We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 ≲ r _H ≲ 16 au. The comet continues to be characterized by a coma of submillimeter-sized and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross section of the coma, C, is related to the heliocentric distance by a power law, , with heliocentric index s = 1.14 0.05. This dependence is significantly weaker than the r _H ⁻² variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r _H ∼ 35 au is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass-loss rates in dust at 10 au are of order 10³ kg s⁻¹, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the nongravitational acceleration to the local solar gravity, α′, may, depending on the nucleus size, attain values of ∼10⁻⁷ ≲ α′ ≲ 10⁻⁵, comparable to values found in short-period comets at much smaller distances. Nongravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.

我们研究了来袭的长周期彗星 C/2017 K2 在日心距离范围 9 ≲ r _H ≲ 16 au 内活动的发展。彗星继续以低速喷射的亚毫米级和更大粒子的彗发为特征。在围绕原子核的固定共同移动体积中，我们发现彗发的散射截面C与日心距离通过幂律相关，日心指数s = 1.14 0.05。这种依赖性明显弱于r _H ^-2 两种效应导致的日照变化。首先是尘埃速度的日心依赖性，其次是由于在观测之前很久就喷射出非常缓慢移动的粒子而产生的滞后效应。光度测量的蒙特卡罗模型表明，需要从r _H ∼ 35 au开始产生灰尘以匹配测量的日心指数，而对粒度分布只有轻微的依赖性。10 au 处灰尘的质量损失率约为 10 ³ kg s ^-1，而气体中的损失率可能要小得多，这取决于未知的灰尘与气体比率。因此，非重力加速度与当地太阳引力之比，α ', 取决于原子核的大小，可能达到 ~ 10 ^-7 ≲ α ' ≲ 10 ^{-5 的}值，与在更短距离的短周期彗星中发现的值相当。C/2017 K2 和类似遥远彗星的非重力加速度，虽然目前尚未测量，但可能会限制我们从长周期彗星的轨道推断奥尔特云特性的准确性。