ASPERA-4 (Analysis of Space Plasmas and Energetic Atoms,
version 4) consists of an ensemble of instruments to study the interaction
between the solar wind and the Martian atmosphere and to characterize the
plasma and neutral gas environment. The instruments consist of a Neutral
Particle Imager (NPI), a Neutral Particle Detector (NPD), an Electron
Spectrometer (ELS), and an ion Mass Analyzer (IMA). NPI provides good
ENA arrival direction resolution but little energy or mass resolution,
while NPD provides energy and mass resolution but little directional
resolution.