Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA.
Nature. 2018 Jan 10;553(7687):186-188. doi: 10.1038/nature25150.
Cometary outgassing can produce torques that change the spin state of the cometary nucleus, which in turn influences the evolution and lifetime of the comet. If these torques increase the rate of rotation to the extent that centripetal forces exceed the material strength of the nucleus, the comet can fragment. Torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist the nucleus can eventually reorient itself and the rotation rate can increase again. Simulations predict that most comets go through a short phase of rapid changes in spin state, after which changes occur gradually over longer times. Here we report observations of comet 41P/Tuttle-Giacobini-Kresák during its close approach to Earth (0.142 astronomical units, approximately 21 million kilometres, on 1 April 2017) that reveal a rapid decrease in rotation rate. Between March and May 2017, the apparent rotation period of the nucleus increased from 20 hours to more than 46 hours-a rate of change of more than an order of magnitude larger than has hitherto been measured. This phenomenon must have been caused by the gas emission from the comet aligning in such a way that it produced an anomalously strong torque that slowed the spin rate of the nucleus. The behaviour of comet 41P/Tuttle-Giacobini-Kresák suggests that it is in a distinct evolutionary state and that its rotation may be approaching the point of instability.
彗发物质的释放可以产生力矩，从而改变彗核的自转状态，这反过来又影响彗星的演化和寿命。如果这些力矩使自转速率增加到足以使向心力超过彗核物质强度的程度，那么彗星就会碎裂。使自转速率减缓的力矩会使自转状态变得不稳定，但如果力矩持续存在，彗核最终可能会重新定向，自转速率也会再次增加。模拟预测，大多数彗星都会经历一个自转状态快速变化的短暂阶段，之后变化会在更长的时间内逐渐发生。在这里，我们报告了对彗星 41P/Tuttle-Giacobini-Kresák 在接近地球时（2017 年 4 月 1 日，距离约为 0.142 天文单位，即 2100 万公里）的观测结果，这些观测结果显示其自转速率迅速下降。在 2017 年 3 月至 5 月期间，彗核的视自转周期从 20 小时增加到超过 46 小时——这一变化速率比迄今为止测量到的要大一个数量级以上。这种现象一定是由彗星释放的气体以一种特殊的方式排列产生的，这种方式产生了异常强大的力矩，从而减缓了彗核的自转速度。41P/Tuttle-Giacobini-Kresák 彗星的行为表明它处于一种独特的演化状态，其自转可能即将达到不稳定点。
