Vokrouhlický David, Nesvorný David, Bottke William F
Institute of Astronomy, Charles University, Prague, Czech Republic.
Nature. 2003 Sep 11;425(6954):147-51. doi: 10.1038/nature01948.
Collisions have been thought to be the dominant process altering asteroid rotations, but recent observations of the Koronis family of asteroids suggest that this may be incorrect. This group of asteroids was formed in a catastrophic collision several billion years ago; in the intervening period their rotational axes should have become nearly random because of subsequent collisions, with spin rates that follow a maxwellian distribution. What is seen, however, is that the observed family members with prograde spins have nearly identical periods (7.5-9.5 h) and obliquities between 42 and 50 degrees, while those with retrograde spins have obliquities between 154 and 169 degrees with periods either <5 h or >13 h. Here we show that these non-random orientations and spin rates can be explained by 'thermal torques' (arising from differential solar heating), which modify the spin states over time. In some cases, the asteroids become trapped in spin-orbit resonances. Our results suggest that thermal torques may be more important than collisions in changing the spin states (and possibly shapes) of asteroids with diameters <40 km.
碰撞一直被认为是改变小行星自转的主要过程，但最近对科罗尼斯小行星家族的观测表明，情况可能并非如此。这组小行星是在数十亿年前的一次灾难性碰撞中形成的；在此期间，由于后续碰撞，它们的自转轴应该变得几乎随机，自旋速率遵循麦克斯韦分布。然而，实际观测到的情况是，正向自旋的家族成员周期几乎相同（7.5 - 9.5小时），倾角在42度至50度之间，而反向自旋的家族成员倾角在154度至169度之间，周期小于5小时或大于13小时。我们在此表明，这些非随机的方向和自旋速率可以用“热扭矩”（由太阳加热差异引起）来解释，热扭矩会随时间改变自旋状态。在某些情况下，小行星会陷入自旋 - 轨道共振。我们的结果表明，在改变直径小于40千米的小行星的自旋状态（以及可能的形状）方面，热扭矩可能比碰撞更为重要。
