Death Roids: What's the minimum time to do anything?

[Sea Skimmer]

If you set off a nuke above the atmosphere the fallout will be very highly dispersed. Stratospheric winds will blow it across a large portion of the planets surface. It’d be nowhere near as bad as what the US was subject too from open air tests in Nevada or the USSR and China from similar open air tests. Detectable fallout would exist, but it’d be pretty irrelevant. Certainly less dangerous then living next to a coal power plant or an oil refinery or anything like that.

[Sky Captain]

Anyway we have fair reason to believe that we could score a direct hit on a rock that was at least 200-300 meters across (free falling ICBM warheads should actually be more accurate bursting at very high altitude because the low level wind does cause inaccuracy), and its doubtful we'd decide to nuke something much smaller then that.

IMHO rock that big would be detected well in advance allowing for dedicated solutions to be planed. Also collisions with objects that large are relatively rare. More likely scenario would be: panicked astronomers calling the military - OMG an asteorid about 40 - 70 m in size (roughly the same size that exploded over Tunguska) is going to strike densely populated area in few days. In that case attacking it with stock ICBM's and ABM systems would be only option. (or you could evacuate the affected area and accept hudreds of billions worth of property damages)

[Sea Skimmer]

IMHO rock that big would be detected well in advance allowing for dedicated solutions to be planed. Also collisions with objects that large are relatively rare. More likely scenario would be: panicked astronomers calling the military - OMG an asteorid about 40 - 70 m in size (roughly the same size that exploded over Tunguska) is going to strike densely populated area in few days. In that case attacking it with stock ICBM's and ABM systems would be only option. (or you could evacuate the affected area and accept hudreds of billions worth of property damages)

I suspect anything over 100 meters or so would indeed be noticed, but with budgets for that kind of search being cut, and detection methods past about 30,000 miles being limited to optical telescopes and just a few remaining interplanetary radar stations we've got no 'gods eye' kind of defensive search and no reason to be confident. Ideally we'd have telescopes outside the asteroid belt to help out but rational spending like that is no match for farm subsidies.

[Skgoa]

Adding a fourth stage is not going to get much more deltaV, even if we ignore the added weight it would most likely result in. Rockets are designed for very precise weight/speed capabilities and most importantly the weight ratios are tuned for maximum effectiveness.

I think he was talking about dumping some of the multiple warheads an ICBM is supposed to carry and place a fourth stage (or maybe a fuel tank for the aiming thingy to make it become the fourth stage with little effort)

I am gonna shoot from the hip and say that this new stage will not be powerfull enough to achieve any significant gains in velocity. (It might not generate enough thrust, it might not have enough time before the highest point is reached...) At the first glance, it would add a bit to the maximum height, IF you just use the weight you save by launching only one nuke.
The question that comes to mind, though, is wether it wouldn't be even more effective to just keep that weight saved.

[Chaotic Neutral]

Ideally we'd have telescopes outside the asteroid belt to help out but rational spending like that is no match for farm subsidies.

Not to derail, but asteroid collisions are on the rare side, by the time of the next major collision it could easily be 4000 A.D. Not exactly the most urgent priority.

[Skgoa]

Yeah, apart from that huge piece of stone and iron that might hit us in 2029, we might be ok.

[someone_else]

The question that comes to mind, though, is wether it wouldn't be even more effective to just keep that weight saved.

Uhm. A good question.
I can say this will need less modifications to the other stages, because they will do what they were supposed to do, and then the 4th stage will ignite and do its businness after they were discarded.
But I don't really know.

Yeah, apart from that huge piece of stone and iron that might hit us in 2029, we might be ok.

You mean the rock this guy is talking about?

[Sea Skimmer]

Not to derail, but asteroid collisions are on the rare side, by the time of the next major collision it could easily be 4000 A.D. Not exactly the most urgent priority.

Surely it’s still more urgent then spending billions on ethanol farm subsidies though? Subsidies which havehad the main effect of raising world food prices and encouraging over cultivation and ever more rapid depletion of US groundwater supplies. The other farm subsidies are even worse when you look at how much money goes to massive farms. But anyway, the point I made earlier was we just do not have the warning system to KNOW for certain if we will be hit or not. We can’t even reliably track all the debris we ourselves have shot into space some of which are rather large like Saturn V upper stages. Some fairly large rocks have air burst in recent years, IIRC one over the Mediterranean in the early 2000s was rated in the tens of megatons and no one saw it coming at all. So however slim the threat is real and of course highly persistent.

A hit on somewhere likethe US or Europe would be bad, but assuming a moderate size rock we’d recover. The effects are more or less known. A much more dangerous situation is what happens if say an asteroid fell on Pakistan right now. The whole country could easily disintegrate or a nuclear war could break out (or both) in the chaotic aftermath. Other nations fall somewhere in-between the US-Euro and Pakistan cases. But the reality is only the US-Russia-China-EU axis has nations with both the required space launch and nuclear weapons to make a defense a reality at a sane cost. Even if Pakistan could afford a defense.... I think we'd rather they didn't build it. So someone standing on the top of the world damn well should act before we regret it.

The insurance policy argument used in favor of ABM works for this too. However unlikely, the consequences of an asteroid strike do have calculable costs. So does a defensive system. You divide the cost of the defensive system over the life of the defensive system and that’s your insurance premium. Then compare the premium to the value of what it will defend (this varies of course depending on if the defense has global coverage or regional coverage, and how big a rock it can kill). Somehow I suspect that any way you work the numbers (within reason) this would work out to anti asteroid defense being a very very cheap kind of insurance, and one with technological and scientific benefits.