When faced with impending doom, optimism and pessimism are both equally fine sentiments. The caveat is that only one of them accurately reflects reality, not that it matters at that point.

I also suppose that industriousness would be helpful for taking one's mind away from the reality of that impending doom.

But hell, nukes to deflect a comet? Sure, we may as well try it -- but I only suggest doing this if we are 100% certain that the comet will both hit us and destroy us. We could conceivably cause our own extinction via a false positive provided by some faulty monitoring device - wait, did that already happen? Nevermind.

If there is a significant chance that a comet will hit and destroy us, it makes sense to do everything we can to prevent it from doing so. Given the stakes, .1% chance should be more than enough for us to take it seriously. It does not seem plausible that we could harm ourselves much by nuking a comet that wasn't going to hit us anyway.

It does seem plausible that we could in fact harm ourselves by attempting to launch a nuclear device out of Earth's orbit. We do have a record of failed launches, several of them recent. Once the nuclear device is out of orbit however, I agree that the harm it can do to us is negligible.

Edit: Also, it just occurred to me the possibility of deflecting a comet (which was never in our path) into our path, instead of away from it, which is really great material for a Hollywood comedy.

Good point, but the chances of a failed launch are fairly small, and the damage from accidentally detonating a large nuke on the Earth's surface, while bad, would be small compared to human extinction.

If we are sufficiently uncertain about the comet's path, we might accidentally deflect it towards us. Obviously, if this is just as likely as deflecting it away from us, it would not be worthwhile to try. But most likely we would be able to reduce the probability of a catastrophic impact even after taking into account the possibility of such an error.

We've intentionally detonated hundreds of nukes in the atmosphere.. Even over heavily populated cities. Sure, it was bad, but not earth-ending bad.

If a lunch failed and "nuked" the launch pad, I hope we'd just move on to another continent and try again (and again)

If a billion-ton comet is about to hit our planet, a failed attempt to launch a nuke doesn't matter in the slightest.

Nuclear weapons have fallen out of the sky before. No, they won't explode -- it isn't easy to make one go boom.

But I would not be surprised if Green political parties argue that risking a few km^2 contamination is worse than a certain dinosaur killer... :-)

http://en.wikipedia.org/wiki/1966_Palomares_B-52_crash

"The non-nuclear explosives in two of the weapons detonated upon impact with the ground, resulting in the contamination of a 2-square-kilometer (490-acre) (0.78 square mile) area by plutonium. [... another which fell] into the Mediterranean Sea, was recovered intact"

It's not Hollywood, it's ballistics.

The critical figures here are what the relative speeds are, when the asteroid/comet is detected, what it is made of and how masssive it is.

After you've figured all those out the stage is set and you get a menu:

  - party like it's 2012

  - attempt to deflect it 

  - brace for impact and deal with the aftermath

Any attempt to deflect it has a large degree of uncertainty, including the trouble of getting humanity to pull in one direction for any length of time.

You'd have to develop the vehicle (and the fuel expenses to get the right velocity / direction are going to be terrible, they may be beyond current tech to make a manoeuvre like that, imagine meeting an oncoming bullet by matching speeds and direction where the bullet can come from any direction and you're sitting on a spinning ball), a plan to deflect the mass (by then you'll have burned up a lot of your time budget) and so on.

Maybe, for very small values of maybe.

In context, if we look at the current state of humanity; sure some of our great minds have helped some (not all) of us to live pretty nice lives here on Earth. But on the whole, we're pretty good at screwing up basic things. Non-basic things, like for example having a Mars orbiter not crash into the planet, yeah, we screw those up too.

Anyway, I thought the OPs comment was likely more facetious than anything. Only in Hollywood can we expect a perfect ending.

Um, global melting is happening, and squat is being done about it. I wish I could be as optimistic as you. Maybe because a comet is a thing, a tangible menace, we would emotionally respond.