In the year 2071 the Dysnomia incident happened. The Dysnomia (a mining ship) suffered a catastrophic failure and violently exploded sending its payload, an aged comet, hurtling towards earth. The comet proceeded to rip apart in the atmosphere distributing a massive amount of dust in the stratosphere leading too catastrophic global cooling. This global cooling forces a large portion of the human race to move into arcologies and urban areas, mostly abandoning the country side.

But is this series of events even plausible? That's what I'm stuck on is how to physically have these series of events unfold without:

  • a massive impact (smaller ones are acceptable)
  • blocking out the visibility of the sun (dimming is fine)
  • Not too severe cooling that GMOs couldn't be bred within a few years to adapt to the new climate (most agriculture would move to vertical greenhouses however)
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    Out of interest, what is an "aged" comet, as opposed to an "unaged" comet? Currently it's considered that all comets formed 4.something billion years ago, there wouldn't be a notion of aged vs unaged/younger, among them. – Stilez Nov 25 at 13:00
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    Some one explained it as having lost it's comet leaving behind a lot of dust inside the head of the comet. – Celestial Dragon Emperor Nov 25 at 13:03
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At the right angle of impact a comet could get reasonably deep into the atmosphere before skimming off back into space. Usually that wouldn't leave behind a lot of material but a comet that had lost most of it's surface volatiles may be coated in a thick, but loose, crust of materials with a higher boiling point. Some of that crust would slog off into the atmosphere due to mechanical friction and heating during the comet's dip into the air. This crust forms naturally on comets as they get exposed to repeated doses of solar radiation and has been observed on an number of objects by comet survey probes.

To cause long term (more than a year or two) cooling the compounds you want to dump into the atmosphere are Sulfur Aerosols. Comets usually do have some sulfur but it's not usually a major component of their chemistry so the ship would have gotten lucky, or unlucky, to find one with that much sulfur in it. The degree of cooling is directly proportional to the concentration of sulfur that eventuates from the atmospheric impact event.

If the angle and momentum of the object is just right, or wrong depending where you're standing, when it first skims the atmosphere it will enter into an unstable orbit with deeper and deeper brushes into the atmosphere until it burns up completely or what's left of it lands, but usually it will hit once and then carry on.

Been there, done that!

This apparently happened already 1,500 years ago. Scientists at Cardiff University, UK, believe they have discovered the cause of crop failures and summer frosts some 1,500 years ago – a comet colliding with Earth.

So the only part that is tricky, other than fine-tuning the size of the comet to come up with just the right amount of sun-dimming dust, is to figure out how a ship would capture a comet. Based on other things I've read, I think capturing an asteroid or two would be a lot easier than a comet. Comets have very large orbits, so they are moving pretty quickly when they get near Earth. Asteroids have more "typical" orbits. There have even been serious real-world proposals to capture & mine small asteroids using near-future technology, though to capture one that would be large enough to cause a nuclear winter would require some significant engineering advances.

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    Working out Dysnomia herself will be a later question, :P – Celestial Dragon Emperor Nov 25 at 0:58
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    Just think about the energy required to capture/change course of the comet. A quick search found 40 km/s for a comet when passing Earth's orbit and 25 km/s for a typical asteroid. In addition, because asteroids stay within the middle of the solar system, you can more easily have time to work on them without zipping out of reasonable communications distance. – manassehkatz Nov 25 at 1:06
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    I remember reading, recently, that there's more evidence to support a really big Icelandic volcanic eruption at that time. However, this doesn't dectract from the fact that a comet (Or asteroid) totally could cause such a series of events. – Andon Nov 25 at 1:06
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    the other problem is coming up with a plausible scenario for a combustion of sufficient magnitude to alter the path of the comet onto an earthbound arc. also, comets are not very dense - hard to blow one up with smashing it to bits. an asteroid works better, but the first problem still adheres. – theRiley Nov 25 at 1:18
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    Comet would be tough. Asteroid much easier. Perhaps some nuclear "engine" of some sort pushing the asteroid towards a near-Earth orbit for mining, but the engine is sabotaged/malfunctions/etc. with the result of the asteroid being blown to bits a little too close to Earth... – manassehkatz Nov 25 at 1:23

First, start with massenkatz' answer above. Then let us change 'comet" to "icy Kuiper Belt Object". Now if we accept what is valuable about this comet is the water, I think we have a possible scenario we can imagine that makes some economic sense.

We are no longer dealing with objects with highly eccentric initial orbits, but we do have the new problem of getting an Icy rock from Pluto's orbit back into the "industrialized space zone" closer to earths orbit. But the water resource is valuable and expected to remain so for generations as water itself is needed by the vast human population for life and the heavy hydrogen isotopes present in the ice are needed for energy production, so governments and companies can take a long view when designing their water supply chain. At any time there will be tens of thousands of icy bodies at various stages of de-orbiting towards earth in a process that takes generations to complete.

They arrive at this: manned or robotic survey ships travel to the belt. They locate good prospects by water/mass ratio, industrial value of non-water content, and viability of revisable orbital dynamics.

They have brought a host of small tugs. These are simple ion drive engines capable of delivering small but constant thrust continuously. They are attached to the target object along with a control module and this package begins the job of returning the object to orbit between mars and earth. Meanwhile, the survey ship moves to find its next target. It has supplies to recover 1000 such objects.

The return orbit of any body is very slow and complex. The ion thrust is very small, so even with an initial boost from another more energetic engine, changing the bodies orbit will take a long time. So, objects use gravity assist whenever possible. First with other Kuiper belt objects, then with planets and asteroids, the planned deorbit involves a series of encounters to first boost velocity and adjust trajectory, then degrade velocity and park the object in an orbit where it can effectively be harvested.

No two deorbits are the same. Companies deploy surveys to take advantage of possible gravity boosts from known high mass Kuiper Belt Objects and outer planets and their moons to return payloads as rapidly as possible using the fewest tugs but only the rarest and most favorable orbital dynamics result in return journeys of less than 250 years.

The incident that eventually sends one of these bodies to earth could happen anywhere along the supply chain, leaving earth with a lot, or almost no warning of the coming catastrophe.

It sounds good but the population would have to move to the countryside rather that urban areas to be closer to food and water and it would be pretty massive.

The dust would likely cause respiratory issues. It would be possible for the dust to be thin enough that the sun shines through but then the dust would settle quickly.

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    The thinner the dust, the longer it stays airborne. What would be massive? It is unclear from your answer. – L.Dutch Nov 25 at 4:05

If you want realism, I'd be a bit concerned about the comet's contents. We have visited a comet now, so we have good information to go on.

If your comet was worth bringing back to earth, then whatever it is made of, must be valuable enough to justify the effort, and also capable of creating a persistent aerosol of dust or other material on atmospheric breakup, that is capable of triggering a cooling episode. Combined with current knowledge of comet contents, that might be quite a limiting problem.

If a warming effect was needed, then methane might be a valid answer. But for cooling - that amount of dust suggests a solid rocky comet, not a frozen one, and I'm not sure how well that fits current scientific knowledge.

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