|MadSci Network: Astronomy|
Though the impact of a large comet on any place on the Earth would be catastrophic to us in terms of what it would do to the ecosystem, it would have a negligible effect on the motion and orientation of the Earth as a planet. The reason is simply the difference in mass between the two objects. The Earth is vastly larger and more massive than a comet, and the two aren't traveling at too grossly different speeds: typical comet velocities may be three or four times the Earth's 30 kilometer per second orbital speed around the Sun. What that means is that something as small as a fifty-kilometer-diameter comet won't have any important effects on the trajectory of the 12,800-kilometer-diameter Earth. (The difference in mass is even greater than this indicates, because the Earth is made of rather denser stuff than comets are.)
A golf ball hitting a moving car doesn't change the motion of the car much by itself: the collision of the small ball with the larger car makes for no real change in the momentum or orientation of the car. It may alarm the car's driver, crack the windshield, or mar the paint, but the car doesn't care, unless the driver freaks out and swerves the car in surprise. Similarly, if you were to collide with a flying insect as you walk down the street, your own movement and posture wouldn't change as a result, unless you are the kind that goes into hysterics when you see a bee.
To significantly alter the orientation or trajectory of the Earth in a single impact, you really would have to hit it with something comparable in mass to the Earth. Something the size of the Moon (1/80 or so the Earth's mass) would do it, but we know about all the objects that big in the inner Solar System, and none of them are on trajectories that will ever bring them close the Earth.
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