The problem statement, all variables and known data: You launch an object into a circular orbit around a spherical, smooth rock, 30 cm in diameter. The rock also has the same mass density as Earth, ρ. How long does it take the object to orbit the rock one time? Attempt at a solution: I know you should start by finding the mass of the rock, which turns out to be: (4/3)pi*r3*ρ I'm not sure what to do after that. Any help?