Modelling of cometary plasma environments in preparation for Comet Interceptor

Abstract

The objective of this research was to model the cometary plasma environment in order to prepare for the Comet Interceptor mission. In other words, the main goal was to determine the outermost plasma boundary to be encountered by Comet Interceptor, i.e. the bow shock, to know the scale size of potential comet targets. The bow shock is the boundary formed where the solar wind first meets the comet. We determine the bow shock stand-off distance potential Comet Interceptor mission targets, if the spacecraft had already been launched. These comets were specifically selected by the project team because they were reachable within the mission's operational lifetime, if it had been already launched.

The study investigated the relationships between tree boundary distances (bow shock, diamagnetic cavity and exobase), heliocentric distance, and outgassing rates of the comets. Furthermore, the Rosetta spacecraft's trajectory has been studied in relation to the bow shock location of its target 67P/Churyumov-Gerasimenko, in order to determine potential interactions with the comet's bow shock boundary. We also modelled theses locations for comet Hale Bopp to get an upper limit to the scale size of potential targets. The stand-off distance of each comet's bow shock was determined using the model developed by Koenders et al. (2013) after having first scaled the model parameters (outgassing rate, ionisation rate, Solar wind parameters) by heliocentric distance. For the diamagnetic cavity, the Cravens et al. (1986) model was used while the Henri et al. (2016) model was applied in order to calculate the exobase location. For comet 67P and the selected dataset, our results clearly show that the bow shock stand-off distance relies on the heliocentric distance. Beyond about 2 AU, the bow shock did not form at all for 67P. For Hale-Bopp, the bock shock was much larger and reached a few times 106 km at a heliocentric distance at 4 AU. Furthermore, we could also study how the stand-off distance increased with the comet's outgassing rate. Regarding the diamagnetic cavity, they have approximatively the same size (around 100 km at 1 AU) as the exobase for the selected dataset of comets according to the models used, while Rosetta observed a difference with a factor 10 between the size of the diamagnetic cavity (≃100 km at perihelion) and the exobase (≃10 km at perihelion) at comet 67P.

Results

The Comet Interceptor mother spacecraft, carrying two IRF instruments probes providing the in situ plasma density measurements prepared in this work, with its two daughter spacecraft in the background, heading towards the target comet. [Image credit: ESA]