A charging model for the Rosetta spacecraft

F L Johansson,G Wattieaux,N Gilet,A I Eriksson,P Henri,M G G T Taylor,F Cipriani,C Imhof

doi:10.1051/0004-6361/202038592

Abstract

Context.The electrostatic potential of a spacecraft,VS, is important for the capabilities of in situ plasma measurements. Rosetta has been found to be negatively charged during most of the comet mission and even more so in denser plasmas.Aims.Our goal is to investigate how the negativeVScorrelates with electron density and temperature and to understand the physics of the observed correlation.Methods.We applied full mission comparative statistics ofVS, electron temperature, and electron density to establishVSdependence on cold and warm plasma density and electron temperature. We also used Spacecraft-Plasma Interaction System (SPIS) simulations and an analytical vacuum model to investigate if positively biased elements covering a fraction of the solar array surface can explain the observed correlations.Results.Here, theVSwas found to depend more on electron density, particularly with regard to the cold part of the electrons, and less on electron temperature than was expected for the high flux of thermal (cometary) ionospheric electrons. This behaviour was reproduced by an analytical model which is consistent with numerical simulations.Conclusions.Rosetta is negatively driven mainly by positively biased elements on the borders of the front side of the solar panels as these can efficiently collect cold plasma electrons. Biased elements distributed elsewhere on the front side of the panels are less efficient at collecting electrons apart from locally produced electrons (photoelectrons). To avoid significant charging, future spacecraft may minimise the area of exposed bias conductors or use a positive ground power system.

Highlights

The European Space Agency’s (ESA) comet chaser, Rosetta, monitored the plasma environment of comet 67P/ChuryumovGerasimenko from August 2014 to September 2016
In our investigation of the correlation of the LAP measured Rosetta Spacecraft potential to the MIP measured densities and characteristic temperatures of two detected cometary electron populations, we find the spacecraft potential to depend more on electron density and much less on electron temperature than expected in the high flux of thermal ionospheric electrons
Comparing the result to 3D PIC Spacecraft-Plasma Interaction System (SPIS) simulations and constructing a simple model bridging the two, we arrive at a system of equations that can readily explain the strong relationship between the Rosetta spacecraft potential and an observed cold (0.1 eV) electron population that sometimes dominate the Rosetta electron environment even when barrier potential effects are considered

Summary

Introduction

The European Space Agency’s (ESA) comet chaser, Rosetta, monitored the plasma environment of comet 67P/ChuryumovGerasimenko from August 2014 to September 2016. The spacecraft often reached negative potentials around and in excess of −15 V, which have a severe effect on in situ measurements of the plasma environment surrounding the spacecraft as electrons are repelled (Eriksson et al 2017) and positive ions are perturbed (Bergman et al 2019, 2020) From this spacecraft potential result, Odelstad et al (2017), with Eq (1), argued that the component dominating the electron flux is a thermal ≈5−10 eV population omnipresent in the parts of the comet coma visited by Rosetta. Normalising the spacecraft potential by e/Tew in Fig. 2 (middle panel) we see that an increase in warm electron density

Exploring what drives Rosetta to negative

Vacuum model for thin circular disk

Biased element in the centre of a solar panel

Spacecraft potential model

Findings

Conclusions