The main scientific objectives of the Rosetta -Virtis spectrometer are: to determine the nature of a solids on the nucleus surface (mineralogical composition, the mean size of grains on surface etc.) to identify the gaseous species, gaseous activity and its spatial distribution and the nature of the dust and gases in coma from the intensity of scattered and emitted radiation in the 0.25-5.0 mum spectral range. This spectral range is diagnostic for ices, minerals and organic compounds and detection of various gases for example H₂O, CO₂, H₂CO, CH₃OH, HCN and CO. The goal of our modelling is to evaluate the influence of the gases, dust particles and surface on intensity and shapes of the spectra. Our model includes contributions from the major gas species as well as the contribution from solar radiation scattered from the dust particles at short wavelengths and the thermal emission from dust at long wavelengths. The spectral characteristics of surface materials have been also accounted for. Some subroutines of the MODTRAN code, expressly modified for application to the comet environment, have been used for the modelling.