The Earth is not a Catcher's Mitt
And, notwithstanding the millenarian theories of some of the odder denizens of the planet, Nobody is aiming meteors, comets, or asteroids at it. There are a lot of rocks in the solar system, and they tend to follow the trajectories dictated by the gravity of Newton with the tiny modifications discovered by Einstein and the possibly even tinier perturbations hinted at by the Pioneer I0/11 anomaly. From time to time, the orbits of those rocks intersect those of Earth, with effects ranging from unnoticeable to devastating.
A space rock killed the dinosaurs, or so we reasonably believe. Space rocks have wiped out species and, at least in the early years, reshaped the planet. Can it happen again? Yes. Will it happen again? Maybe. Should you worry about it? No. And that "No" is the point of this particular blogitem, since there are people who are worried about it, and I actually know a couple of them. This blog is for you, Mr. and Ms. Spacelab Hat.
Two reasons to be cheerful:
Reason the first: We live in an interregnum, so to speak. A century ago the actual nature of the spacerock threat wasn't understood. Yes, plenty of people were worried about sudden doom from comets, but it never occurred to them that something could be done about it. It was God's will and that was that. A century from now, I believe and trust, nuclear-powered rockets will be available and space engineering projects will be routine. Right now we're on the borderline of being able to find the threats and mitigate them. The "finding" problem is susceptible to increasing the power of sensors and computers, something which is accomplished on a continuing basis just by the advance of technology. The "mitigation" problem is more difficult, since progress in chemical rocketry is bounded by natural laws whose limitations we are approaching. Nonetheless, progress is being made: Nuclear electric (ion engine) rockets are clearly feasible, and more advanced nuclear rockets are at least in theoretical prospect.
So how does that help us now? By giving us perspective. There are plenty of candidate disasters predicted for the next hundred years - global warming, pandemic, nuclear accident. None of these is as likely to wipe out humanity as a good asteroid impact might, but they all have imminent time scales. We have reason to believe that species-destroying asteroid impacts occur roughly every 64 million years. The odds that it will be in the next hundred years are almost a million to one.
Reason the second: For lesser spacerocks, we are still a lousy target, and you, personally, are even a poorer one. Although there are a couple of cases, possibly apocryphal, of people's houses and cars having been hit by meteorites, nobody has been known to have been injured by one. Contrast this with the fact that there are documented cases of people winning the lottery — real lotteries, too, not just the RIKL Lottery in which everybody wins. Do you think you could possibly be unlucky enough to lose the lottery and yet be hit by a meteorite?
And one sidelight:
I always find it interesting that when a NEO (Near Earth Object) is discovered, it's orbit is calculated and we're told "it will pass less than 100,000 miles from the earth." (Or whatever the number might be.) This engenders some handwringing: <Thought balloon>"Wow! The earth is 8,000 miles in diameter, so it's got almost a one-in-ten chance of hitting us."</Thought balloon>
While that is manifest nonsense, when I started to think about what the actual odds of such an encounter are, I realized it isn't a trivial problem. For example, is it more reasonable to compare areas or volumes? I don't think either is quite correct, and so have given myself a calculation to do in my spare time. Doing math in one's alleged mind is said to be a method of postponing dotage, but somehow I don't think it will postpone it long enough to allow a meteoritic encounter with my capital unit.
This just in! From Jeff the Oddities dude, a report of a record meteorite hitting Norway. I was actually in the midst of writing this blogitem when I received it. Absent additional information, it would seem to militate against my theory of odds, but given the size of the explosion ("If the meteorite was as large as it seems to have been, we can compare it to the Hiroshima bomb"), it would seem to validate my point about poor targeting. ("There were ground tremors, a house shook and a curtain was blown into the house".)