Morrison David
NASA Astrobiology Institute, Mountain View, California 94035, USA.
Astrobiology. 2003 Spring;3(1):193-205. doi: 10.1089/153110703321632516.
The discipline of astrobiology includes the dynamics of biological evolution. One of the major ways that the cosmos influences life is through the catastrophic environmental disruptions caused when comets and asteroids collide with a planet. We now recognize that such impacts have caused mass extinctions and played a major role in determining the evolution of life on Earth. The time-averaged impact flux as a function of projectile energy can be derived from lunar cratering statistics as well as the current population of near Earth asteroids (NEAs). Effects of impacts of various energies can be modeled, using data from historic impacts [such as the Cretaceous-Tertiary (KT) impactor 65 million years ago] and the observed 1994 bombardment of Jupiter by fragments of Comet Shoemaker-Levy 9. It is of particular interest to find from such models that the terrestrial environment is highly vulnerable to perturbation from impacts, so that even such a small event as the KT impact (by a projectile 10-15 km in diameter) can lead to a mass extinction. Similar considerations allow us to model the effects of still smaller (and much more likely) impacts, down to the size of the asteroid that exploded over Tunguska in 1908 (energy approximately 10 megatons). Combining the impact flux with estimates of environmental and ecological effects reveals that the greatest contemporary hazard is associated with impactors near 1 million megatons in energy (approximately 2 km in diameter for an asteroid). The current impact hazard is significant relative to other natural hazards, and arguments can be developed to illuminate a variety of public policy issues. The first priority in any plan for defense against impactors is to survey the population of Earth-crossing NEAs and project their orbits forward in time. This is the purpose of the Spaceguard Survey, which has already found more than half of the NEAs >1 km in diameter. If there is an NEA on a collision course with Earth, it can be discovered and the impact predicted with decades or more of warning. It is then possible to consider how to deflect or disrupt the NEA. Unlike other natural hazards, the impact risk can be largely eliminated, given sufficient advanced knowledge to take action against the threatening projectile.
天体生物学学科包括生物进化的动力学。宇宙影响生命的主要方式之一是彗星和小行星与行星碰撞时造成的灾难性环境破坏。我们现在认识到，此类撞击导致了大规模物种灭绝，并在决定地球上生命的进化过程中发挥了重要作用。作为射弹能量函数的时间平均撞击通量可从月球撞击坑统计数据以及当前近地小行星（NEA）的数量推导得出。利用来自历史撞击（如6500万年前的白垩纪 - 第三纪（KT）撞击体）的数据以及1994年苏梅克 - 列维9号彗星碎片对木星的观测撞击，可以模拟各种能量撞击的影响。从这些模型中特别有趣地发现，地球环境极易受到撞击的扰动，以至于即使像KT撞击（由直径10 - 15公里的射弹造成）这样的小事件也能导致大规模物种灭绝。类似的考虑使我们能够模拟更小（且更有可能）撞击的影响，直至1908年在通古斯卡上空爆炸的小行星大小（能量约为1000万吨）。将撞击通量与环境和生态影响的估计相结合表明，当代最大的危害与能量接近100万吨级的撞击体有关（对于小行星来说直径约为2公里）。相对于其他自然灾害，当前的撞击危害相当大，并且可以提出论据来阐明各种公共政策问题。任何针对撞击体的防御计划的首要任务是调查穿越地球的近地小行星数量，并及时预测它们的轨道。这就是太空卫士调查的目的，该调查已经发现了一半以上直径大于1公里的近地小行星。如果有一颗近地小行星处于与地球的碰撞轨道上，它可以被发现并且可以提前数十年或更长时间预测撞击。然后就可以考虑如何使近地小行星偏转或使其解体。与其他自然灾害不同，鉴于有足够的预先知识来对威胁性射弹采取行动，撞击风险在很大程度上可以消除。
