> With the masses of the comet and the spacecraft being relatively close
No, they're not close at all. The comet is three by five kilometers across, and the mass is around 3.14 * 10^12 kg. The orbiter at launch weighs in at 2900 kg and the lander is 100 kg.
http://en.wikipedia.org/wiki/Rosetta_(spacecraft) http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimen...