1. The problem statement, all variables and given/known data A satellite circles planet X once every 99 min at a mean altitude of 530km. Calculate the mass of the Planet, if its radius is 5.78 * 10^6 So this is the Question i am having trouble with, these gravity questions are really killing me. 2. Relevant equations 4 pi2R2/ T2 = GM/R 3. The attempt at a solution So i changed that equation in the following 4 pi2R3/ G T2 = M and then i change the 99 minuets in to seconds and then i change the 530Km in to M . Now this is where a friend told me that i need to at 530 000m in the the given radius of 5.78 * 10^6. I couldnt understand why he said that though. So going on with what i was told i plugged everything in to the equation and i got 5.246804357 * 1017Kg as the mass of the planet. Now i have doubts about this because i didn't quite know what to do with the 530 000m. I hope someone can correct my mistakes and show me the right solution. These gravitational questions are really giving me a tough time today. thanks.