1. The problem statement, all variables and given/known data Mercury's orbital speed varies from 38.8km at aphelion to 59.0km at perihelion. If the planet is 6.60*10^10m from the Sun's center at aphelion, how far is it at perihelion? 2. Relevant equations v = sqrt(GM/r) (speed,circular orbit) 3. The attempt at a solution V_a = velocity of aphelion = 38.8km = 3.88 * 10^4m V_p = velocity of perihelion = 59.0km = 5.9 * 10^4m V_a = sqrt(GM/r) 3.88 * 10^4m = sqrt(GM/r_a) and solving for M gives me (I think) M = (3.88 * 10^4m*r_a)^2 / G V_b = sqrt(GM/r_p) ; where W_b = 5.9 * 10^4m solving for r_p gives : r_p = (3.88 * 10^4m *r_a ) / 5.9 * 10^4m ; where r_a = 6.60*10^10m; and r_p = 4.60 * 10^10 is this right?