12. If a satellite of mass ms moves in a circular orbit around a planet of mass Mp, we can set the equation for centripetal force of the satellite equal to the equation for gravitational force between the satellite and the planet and solve for the speed of the satellite orbiting at a particular radius, r. 12a. Using the information above, derive an equation for the speed as a function of radius. 12b. Given that we know that Venus orbits the sun in a "near circle" at a radius of 1.08 x 1011 m with an orbital period of 228 days. Use the circumference, orbital period, and the equation for the speed derived above to estimate the mass of the sun in kg.