A. V. Rodionov

The near-nuclear coma of comet Halley in march 1986

The cameras carried onboard the flyby missions to comet P/Halleyin 1986 imaged the near nuclear jet activity fromseveral spatial directions. The observed, very structured near nucleardust jets were considered at that timeas the result of dust emission from well localized active surface regions(without supporting 3-D model computations, however).Based on the first, recently developed 3-D gas dynamical model ofP/Halleys activity,we have been shown that jet features can be reproduced assuming ahomogeneous dusty icenucleus surface. The dust in the collisional near nuclear comais concentrated along the gas flow discontinuities resulting from thecomplicated surface orography, creating the visual impression ofdust jets. We present here the results of these calculations forthe near nucleus dust distributions,and we compare them with the direct observations made during thethree Halley flybys (Vega 1, Vega 2, and Giotto).

Dynamical effects of comet P/Halley gas production

We revisit the rotation of P/Halley taking into account the most essential observational constraints, as well as the external torques aecting the nucleus, in particular the outgassing torque. We solve the dirty ice sublimation equations at each point of the sunlit surface, using, for the rst time, the surface shape derived from the 1986 flybys imaging data. We assume that the nuclear surface is homogeneous in composition, thus reducing the number of model free parameters to one only: the dust-to-ice ratio on the surface. Our derived rotation model is a short-axis mode; it is consistent both with the 1986 nucleus imaging data, with the estimated non gravitational force, and with the observed time variations of the nucleus production rates. The outgassing torque results in a signicant variation of the angular momentum vector { for the assumed nucleus density of 0.5 g/cm 3 .

Comparison between Navier‐Stokes and DSMC Simulations of the Rarefied Gas Flow from Model Cometary Nuclei

We compare two fundamental ways of modeling the steady gas flows in the near‐nuclear atmosphere (coma) of comets. Several heliocentric distances and several homogeneous non‐rotating model nuclei — spherical and aspherical — were considered, in order to cover a wide range of boundary conditions at the surface of the nucleus and of gas flow rarefaction in the coma. Two methods were used for simulations: (1) the Direct Monte‐Carlo Simulation (DSMC), (2) the so‐called “BE‐NSE” method, which self‐consistently solves the Boltzmann equation (BE) in the immediate nonequilibrium vicinity of the nucleus surface, and the Navier‐Stokes equations (NSE) in the downstream region. For the BE solution, a locally plane‐parallel approximation is made, using Cercignani’s (1981) algorithms. The two methods were found to agree over an unexpectedly large parameter range. To maximize the modeling efficiency, a hybrid approach combining the BE‐NSE approach in the denser regions, with DSMC in the more rarefied regions is also desc...

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

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 c20 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 c22 may require using the true pressure field instead of the mean pressure field.

The near-nucleus gas coma of comet 67P/Churyumov-Gerasimenko prior to the descent of the surface lander PHILAE

Context. The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data. Methods. The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a Direct Simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times. Results. We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1 – November 30 of 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in-situ measurements.

The prediction of the gas environment of the PHILAE probe during its 2014 descent to the nucleus of the comet 67P

One of the objectives of the ESA ”ROSETTA” mission to the comet 67P was to insert, in August 2014, an orbiter probe around the so-called nucleus of the comet, and to deposit the ”PHILAE” lander at the surface of the nucleus in November 2014. The selection of the landing site and the definition of the release point and initial descent velocity vector were made in the period August to October 2014 on the basis of simulations of the descent trajectory. This requested an assessment of the gravitational and aerodynamic forces on PHILAE. We here describe the so-called RZC model developed to predict the gas environment of 67P in November 2014 and compute the aerodynamic force. We first outline the unusual diffculties resulting from (1) the complexity of the nucleus surface on all scales, (2) the absence of direct measurements of the gas flux at the surface itself, (3) the time-dependence of the gas production induced by the fast nucleus rotation, (4) the need to perform the whole program within less than three m...