A numerical model of dust particle impacts during a cometary encounter with application to ESA's Comet Interceptor mission
Authors: Haslebacher Nico , Gerig Selina-Barbara , Thomas Nicolas , Marschall Raphael , Zakharov Vladimir , Tubiana Cecilia 
Published: July 2021

IntroductionComet Interceptor is the first F-class mission developed by the European Space Agency (ESA). The goal of the mission is to intercept a long period comet or an interstellar object. The novelty of Comet Interceptor is, that it will be launched before its main target has been found. Because the target is unknown the spacecraft and its instruments need to be designed such that they can handle a wide range of targets, encounter geometries and potentially hazardous environments [1]. We study the attitude perturbations caused by the impacts of large dust particles during a cometary encounter. Specifically, a numerical model is used to make predictions in relation to Comet Interceptor and its main imaging system called Comet Camera (CoCa).MethodBecause Comet Interceptor is in an early phase we use a generic approach. The dust model is based on force-free radial outflow modelled after comet 1P/Halley. To compare our modelling of the dust coma we use the Engineering Dust Coma Model (EDCM), which will be used by ESA and the industrial consortia designing the Comet Interceptor spacecraft. For simplicity the GNC of our model is idealized, which means that it is able to correct any attitude perturbations instantaneously. Currently there is no knowledge about the implementation of the GNC available and we consider the modelled GNC to be a best case. Further, we assume that the spacecraft has a homogenious mass distribution. To get a statistical distribution of possible outcomes each scenario is simulated 1000 times.Comparison to GiottoTo validate our model it was applied to the Giotto mission and compared to the measurements acquired during the approach to comet 1P/Halley.Percentile
Total &#916;v [cm/s]
Nutation angle at t = 50 s [&#176;]
50th
13.27
0.017
75th
45.95
0.87
Measurement Giotto
23.05
&#8764;0.07
&#160;In the table above the results of our model are compared to the total change in velocity &#916;v [2] and the nutation angle 50 seconds before closest approach of Giotto [3]. This shows that our model is able to produce results that are in the same order of magnitude than what Giotto measured.&#160;Comparison with EDCMThe EDCM contains a 1th, 5th, 10th, 25th, 50th, 75th, 90th, 95th and 99th percentile of the local dust number density at the specific point along the spacecraft trajectory. To compare our dust model with the EDCM we used the local dust density of a given percentile along the whole trajectory. As shown in the table below, this analysis showed, that our dust model lies in between the 50th and 75th percentile of the EDCM.&#160;
Our Model
EDCM 50th percentile
EDCM 75th percentile
Median &#916;v [cm/s]
13.27
3.88
37.88
&#160;Free input parametersThe free parameters of our model are radius, height and mass of the spacecraft, dust production rate, relative velocity at the encounter, distance to the nucleus at closest approach and time interval between attitude correction. For target objects similar to comet 1P/Halley, we will show that without attitude control the nucleus is shifted out of the field of view of CoCa at approximately 40 seconds before closest approach.We will show that out of the free input parameters the most crucial parameters are the encounter velocity, the spacecraft radius and the time interval between attitude control. Further, scaling laws of the free parameters will be shown. As an example, in Figure 3 the attitude perturbations in relation to the time interval between attitude correction and its scaling law fit is shown.ConclusionBased on our analysis we think that there is a high risk of loosing a few images, because the impact of a large particle shifts the nucleus partially or completely out of the field of view of CoCa. We will show that the rate of attitude corrections needs to be