Modelling dust particle dynamics in the coma of 67P/Churyumov-Gerasimenko

Comets are among the oldest populations in the Solar System, thus providing information about the conditions of the environment where it was formed. The European Space Agency's Rosetta mission, which escorted comet 67P/Churyumov-Gerasimenko (hereafter 67P) for approximately two years, represented a milestone in the study of these bodies, providing the most detailed data of the nucleus and inner regions of the coma to date. In particular, images taken by OSIRIS,  the main imaging system on board Rosetta, provided the opportunity to study dust particles present in the coma. While the smallest particles have the biggest contribution to the coma brightness observed in these images, larger particles can be detected and analyzed individually.

In this work, the motion of individual dust particles in the coma of the comet 67P is investigated. This is done by analyzing sets of images taken by OSIRIS in which these large dust particles can be seen as bright tracks instead of points sources as result of the combination of motions of both particles and spacecraft. Since the distance from the particles to the camera is unknown, most of their physical and dynamical properties such as size and velocity cannot be uniquely determined. Previous works dealing with the same issue focused on obtaining the distance to individual detected particles in the image, which could be done in specific cases, reducing the number of analyzed particles. 

In this work we use a different approach to analyze the dynamics of the dust. It is based on the statistical comparison between the images obtained by OSIRIS and synthetic images generated from dynamical modelling of the dust in the coma. The main advantage of this approach is to bypass the distance determination to the particles, which lifts the strict requirement on the observation conditions that were imposed by the earlier methods and allows us to analyze a much larger set of tracks. The dynamics of the particles, and then the distribution of tracks in the synthetic images, depend on certain parameters as dust size, density and initial velocity, so the comparison between observations and different synthetic images can help to constrain the properties of the dust. 

The main steps involved in the method are: (i) detection of dust tracks on the OSIRIS images using an algorithm based on the Hough transform method, (ii) dynamical modelling of the trajectories of dust particles and generation of synthetic images using information about spacecraft position and orientation and (iii) solving the inverse problem by finding the dust parameters that generate synthetic images most similar to real ones. We present results from several image sets taken at different moments during the orbit of 67P using different dynamical models, in order to analyze the evolution of the dust properties and compare the relevance of different forces in the particles trajectories.