Charging of spacecraft in terrestrial or planetary ionospheres and magnetospheres is a well-known effect which may have detrimental consequences for in-situ plasma measurements. The case of the Rosetta mission is of particular importance because the expected temperatures of cometary ions and electrons in the inner coma of comet Wirtanen are very low. For this reason, we have undertaken a numerical simulation study of the electrical equilibrium of the Rosetta spacecraft in various conditions during its operational phase in order to determine its floating potential and the structure of the surrounding plasma sheath. The results of this study demonstrate that the electric equilibrium of the spacecraft is essentially controlled by the photoelectron current emitted by its very large solar panels. Only very close to the nucleus the expected plasma density is large enough to allow the spacecraft to collect enough thermal electrons to stay within a few kTe from the plasma potential. At larger distances it will float positive and at high potentials compared with the characteristic energy of cometary particles, thus hindering reliable ion measurements. A possible way to circumvent this effect is suggested by our numerical model. It relies on disconnecting the sunlit surface of the solar panels from the spacecraft ground. With such a grounding scheme the equilibrium electric potential of the spacecraft would stay at moderate values compatible with thermal ion measurements.