Deep Impact scenario

[OmegaChief]

Adam, this may be me misinterpriting my post here, but are you suggesting using Orion propulsion rockets to move things into orbit from the surface?

As great as an Orion would be in space, using it as a surface to orbit launch platform just is not a good idea, what with the massive collateral damage such a launch would create.

Not to mention that inside an atmosphere nukes have a much greter damage potential, what with the blastwave and superheated atmosphere and all.

[Samuel]

Yep. They work best as heavy lifter to get as much into orbit as possible. As for collateral damage, that is what the ocean is for!

[Temujin]

There are some ideas on how to mitigate that problem. Link from Atomic Rockets.

[Sea Skimmer]

Five years is not a enough to build an Orion craft, or any other nuclear powered spacecraft. Way too many engineering challenges need to be overcome to make any of that stuff work with any useful reliability, and you can't build it before you know how to build it. The shock absorbers required for Orion may not be possible at all for example, and you'd need one hell of a control system to hold it stable while it rides the shock wave. The only realistic option I can see would be a 'big dumb booster' kind of setup in which we rush to mass produce known rocket technology to get as many nukes on target as we can. Like we take our largest launchers now, and strap on a dozen shuttle solid rocket boosters on top of it all. Luckily 5 years would be more then enough to build a bunch of new very high yield nukes for the job. A string of nuclear weapons aimed at the same point over time would at least have a chance of diverting a comet that huge.

5 years is also way more realistic for creating deep underground shelters that could protect both a secure gene pool worth of population, and enough machinery and key raw materials to ensure that when they come back out they can reestablish a basic industrial economy, instead of living medieval peasants. Doing that in 2 years as in the movie isn't totally out of reason (minus the secret part... yeah right) but it would be very hard.

[Sky Captain]

Another advantage of using nukes to deflect asteorid is that you don't need to match velocities with impactor which in case of a long period comet falling into sun's gravity well would be very fuel intensive/impossible with off the shelf rocket technology. A nuke just needs to pass at what standoff distance is most optimal and blow up at right point. The fact that nuke and comet may have 20+ km/s relative velocity shouldn't affect things too much.