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Scientists, using radio and orbital data from Rosetta, have made their first estimate of the mass of Comet 67P/C-G.
For the Rosetta mission, this task comes under the umbrella of the Radio Science Investigation (RSI). Using 80 hours worth of tracking data between 6 August, since arriving at the comet, and up to 9 August, the RSI team made a first estimate of the comet’s mass as approximately 1×10^13 kg +/-10%, or about 10 trillion kilograms.
This is very much a preliminary number which will be refined significantly as the spacecraft continues its travels with the comet.
More here. This mass, when considered in conjunction with the nucleus’s size, suggests a very low density. If you could find an ocean large enough it would float.