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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?

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Indeed it's extremely difficult to accurately determine the mass of a comet without flying something past it at close range. Even if you assume an average density (which is not particularly safe, as little is yet known about the internal makeup of comets and how that varies from body to body), no earthbound telescopes (including Hubble) were able to resolve the nucleus before the mission to give an accurate volume estimate. Researchers had to go by the magnitude and an assumed albedo, which is similarly poorly-characterized across the comet population.

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.

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  • $\begingroup$ Thank you for your answer. Could you comment on what broke in Maquet et al's estimate? Did they use the wrong albedo? Or the wrong density? $\endgroup$ – E.P. Nov 15 '14 at 16:49

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