# What was known about 67P/C-G's mass before the arrival of Rosetta?

The determination of the mass of a celestial object is usually between hard and impossible unless one can observe something that orbits it. In the case of comet 67P/Churyumov–Gerasimenko, one of Rosetta's first tasks was to determine the comet's mass and gravitational field. However, in planning the mission, presumably some knowledge of this mass would have been necessary to plan things like Philae's descent.

How much did we know about this prior to launch? How was this knowledge obtained? Did it come from brightness or size measurements from imagery? If so, how reliable are these known to be?

One pre-encounter estimate by Maquet et al in 2012 gave $3.14 \pm 0.21 \times 10^{12}$ kg.
The most recent publicly-available estimate using Rosetta orbital data is roughly three times as great: $1 \pm 0.1 \times 10^{13}$ kg.