Decisions and Trade-Offs in the Design of a Mass Spectrometer for Jupiter's Icy Moons

This paper describes the Neutral gas and Ion Mass spectrometer (NIM) onboard ESA's JUpiter ICy moon Explorer (JUICE). Instead of detailing the nuts and bolts of the design, we recount it in terms of `given', architectural, and design decisions. `Given' decisions are relative to elements of the instrument defined at the outset of the project, which bound the solution space to be explored by the development team. These decisions comprise the definition of the instrument concept (time-of-flight), sample ionization, MCP detector, ion optics technology, radiation shielding, and allocation of electronics' functions. Architectural and design decisions regard other elements of the instrument defined during the preliminary and detailed design phases, after trade-offs accounting for both technical and programmatic factors. These decisions regard the instrument's ion source, mechanism, FPGA, emitters, PCB interconnect, detector, materials, circuitry of the emission controller and high-voltage pulser, and software. Lessons learned are discussed throughout, including decisions that, ex-post, should have been made differently. This recollection ends with the first mass spectrum acquired with the flight model.