A numerical model of dust particle impacts during a cometary encounter with application to ESA's Comet Interceptor mission

Introduction

Comet 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).

Method

Because 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 Giotto

To 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 Δv [cm/s]	Nutation angle at t = 50 s [°]
50th	13.27	0.017
75th	45.95	0.87
Measurement Giotto	23.05	∼0.07

 

In the table above the results of our model are compared to the total change in velocity Δv [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. 

Comparison with EDCM

The 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.

	Our Model	EDCM 50th percentile	EDCM 75th percentile
Median Δv [cm/s]	13.27	3.88	37.88

 

Free input parameters

The 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.

Conclusion

Based 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 <10 seconds and that the total change in angular velocity that needs to be corrected is in the order of 10 °/s. To provide more insightful requirements the GNC needs to be modelled in more detail in the future.

Acknowledgement

This work has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation. The authors acknowledge the financial support of the SNSF.

References

[1] Colin Snodgrass and Geraint H. Jones. The european space agency’s comet interceptor lies in wait. Nature Communications, 10(1):5418, 2019.

[2] P. Edenhofer, M. K. Bird, J. P. Brenkle, H. Buschert, E. R. Kursinki, N. A. Mottinger, H. Porsche, C. T. Stelzried, and H. Volland. Dust Distribution of Comet p/ Halley’s Inner Coma Determined from the Giotta Radio Science Experiment. , 187:712, November 1987.

[3] W. Curdt and H.U. Keller. Large dust particles along the giotto trajectory. Icarus, 86(1):305 – 313, 1990.