Solving for orbital period when at the surface, where d is the density of rock (googled): d ~ 2500 kg/m^3 T^2 = (4 * pi^2 * r^3) / (G * M) M = d * 4/3 * pi *

Solving for orbital period when at the surface, where d is the density of rock (googled): d ~ 2500 kg/m^3 T^2 = (4 * pi^2 * r^3) / (G * M) M = d * 4/3 * pi * r^3 substituting M into the equation for T^2 = (3 * pi) /(d * G)