A satellite is orbiting a strange space rock at a distance of 3.5 billion km from Earth. The space rock has a mass of $1.5\times 10^{14}kg$ and can be modeled as a sphere with a radius of $2.5\times 10^{3}m$. The satellite orbits at an altitude of 4.0 km above the surface of the space rock. Find the orbital period of the satellite about the space rock.