An analysis of outgassing pressure forces on the Rosetta orbiter using realistic 3D+t coma simulations

Abstract

A model for the interaction between a multicomponent Maxwellian atmosphere and a spacecraft is described. Multidimensional, time-dependent gasdynamical simulations of the gas coma around the recently reconstructed aspherical rotating nucleus of comet 67P/C-G is used to analyze the outgassing pressure forces on the ESA spacecraft Rosetta. The forces were in general found to be directed significantly away from the cometocentric position vector of the spacecraft. It was also found that in a maximum outgassing scenario at comet rendezvous, the outgassing pressure force exceeds the gravitational attraction from the nucleus in the cometocentric direction of the Sun. Furthermore, the highly non-spherical pressure field was found to undergo very large changes as the nucleus rotated. Still, it was possible to represent the mean pressure field experienced by Rosetta by a fairly simple model, which can be used for the determination of the comet mass and the so-called oblateness coefficient c 20 from the spacecraft Doppler signal. The oblateness coefficient represents a type of asphericity of the gravity field. The determination of the so-called triaxiality coefficient of the gravity field c 22 may require using the true pressure field instead of the mean pressure field.

Orbital Motion in Strongly Perturbed Environments

“…A current description of some of these modeling developments can be found in [115]. Improved models of comet outgassing are currently under development and will be tested and refined once the Rosetta spacecraft arrives at comet Churyumov-Gerasimenko.…”

“…There have been only a relative few analyses of spacecraft with outgassing comets, essentially starting with Scheeres et al [172], the thesis and related study by Byram [21,23,22], and the more recent analyses by Mysen [115]. For these simple analytical models there are a number of results concerning how a spacecraft responds to this environment.…”

Evolution of orbits about comets with arbitrary comae

“…The quantity and behavior of the gases in the coma cannot be predicted accurately and are not known a priori and are, in general, poorly understood without dense datasets. (Mysen et al 2010;Fougere et al 2016;Combi et al 2020) The most sophisticated coma models to date use discrete simulation Monte Carlo (DSMC) techniques to model gas kinetics, non-spherical nucleus effects, accurate insolation geometry, and even differing activity levels on the surface. (Fougere et al 2016) These models have nearly infinite degrees of freedom and require careful calibration with real data to produce meaningful results.…”

“…The spherical harmonic representation of the dynamic pressure field is only a choice of basis function, not a solution to the governing equations. Under the assumption of radial outgassing, any gas distribution should be well modeled to the surface (Scheeres et al 2000); however, for non-spherical nuclei one can readily expect that the near nucleus gas velocity field will have significant non-radial components that tend toward radial as the distance to the nucleus increases (Fougere et al 2016;Mysen et al 2010).…”

“…Variations with angular position can be expected due to the slow rotation rate of nuclei relative to their thermal inertia (i.e., the nucleus will not, in general, be isothermal), and nucleus shape effects (Groussin et al 2007). Deviations from a spherically symmetric coma are of interest due to the fact that actual comae are complex (e.g., Combi et al 2020;Mysen et al 2010), and introduce additional perturbations to orbits, discussed in detail throughout this paper, necessitating the use of more complex, analytically tractable, models. In this paper, a Fourier series in the argument of latitude modulated by the inverse square of the spacecraft-comet distance is used to model the coma, discussed in detail in Sect.…”