Observations Relevant to Cometary Hyperactivity (The Year of 46P/Wirtanen)

在2018年秋季，非常明亮、非常活跃的彗星46P/Wirtanen将与另外两颗明亮的彗星21P/Giacobini-Zinner和64P/Swift-Gehrels共享天空，因为它们将到达离太阳最近的地方。46P/Wirtanen彗星将会特别明亮。研究人员将收集这三颗小彗星的图像和光谱，它们的轨道由木星的存在控制。今年它们经过太阳附近时的观测将为未来的研究提供重要数据，以量化彗星周围气体和尘埃的组成和物理特征，调查冰粒的存在，并确定彗星之间的共同点或关键差异。这些观测对时间至关重要，因为这些良好的观测条件对21P彗星来说在十多年内不会再出现，对46P和64P彗星来说在二十多年内不会再出现。研究人员将通过各种公开演讲与更广泛的社区分享他们对测量到的彗星活动的解释，以及这些彗星预期的令人震惊和信息丰富的图像。与极度活跃的彗星46P不同，大多数木星家族的彗星表现出一定程度的活动，这可以通过其表面一小部分的升华来解释。另一方面，彗星103P/哈特利2号（美国宇航局太空探测器“深度撞击”的目标）是过度活跃的群体中的一个代表成员，矛盾的是，活跃的比例是100%。在飞越哈特利2号的过程中，“深度撞击”观测到一股巨大的二氧化碳喷流，其中充满了小颗粒的水冰，来自彗星的一个叶状结构。从彗核中逸出的二氧化碳很可能将地下的水冰粒拖进了彗发。一旦进入昏迷状态，分布的冰粒显著增加了水蒸气升华的表面积，这解释了哈特利2号的过度活跃。这个模型表明，极度活跃的彗星是由具有低升华温度的地下挥发物控制的，这些挥发物携带冰粒产生异常高的产水率。然而，自“深度撞击”测量以来，没有任何其他活跃的彗星被充分观察到，以证实哈特利2号模型的适用性；这将随着46P的观测而改变。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

During the fall of 2018, the very bright, very active comet 46P/Wirtanen will share the sky with two other bright comets, 21P/Giacobini-Zinner and 64P/Swift-Gehrels, as they reach their closest approach to the sun. Comet 46P/Wirtanen will be especially bright. The investigators will collect images and spectra of these three small comets whose orbits are controlled by Jupiter's presence. The observations during their passages near the sun this year will provide future studies with significant data to quantify compositions and physical characteristics of gases and dust around the comet, investigate the presence of icy grains, and determine common aspects or key differences among the comets. These observations are time critical as these good observing conditions will not present themselves again for over a decade for comet 21P, and more than two decades for comets 46P and 64P. The investigators will share their interpretations of the measured cometary activity, as well as the expected stunning and informative images of these comets, with the broader community through various public talks.Unlike the hyperactive comet 46P, most Jupiter-family comets display a level of activity that can be accounted for by sublimation from a small fraction of their surface area. Comet 103P/Hartley 2 (the target of the NASA space probe Deep Impact), on the other hand, is a representative member of a hyperactive population that, paradoxically, exhibit active fractions 100%. During its flyby of Hartley 2, Deep Impact observed a large carbon dioxide jet, suffused with small grains of water ice, originating from one lobe of the comet. The carbon dioxide escaping from the nucleus likely dragged subsurface water-ice grains into the coma. Once in the coma, the distributed ice grains dramatically increased the surface area for water vapor sublimation, explaining Hartley 2's hyperactivity. This model suggests that hyperactive comets are governed by subsurface volatiles with low sublimation temperatures that entrain ice grains to produce anomalously high-water production rates. Since the Deep Impact measurements, however, no other hyperactive comet has been sufficiently observed to confirm the applicability of the Hartley 2 model; this will change with observations of 46P.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.