estimation under a range of bolide scenarios.

astro-ph.EP↗

arXiv2022

Radiation Effects from ISM and Cosmic Ray Particle Impacts on Relativistic Spacecraft

Relativistic spacecraft, like those proposed by the NASA Starlight program and the Breakthrough Starshot Initiative, will have to survive radiation production that is unique when compared to that experienced by conventional spacecraft. In a relativistic interstellar spacecraft's reference frame, the interstellar medium (ISM) will look like a nearly mono-energetic beam of charged particles which impinges upon the leading edge of the spacecraft. Upon impact, ISM protons and electrons will travel characteristic lengths through the spacecraft shield and come to a stop via electronic and nuclear stopping mechanisms. As a result, bremsstrahlung photons will be produced within the spacecraft shield. In this work, we discuss the interstellar environment and its implications for radiation damage on relativistic spacecraft. We also explore expected radiation doses in terms of on-board device radiation tolerance.

physics.space-ph↗

arXiv2022

Don't Forget To Look Up

We discuss a hypothetical existential threat from a 10 km diameter bolide discovered 6 months prior to impact with one case being a comet and the other being an asteroid. We show that an extension of our work on bolide fragmentation using an array of penetrators but modified with small nuclear explosive devices (NED) in the penetrators, combined with soon-to-be-realized heavy lift launch assets with positive $C_3$ such as NASA SLS or SpaceX Starship (with in-orbit refueling) is sufficient to mitigate this existential threat. A threat of this magnitude hitting the Earth at a closing speed of 40 km/s for the comet and 20 km/s for the asteroid would have an impact energy of roughly 65 Teratons TNT, or about ten thousand times larger than the current combined nuclear arsenal of the entire world. This is similar in energy to the KT extinction event that killed the dinosaurs some 66 million years ago. Such an event, if not mitigated, would be an existential threat to humanity. We show that mitigation is conceivable using existing technology, even with the short time scale of 6 months warning, but that the efficient coupling of the NED energy is critical.

astro-ph.EP↗

arXiv2022

The Economics of Interstellar Flight

Large scale directed energy offers the possibility of radical transformation in a variety of areas, including the ability to achieve relativistic flight that will enable the first interstellar missions, as well as rapid interplanetary transit. In addition, the same technology will allow for long-range beamed power for ion, ablation, and thermal engines, as well as long-range recharging of distant spacecraft, long-range and ultra high bandwidth laser communications, and many additional applications that include remote composition analysis, manipulation of asteroids, and full planetary defense. Directed energy relies on photonics which, like electronics, is an exponentially expanding growth area driven by diverse economic interests that allows transformational advances in space exploration and capability. We have made enormous technological progress in the last few years to enable this long-term vision. In addition to the technological challenges, we must face the economic challenges to bring the vision to reality. The path ahead requires a fundamental change in the system designs to allow for the radical cost reductions required. To afford the full-scale realization of this vision we will need to bring to fore integrated photonics and mass production as a path forward. Fortunately, integrated photonics is a technology driven by vast consumer need for high speed data delivery. We outline the fundamental physics that drive the economics and derive an analytic cost model that allows us to logically plan the path ahead.

econ.GN↗

arXiv2021

The First Interstellar Astronauts Will Not Be Human

Our ability to explore the cosmos by direct contact has been limited to a small number of lunar and interplanetary missions. However, the NASA Starlight program points a path forward to send small, relativistic spacecraft far outside our solar system via standoff directed-energy propulsion. These miniaturized spacecraft are capable of robotic exploration but can also transport seeds and organisms, marking a profound change in our ability to both characterize and expand the reach of known life. Here we explore the biological and technological challenges of interstellar space biology, focusing on radiation-tolerant microorganisms capable of cryptobiosis. Additionally, we discuss planetary protection concerns and other ethical considerations of sending life to the stars.

astro-ph.EP↗

arXiv2021

Damage to Relativistic Interstellar Spacecraft by ISM Impact Gas Accumulation

As part of the NASA Starlight collaboration, we look at the implications of radiation effects from impacts with the interstellar medium (ISM) on a directed energy driven relativistic spacecraft. The spacecraft experiences a stream of MeV/nucleon impacts along the forward edge primarily from hydrogen and helium nuclei. The accumulation of implanted slowly diffusing gas atoms in solids drives damage through the meso-scale processes of bubble formation, blistering, and exfoliation. This results in macroscopic changes to material properties and, in the cases of blistering and exfoliation, material erosion via blister rupture and delamination. Relativistic hydrogen and helium at constant velocity will stop in the material at a similar depth, as predicted by Bethe-Bloch stopping and subsequent simulations of the implantation distribution, leading to a mixed hydrogen and helium system similar to that observed in fusion plasma-facing components (PFC's). However, the difference in location of near-surface gas atoms with respect to the direction of exposure means that previously developed empirical models of blistering cannot be used to predict bubble formation or blistering onset. In this work, we present a model of the local gas concentration threshold for material blistering from exposure to the ISM at relativistic speeds. Expected effects on the spacecraft and mitigation strategies are also discussed. The same considerations apply to the Breakthrough Starshot mission.

physics.space-ph↗