The Rosetta mission will begin its comet rendezvous and lander mission in mid 2014. The plasma instruments onboard Rosetta will provide detailed measurements of the plasma environment of comet 67P/Churyumov-Gerasimenko and its responses to solar wind interaction. In this chapter the basic scale lengths connected to the dimension of the radial-expanding magnetic field-free ionosphere and to the boundary separating photoelectrons cooled by collisional interaction with water molecules, respectively, are explained. Their variations at different heliocentric distances along the orbit of comet 67P are described. It is found that the radii of the diamagnetic ionospheric cavity (35 km at perihelion) and that of the cold photoelectron zone (350 km at perihelion) are all much smaller than those of comet Halley. These theoretical estimates will be tested by the upcoming in situ plasma measurements and remote-sensing observations onboard the Rosetta spacecraft.