The near-Earth object (NEO) population includes both asteroids (NEAs) and comet nuclei (NECs) whose orbits have perihelion distances q<1.3 AU and which can approach or cross that of the Earth. A NEA is defined as a “potentially hazardous asteroid” (PHA) for Earth when its minimum orbit intersection distance (MOID) comes inside 0.05 AU and it has an absolute magnitude H<22 mag (i.e. mean diameter > 140 m). These are big enough to cause, in the case of impact with Earth, destructive effects on a regional scale. Smaller objects can still produce major damage on a local scale, while the largest NEOs could endanger the survival of living species. Therefore, several national and international observational efforts have been started (i) to detect undiscovered NEOs and especially PHAs, (ii) to determine and continuously monitor their orbital properties and hence their impact probability, and (iii) to investigate their physical nature. Further ongoing activities concern the analysis of possible techniques to mitigate the risk of a NEO impact, when an object is confirmed to be on an Earth colliding trajectory. Depending on the timeframe available before the collision, as well as on the object’s physical properties, various methods to deflect a NEO have been proposed and are currently under study from groups of experts on behalf of international organizations and space agencies. This paper will review our current understanding of the NEO population, the scientific aspects and the ongoing space- and ground-based activities to foresee close encounters and to mitigate the effects of possible impacts.

中文翻译：

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近地天体及其对我们星球的潜在威胁

近地天体 (NEO) 群包括小行星 (NEA) 和彗核 (NEC)，它们的轨道近日点距离 q<1.3 AU，可以接近或穿过地球。当 NEA 的最小轨道交叉距离 (MOID) 在 0.05 AU 以内并且绝对星等为 H<22 mag（即平均直径 > 140 m）时，NEA 被定义为对地球来说是“潜在危险小行星”（PHA）。在撞击地球的情况下，它们大到足以在区域范围内造成破坏性影响。较小的物体仍然可以在局部范围内造成重大破坏，而最大的近地天体可能危及生物物种的生存。因此，已经开始了一些国家和国际观测工作：(i) 探测未发现的近地天体，尤其是 PHA，(ii) 确定并持续监测它们的轨道特性及其撞击概率，以及 (iii) 调查它们的物理性质。进一步正在进行的活动涉及分析可能的技术，以在确认物体位于地球碰撞轨道上时减轻近地天体撞击的风险。根据碰撞前可用的时间范围以及物体的物理特性，已经提出了各种偏转近地天体的方法，目前代表国际组织和空间机构的专家组正在研究这些方法。本文将回顾我们目前对近地天体种群、科学方面以及正在进行的空间和地面活动的了解，以预见近距离接触并减轻可能撞击的影响。