yThe Mercury sodium atmospheric spectral imager for the MMO spacecraft of Bepi-Colombo

The Mercury Sodium Atmosphere Spectral Imager (MSASI) on the Mercury Magnetospheric Orbiter (MMO) of the JAXA/ESA Bepi-Colombo (BC) Mission will address a range of fundamental scientific questions pertaining to Mercury's exosphere. The measurements will provide new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral region near the sodium D2 emission (589 nm), a major constituent of the Mercury exosphere. A single high-resolution Fabry–Perot etalon is used in combination with a narrow-band interference filter to achieve a compact and efficient instrument design. The etalon and filter are extremely stable with respect to long-term aging and temperature variations. Full-disk images of the planet are obtained by means of a single-axis scanning mirror in combination with the spin of the MMO spacecraft. This paper presents an overview of the MSASI and the design of the Fabry–Perot interferometer used as its spectral analyser.It is concluded that:(1)The MSASI optical design is practical and can be implemented without new or critical technology developments.(2)The thermally stable etalon design is based on concepts, designs and materials that have a good space heritage.(3)The MSASI instrument will achieve a high signal-to-noise ratio (SNR) (>10) in the range of 2 K–10 M Rayleigh.