小天体撞击灾害

由小天体与地球相撞而造成的撞击灾害是发生在太阳系尺度上的特大自然灾害：所谓小天体，是指那些可能与地球相撞的近地小行星、短周期彗星和长周期彗星，撞击灾害比洪水、地震和其他自然灾害更少发生，可是撞击灾害的后果却严重得多，甚至严重到可以引起地球生物大规模绝来的程度，并非所有撞击地球的小天体都能造成撞击灾害，能量低于10Mt的小天体就不能形成撞击灾害，而能量接近全球撞击灾害阈值（10^5-10^6Mt）的撞击灾害风险最大，全球撞击灾害是目前我们唯一知道能够造成世界上当大部分死亡的自然灾害，并且它的风险水平与我们熟悉的许多自然灾害相当。

The impact hazard of small objects

The impact hazard caused by impacts of small objects is the greatest natural disaster on the scale of solar system. The small objects are the near-earth asteroids, short-period and long-period comets striking the Earth. Impact hazards are rare compared with floods, earthquake, and other more mundane natural disasters; but they may have so serious consequences that they would be sufficient to cause mass extinction of lives on Earth. All the small objects striking the Earth cannot make impact hazards; the small objects which energy is below 10 Mt are unable to cause impact hazards, while the greatest risks are associated with impacts which energies are close to the global impact threshold between 105 and 106 Mt. The global impact catastrophe is the only known natural disaster capable of killing substantial fraction of the world population, with its risk level that is comparable to that of many other more familiar natural disaster.