Thanks for pointing out my SMALL oversight. I did a rough calculation which suggests that from a distance of 1 mile above earth and mars respectively the addition velocities would be about 392 mph and 218 mph. As you say small beer compared to the original velocity of the asteroid . I estimate , using 1/2 m V^2 that the kinetic energy in both cases would be increased byabout 8K M and 2.4K M respectively. For the sake of comparison with your figures, I have translated the velocities into metric measure on the basis of 1m=3'. They work out as 191m/sec and 106m/sec respectively. Obviously. these value will be larger if we were to calculate them from the time the asteroid enters earth's atmosphere as opposed to my values based on a height of 1 mile. They would, however, still be insignificant , in relation to 17-20km/sec.