It seems clear that the earth has been hit many times by asteroids, meteorites, or other space objects. If you have any doubts, just click through the many web sites that deal with earth’s geological history, and the process of predicting future possible collisions. For information on asteroids whose orbit crosses earths, click on:
http://csep10.phys.utk.edu/astr161/lect/asteroids/collisions.html
Hollywood's answers do not satisfy me. I can just imagine the terror and panic that would ensue if we learned that 10 years from now an asteroid the size of Texas was going to collide with earth. That single event would change everything that would happen on earth for those next 10 years. I hope the more imaginative of our contributors will speculate on what it might be like. See speculations on waiting for the blow.
I for one, do not want to wait helplessly for such a potential disaster. If for no other reason than a potential collision, I believe the present generations should do our best to assure the continuation of our species far out into the future. Many scientists think that some sort of collision led to the extinction of the dinosaurs. I don't want it to happen to human kind, even if it isn't a risk in the foreseeable future.
We may discover ways in which we could use thrusters to ease the earth out of the way of a potential collision. If we can't find a way to move the entire earth, we may find that we can pick where the space object hits the earth, so as to minimize the overall damage. We could possibly change the orientation by changing the spin rate, or shifting the mantle around the molten core.
Let our scientists’ struggle with the question, "If we have to get hit, where would it do the least damage, overall to earth and its citizens?"
I rather favor a desert, like the Mojave, or the outback in Australia, or the Sahara, but this is just uniformed speculation by someone who is ignorant on the subject.
It may turn out we're better off having the object strike in the mountains, and we have those all over the globe. We'll have to see.
But, I am clear that we're not best off having the object strike an ocean where it would wipe out Island nations, or the largest cities in the world, which are often near sea ports. I don't think we would want the object to strike near high concentrations of people. Deserts and Mountains would be easier to evacuate to protect life and property.
An asteroid, or comet, or other huge space object doesn't have to hit the earth in order to wreak havoc. If a huge space object comes close to earth, it could have a considerable effect on our orbit and even perhaps, our orientation.
Harpers Magazine had an interesting quote from NASA that I haven't yet traced back. Perhaps you could help me find it.
If this is valid and true, then it suggests that there are fairly simple ways in which the earth's rotation can be affected.
Suppose every 100,000 years, a huge "moon sized" object intersected earth's orbit and did a fly-by within a couple of hundred thousand miles. Further suppose that every time that happened, it threw the earth out of its almost circular orbit, and into an orbit that was quite eccentric, or oval. Suppose that each time that happened, an ice age was started, and that the Suns gravity and earth's gravity take 90,000 years to bring the earth back into an almost circular orbit.
If we supposed these things, we might have another alternative to the cycles of ice ages first proposed by Milankovitch.
Ignoring this wild speculation for a moment, it is possible that periodic "fly-by's" from large space objects could have a devastating effect on earth's orbit. This is just one more of the potential dangers that earth faces. Another danger that having control over our orbit would allow us to fix.