The Advanced Small Analyzer for Neutrals (ASAN) on the Chang'E-4 Rover Yutu-2

The Advanced Small Analyzer for Neutrals (ASAN) is a compact mass resolving energetic neutral atom analyzer flown on the Yutu-2 rover of the Chinese Chang'E-4 mission to the Moon. ASAN measures energetic neutral atoms in the energy range from 10 eV to 10 keV with typically 30% energy resolution. The field-of-view is a single angular pixel of 37 degrees x 30 degrees pointing at lunar surface. Energetic neutral atoms enter the instrument through a charge particle deflection system and are then ionized on a diamond-like carbon conversion surface. Positive ions from the conversion surface are energy analyzed using an electrostatic analyzer and their velocity is subsequently determined in a time-of-flight section using ceramic channel electron multipliers as detectors. The obtained mass resolution m/Delta m is about 2, comfortably separating neutral hydrogen, helium, and the oxygen-group (>= 16 amu). A full energy spectrum for energetic neutral atoms is obtained in 3 seconds. Switching the charged particle deflection system off, ASAN also directly measures positive ions with similar performance as for energetic neutral atoms. Including electronics the instrument weighs 970 g and consumes 3.4 W.