This is a past paper exam question: 1. The problem statement, all variables and given/known data One of the moons of Saturn is in an orbit which has approximately the same radius as that of the Earth's moon. Given that the speed of the saturn moon is ten times the speed of the Earth's moon, calculate a value for the mass of Saturn. (Mass of the Earth=6x10^24kg) 2. Relevant equations GmmE/r^2=mv^2/r 3. The attempt at a solution Mass of earth=6x10^24 r satmoon=r earthmoon v satmoon=v earthmoon/10 I did try this, but it's obviously wrong: tau^2=4pi^2r^3/Gme G=(6.67x10^-11) me=(6x120^24) period of revolution of the moon=24hrs hence (24)^2=4pir^3/4x10^14 576=4pi r^3 144=pir^3 even without going further, you can tell that it's going to be faaar too small to represent the radius of the moon. Without being given anything else-- do you need to memorise the radius of the moon's orbit for the exam? I can't really see any other way! I'm stumped...please help!