Observing and monitoring low-energy charged particles—from sub-MeV up to tens of MeVs—has
become increasingly important for several reasons, impacting various fields of science. These
range from radiation protection and the study of magnetosphere–lithosphere interactions to in-
vestigations of space weather and the interplay between the heliospheric environment and the
magnetosphere. To address this wide array of scientific objectives, the Low Energy Module
(LEM) onboard the NUSES space mission has been developed.
NUSES is a planned space mission designed to test innovative approaches for studying low-energy
cosmic rays, gamma rays, astrophysical neutrinos, space weather phenomena, and models of mag-
netosphere–ionosphere–lithosphere coupling. The NUSES satellite carries two payloads: Terzina
and Zirè. Zirè, which measures protons and electrons up to a few hundred MeV, incorporates the
LEM, a compact particle spectrometer mounted directly on the satellite’s external top panel. The
LEM is dedicated to measure fluxes of low-energy electrons (0.1–7 MeV) and protons (3–50 MeV)
in the low-Earth orbit environment. Its compact design and limited acceptance enable event-based
particle identification even in high-radiation regions such as the South Atlantic Anomaly and the
inner Van Allen belt, where electron fluxes can reach approximately $10^{6}$ $cm^{-2}$ $s^{-1}$ $sr^{-1}$ . The
innovative features of the LEM include its extremely compact design—fitting within a 10×10×10
$cm^{3}$ volume—and its active collimation technique, which effectively mitigates the challenges of
multiple scattering (that typically prevent the use of standard tracking techniques) at low energies.
In this contribution, we present the geometry and detection concept of the detector flight model,
its expected scientific performance, the data products that will be available in orbit.