Analysis of Charging of the HTV-4 Based on On-Orbit Data

After three H-II transfer vehicles (HTVs) had finished their mission to resupply the International Space Station (ISS), NASA requested data of the HTV's potential to evaluate the charging/discharging process that occurs when the HTV docks to the ISS. To measure these data, a new instrument was installed on the fourth HTV. This instrument allows us to measure the HTV-4 surface potential relative to the surrounding plasma, and is called advanced technology on-orbit test instrument for space environment-mini (ATOTIE-mini). The ATOTIE-mini observed the HTV's local potential in the orbit for more than one month. The measured potential during the HTV solo-flight phase varied between -30 and -60 V in sunlight and was about 0 V in eclipse conditions. The HTV's potential during the time when it was docked to the ISS followed the ISS's potential with an almost constant offset of about 10 V. The data measured by ATOTIE-mini are consistent with those measured by the floating potential measurement unit on the ISS, and thus are considered reliable. The HTV's potential level itself was acceptable for ISS. Note that the solar array panels can generate up to approximately 120 V, which is much larger than the absolute potential range in sunshine. We analyze the potential distribution on the HTV surface by a multi-utility spacecraft charging analysis tool, because ATOTIE-mini can only observe one point on the HTV surface. The analysis results are discussed with respect to the flight attitude.