Laboratory measurement on impact ionization by neutrals and floating potential of a spacecraft during encounter with Halley's Comet

Abstract

A spacecraft penetrating into the dense cloud of ambient gas and dust particles in the coma of Halley's Comet, is exposed to a bombardment by these particles having a high kinetic energy due to the large velocity of the spacecraft relative to the cometary coma. The interaction of the spacecraft and cometary neutral particles was simulated by using neutral beams of different species directed towards various target materials such as aluminium, gold and the white conductive paint PCB-Z. The kinetic energy of the primary beam covered, depending on the species, the range from about 700 eV up to more than 3 keV and contained, except for H 2O, the expected specific ram energy of 24 eV amu . The highest achievable density corresponded to a distance of slightly less than 10 4 km off the nucleus. Upon impact on the surface of the target, emission of charged as well as neutral secondary particles was initiated. The yields of the charged particles were derived from measurements of the electrical current produced by secondary ions or electrons. The obtained results for the yields complement other measurements performed in parallel to this work. The derived floating potentials show somewhat lower voltages than obtained by model calculations. It was found that for the metallic targets, the acquired charging potential due to neutral gas impact lies between 4 and 6 V. In the case of PCB-Z, the averaged floating potential amounts to about 14 V.