as near as i can figure, you need to use newton's law of universal gravitation, and set that equal to normal acceleration, solve for tangential velocity. probably overly complicated, but that'd do it. once you know the tangential velocity, you can find angular velocity, divide by 2pi to get rotations/sec, take the inverse for period. might want to convert to something longer than a second, though.
G((m1*m2)/r^2) = abs(v^2)/r
v = sqrt(G*m1*m2/r)
Last edited by cww516 : 12-11-2009 at 10:22 PM.