反向自转：坦普尔-吉亚科比-克雷萨克彗星
Reversal of Spin: Comet 41P/Tuttle-Giacobini-Kresak
AI总结：
研究发现坦普尔-吉亚科比-克雷萨克彗星核在2017年经历自转反转，其有效半径为500±100米，活跃分数从2001年的2.4降至2017年的0.14，表明长期表面变化。
摘要：
彗星核的自转已知会因出气扭矩而发生变化。坦普尔-吉亚科比-克雷萨克彗星（41P/Tuttle-Giacobini-Kresak）在2017年4月接近近日点时表现出特别剧烈的自转变化。在这里，我们利用2017年12月的哈勃空间望远镜档案观测数据，研究彗星核的近日点后光变曲线，并评估核的大小。通过哈勃光度计和非重力加速度测量，我们发现一个尺寸小的核，有效半径r=500±100米。系统光学变化与一个双峰（即旋转对称）光变曲线一致，周期为0.60±0.01天，与2017年早些时候测量的周期显著不同。核的自转可能在2017年4月的近日点和12月之间反转，这是由于强烈的出气扭矩所致。我们推断无量纲力臂k=0.013，大约是短周期彗星中位值的两倍。光变幅度为0.4等，表明投影核轴比大于1.4:1，而核的活跃分数从2001年的2.4（表明由升华 coma 冰粒增加气体产生）降至2017年的0.14，这是长期表面修改的结果。我们发现这个小核的自转加速寿命比当前轨道报告的1500年动力学时间要短。两种可能的解释是：核可能处于异常强烈的活动状态，导致我们高估了平均质量损失率和出气扭矩，从而低估了物理寿命；或者，核可能是曾经更大体的残余，其出气扭矩在改变自转方面效果较小。
英文摘要：
The rotations of cometary nuclei are known to change in response to outgassing torques. The nucleus of comet 41P/Tuttle-Giacobini-Kresak exhibited particularly dramatic rotational changes when near perihelion in 2017 April. Here, we use archival Hubble Space Telescope observations from 2017 December to study the post-perihelion lightcurve of the nucleus and to assess the nucleus size. From both Hubble photometry and non-gravitational acceleration measurements we find a diminutive nucleus with effective radius r = 500+/-100 m. Systematic optical variations are consistent with a two-peaked (i.e., rotationally symmetric) lightcurve with period 0.60+/-0.01 days, substantially different from periods measured earlier in 2017. The spin of the nucleus likely reversed between perihelion in 2017 April and December as a result of the strong outgassing torque. We infer a dimensionless moment arm k = 0.013, about twice the median value in short-period comets. The lightcurve range of 0.4 magnitudes indicates a projected nucleus axis ratio greater than 1.4:1, while the active fraction of the nucleus decreased from 2.4 in 2001 (suggesting augmentation of the gas production by sublimating coma ice grains) to 0.14 in 2017, a result of long-term modification of the surface. We find that the physical lifetime of this small nucleus to spin-up is short compared to the reported 1500 year dynamical time spent in the current orbit. Two limiting reconciliations of this inequality are suggested. The nucleus could be in a state of unusually strong activity, leading us to over-estimate the average mass loss rate and outgassing torque and so to under-estimate the physical lifetime. Alternatively, the nucleus could be the surviving remnant of a once larger body for which outgassing torques were less effective in changing the spin.
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