The MIDAS device is a detector with wristwatch dimensions and mass less than 50g. It consists of a plastic scintillator cube connected to a silicon photomultiplier on its one face, while all the other faces are covered by a titanium box and 2 consecutive layers of fully depleted silicon pixel sensors with in-pixel signal processing electronics. Its purpose is not only to act as a LET spectrometer, but to determine the radiation field parameters near to an astronaut and enable the application of fluence to dose conversion factors for the evaluation of dose and subsequently dose equivalent. We have undertaken a detailed GEANT4 simulation of the device which has played a decisive role in its design. Methods and results of the investigation of the device capabilities will be presented as far as it concerns particle identification and kinetic energy determination.