Apophis 2029年接近地球对小型小行星撞击的敏感性
On the sensitivity of Apophis' 2029 Earth approach to small asteroid impacts
AI总结：
本文评估小型小行星撞击对Apophis 2029年近地轨道的摄动风险，发现显著改变keyhole或掠过距离的概率极低，并指出2027年单次观测可消除大部分风险。
AI中文摘要：
Apophis目前的轨道使其在2029年4月13日以数个地球半径的距离安全掠过地球。本文考虑了Apophis可能与一颗小型小行星相撞的情况，这类小型小行星如同频繁且不可预测地撞击地球的天体一样，并研究由此产生的轨道摄动。研究发现，可能使Apophis相对于其keyholes（锁眼）发生显著位移的撞击概率低于百万分之一，这类撞击需要大于0.3 mm/s的delta-v；而可能使Apophis相对于其2029年的掠过距离发生显著位移的撞击概率低于十亿分之一，需要大于5 cm/s的delta-v。这些概率低于小行星撞击预警系统通常考虑的阈值。Apophis在2021年年中至2027年期间处于白昼天空中，无法观测。要在2027年通过单晚观测确定Apophis是否已在目标平面上充分移动并进入危险keyhole将具有挑战性，因为其相对于标称星历的偏差可能仅有零点几角秒。然而，如果即将发生地球撞击，在大多数情况下，Apophis在2027年相对于其标称星历出现的数十角秒偏差将给出明确信号。因此，尽管少数情况存在一定歧义并得到更详细讨论，但通过2027年对Apophis进行一次观测，大部分撞击风险即可消除。文中还给出了不同情景下Apophis在天空中位置的图表，以供观测者快速评估。
英文摘要：
Apophis' current trajectory takes it safely past our planet at a distance of several Earth radii on 2029 April 13. Here the possibility is considered that Apophis could collide with a small asteroid, like the ones that frequently and unpredictably strike Earth, and the resulting perturbation of its trajectory. The probability of an impact that could significantly displace Apophis relative to its keyholes is found to be less than 1 in $10^6$, requiring a delta-v greater than 0.3 mm/s, while for an impact that could significantly displace Apophis compared to its miss distance in 2029 it is less than 1 in $10^9$, requiring a delta-v greater than 5 cm/s. These probabilities are below the usual thresholds considered by asteroid impact warning systems. Apophis is in the daytime sky and unobservable from mid-2021 to 2027. It will be challenging to determine from single night observations in 2027 if Apophis has moved on the target plane enough to enter a dangerous keyhole, as the deviation from the nominal ephemeris might be only a few tenths of an arcsecond. An impending Earth impact would, however, be signalled clearly in most cases by deviations of tens of arcseconds of Apophis from its nominal ephemeris in 2027. Thus most of the impact risk could be retired by a single observation of Apophis in 2027, though a minority of cases present some ambiguity and are discussed in more detail. Charts of the on-sky position of Apophis under different scenarios are presented for quick assessment by observers.
