1. The problem statement, all variables and given/known data On June 22, 1978, James Christy made the first observation of a moon orbiting Pluto. Until then, the mass of Pluto was not known, but with the discover of its moon, Charon, its mass could be calculated with some accuracy. Explain. 2. The attempt at a solution "By knowing how large the orbit of the moon was, the mass of pluto could be calculated. In order to do this, the radius of the orbit of the moon is used because it shows the amount of gravity pluto is exerting on it. Because mass is proportional to gravity, and gravity to the size of the orbit, the mass can be calculated by looking at the size of the orbit." I was given 4/5 credit for this with no direction as to where I went astray. I'm not sure if I was just too vague or I was simply incorrect, but any input would be greatly appreciated.