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Abstract

Comets, thought to be amongst the most primordial of Solar System objects, are distinguished by their activity, i.e. the insolation driven ejection of gas and dust from their surfaces. The exact mechanisms of the outgassing and dust ejection remain an important open question in planetary science, relating as it does to the structure, composition, and thermophysical properties of the surface material. A directly observable effect of the activity, however, is the resultant non-gravitational force and torque on the cometary nucleus, which can alter its trajectory and rotation state. Understanding the effect of non-gravitational forces on the dynamics of a particular comet therefore gives us a powerful tool to investigate its activity and surface properties.

In this context, the detailed measurements made by ESA’s Rosetta mission at comet 67P/Churyumov-Gerasimenko provide a unique opportunity to discern a ground-truth for models of both non-gravitational acceleration (NGA) and torque (NGT), and outgassing activity. Rosetta collected data at 67P from 2014 to 2016, whilst spacecraft radio tracking combined with optical navigation allowed the comet’s position to be measured with unprecedented precision. Initially, the comet’s trajectory was reconstructed without taking the NGA into account, resulting in discontinuities in the reconstruction and hampering the extraction of the acceleration. Recent work, however, has improved the situation, and it is now possible to extract time-varying curves of the outgassing induced NGA directly from the trajectory. Alongside this, tracking of the nucleus orientation allows a measurement of the outgassing induced torque, while Rosetta’s in-situ instruments have measured the total outgassing rate itself. Together, these data provide significant constraints on both the distribution of activity over 67P’s surface, and the activity mechanism itself. Various comet thermal models are available in the literature, and these produce differing activity curves in space and time that can be directly compared with the combined dataset. 67P, therefore, provides a unique opportunity to constrain thermal-model parameters and near-surface nucleus properties.

This ISSI team will bring together international experts in spacecraft trajectory analysis with those in modelling and observations of cometary activity, in order to better understand the non-gravitational forces present at 67P and their implications for its activity pattern. This usage of the Rosetta data will result in the output of new scientific knowledge regarding cometary activity in general, which will be relevant for the next generation of missions such as Comet Interceptor.