Enhanced Ion Sampling Techniques for In-situ Neutral Gas and Low-energy Ions Exploration of Main-belt Comet

One of the detection objectives of the Chinese Asteroid Exploration mission is to investigate the space environment near the Main-belt Comet (MBC, Active Asteroid) 311P/PANSTARRS. This paper outlines the scientific objectives, measurement targets, and measurement requirements for the proposed Gas and Ion Analyzer (GIA). The GIA is designed for in-situ mass spectrometry of neutral gases and low-energy ions, such as hydrogen, carbon, and oxygen, in the vicinity of 311P. Ion sampling techniques are essential for the GIA’s Time-of-Flight (TOF) mass analysis capabilities. In this paper, we present an enhanced ion sampling technique through the development of an ion attraction model and an ion source model. The ion attraction model demonstrates that adjusting attraction grid voltage can enhance the detection efficiency of low-energy ions and mitigate the repulsive force of ions during sampling, which is influenced by the satellite’s surface positive charging. The ion source model simulates the processes of gas ionization and ion multiplication. Simulation results indicate that the GIA can achieve a lower pressure limit below 10^–13 Pa and possess a dynamic range exceeding 10⁹. These performances ensure the generation of ions with stable and consistent current, which is crucial for high-resolution and broad dynamic range mass spectrometer analysis. Preliminary testing experiments have verified GIA’s capability to detect gas compositions such as H₂O and N₂. In-situ measurements near 311P using GIA are expected to significantly contribute to our understanding of asteroid activity mechanisms, the evolution of the atmospheric and ionized environments of main-belt comets, the interactions with solar wind, and the origin of Earth's water.