小惑星の形状の不規則性が核爆発による軌道変更の効果に及ぼす影響を解析（Asteroid defense hits a lump in the road）

The effectiveness of deflecting Near Earth Objects (NEOs) using pulsed radiation is challenging to quantify because it hinges critically on surface topography, which is often unknown. We demonstrate an efficient computational method for predicting deflection forces on arbitrarily-shaped, kilometer-scale asteroids following a stand-off X-ray mitigation scenario. First, one-dimensional, radiation-hydrodynamics calculations predict gas ejection, accounting for radiation scattering, heating, phase change, shock hydrodynamics, and thermal reemission in a fully-coupled manner. Then, Direct Simulation Monte Carlo (DSMC) gas dynamics calculations, which scale more favorably in three dimensions, are used to predict the vapor evolution and surface pressure over the entire body. The approach improves on earlier methods by accounting for the actual topography of the asteroid fully in three dimensions without unacceptably increasing computational cost or reducing accuracy. As a demonstration, we predict the deflection of three bodies of different shape—asteroid (101955) Bennu, asteroid (25143) Itokawa, and asteroid (6489) Golevka—from both near-field and far-field irradiations. We find that the orientation of the body at time of intercept can increase the deflection velocity by up to nearly 2× for Bennu, 4× for Itokawa, and 5× for Golevka, compared to the minimum response for each body. Implications for changes in rotational velocity are also discussed. Finally, mapping the pressure fields to a loosely-compacted, Itokawa-like boulder assembly shows that solid ejecta from shock wave fracturing and rubble ejecta is also highly asymmetric, further motivating accurate topographic mapping of the vapor evolution for hazardous NEOs.