I assume you calculated the average force and relative velocity experienced in the 450m circular orbit. At the perigee of the polar orbit (when the picture was

I assume you calculated the average force and relative velocity experienced in the 450m circular orbit. At the perigee of the polar orbit (when the picture was taken) the force and the relative speed should be bigger. Also due to the small size of the asteroid the gravity should be quite uneven during the approach.