Non-gravitational Forces and Torques in Comet 67P/Churyumov-Gerasimenko and Determining their Relationship to Cometary Activity

The Rosetta mission to comet 67P/Churyumov-Gerasimenko (hereafter 67P) was the only spacecraft to orbit a cometary nucleus. It produced the most detailed information on the nuclear shape, cometary activity, and evolution of the comet's orbit and the rotational rate of the nucleus as a function of time. Therefore, Rosetta observations provide an excellent opportunity to investigate the nuclear activity and how it manifests in the orbital and rotational motions of the comet.

The activity of a cometary nucleus helps us understand its evolution and is a probe that enables us to peek into the physics and chemistry of the surface layers. Understanding how the activity will manifest in the orbital and rotational motions will help us understand the physics of outgassing. A detailed understanding of this manifestation also provides an accurate deterministic tool to estimate the mass and therefore the bulk density of cometary nuclei of other comets in the future.

The non-gravitational forces and torques affect the orbital and rotational motions of a comet, respectively. Therefore, in order to understand how the activity of comet 67P is related to non-gravitational forces and torques, we will carry out thermal modeling coupled with non-gravitational force and torque modeling. This will be carried out using the publicly available Rosetta observations (e.g., shape model, SPICE kernels, and images available in PDS) and published literature. The models corresponding to different scenarios will be used to derive orbital and rotational changes and they will be compared with mission data; i.e., in the proposed investigation, we use the shape model, surface features, activity on the nucleus, and orbital and rotational information of the nucleus derived from Rosetta data to constrain the model parameters.

In our study, a detailed assessment of the momentum transfer due to sublimating gases and cancellation of non-gravitational forces and torques associated with outgassing originating from different parts of the nuclear surface will be carried out. The cancellation of torques will be assessed in the context of rotational changes of comets discussed in Samarasinha and Mueller (2013, ApJL, 775, L10). Rosetta observations show outgassing from pit walls (e.g., Vincent et al., 2015, Nature, 523, 63-66), and an evaluation of how the resultant forces and torques would manifest in orbital and rotational parameters of the comet will be performed. The proposed work will enable us to realistically assess how the detailed shape of the nucleus and outgassing from it affect the non-gravitational forces and torques acting on comet 67P (e.g., analogous to Statler (2009, Icarus, 202, 502-513) for YORP torques).

The team members have extensive experience with investigations involving non-gravitational studies and thermal modeling. The proposed work will enable us to (a) gain a greater understanding of comet 67P's activity, leading us to crucial clues about the mechanism(s) of cometary activity, (b) provide an in-depth comprehension about the relationship between nuclear activity and non-gravitational forces and torques which will reveal the underlying physics, (c) provide a context vital for better interpretation of Rosetta data for other science tasks, and (d) facilitate possible application of new insights into cometary activity and non-gravitational forces and torques to other comets in future work.