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In an Earth-like world I have long pondered building up, the pre-industrial (roughly colonial) society primarily exists on a single continent, roughly the size of Africa. Another continent has been discovered, and an early colony has been settled there.

This world has multiple moons, which in a freak event are aligned just right as a comet (or a meteor) passes between them, that the gravitational forces break the object apart. The resulting fragments rain down on the primary continent, causing damage, and setting civilization back a ways, but not completely obliterating it. (Leaving the new-continent colony temporarily more technologically advanced, and opening a plot for them to go back and see what happened)

What size comet or meteor could fit this type of criteria? I lean towards comet more because of their porosity, and a bit of 'wow' factor if it happened to be seen as it approached, but a meteor is acceptable if a comet is not plausible. It needs to be large enough to cause a major blow to a continent, but not so large as to completely wipe out all life on it - perhaps (most) cities would shatter and crumble, but some pockets of life will survive.

(I am aware of This Question but it seems to focus on city-sized effect, rather than continental, and I can't figure out how to work the simulator linked therein to meet my request)

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    $\begingroup$ This question in Space Exploration mentions a handy calculator from Purdue that might tell you what you need to know. There are apparently a lot of factors that contribute to impact damage. $\endgroup$
    – papidave
    Commented Jul 29, 2016 at 3:17
  • $\begingroup$ That calculator does look a bit better, but it still is difficult to think of what it's trying to tell me, I think. I suppose the energy and possibly the intensity of the airblast give me a little to go by, but I have a hard time relating those to anything I'm familiar with. I can, however, imagine that I need to approach each individual impact site as a separate meteor, and try to combine the individual masses to get the mass of the original object (plus a bit, for ablation, etc) ? $\endgroup$
    – Rayanth
    Commented Jul 29, 2016 at 4:13

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Running some scenarios through the ICL impact effects calculator, I'd say a comet two to three kilometers in diameter should do the job. A close pass by a moon or three will break it up nicely; since a comet is less homogeneous than an asteroid, you can expect a wider range of piece sizes.

1000 meters: 18 gigatons TNT equivalent; will leave a crater 10 km in diameter and will wreck buildings and knock down trees up to 150 kilometers away.

500 meters: 225 megatons TNT equivalent; will leave a crater 5 km in diameter and knock things down up to 75 km away.

100 meters: 1.8 megatons TNT; won't leave a crater, but the air burst will still have a damage radius of 20 km or so. This size is particularly good for a "what happened to the city?" mystery, since it doesn't cause any changes to the ground.

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  • $\begingroup$ Are the sizes the size of the comet that reaches the planet, or the sizes of the individual pieces after it breaks up ? if I have several 500 meter fragments, It might be enough to cause sufficient damage to a continent, but that means a several-km comet... (just how big ARE comets anyway... to the Google!) $\endgroup$
    – Rayanth
    Commented Jul 30, 2016 at 2:01
  • $\begingroup$ The sizes I give effects for are the sizes of pieces at atmospheric entry. You could reasonably expect a 3-km comet to produce a few dozen 500-meter fragments. $\endgroup$
    – Mark
    Commented Jul 30, 2016 at 2:10
  • $\begingroup$ (Google says: a comet's nucleus is in the sub-10km range, so this works out) Accepting this answer, because it gives a visual breakdown of what each size might accomplish. Calamities of this scale happen rarely enough (aside from perhaps Tunguska) that it's hard to envision properly! $\endgroup$
    – Rayanth
    Commented Jul 30, 2016 at 2:20
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At planetary and interplanetary velocities, there is no effective difference between an asteroid and a comet. You could strike the planet with an equivalent mass of wadded up kleenex at that speed and get the same kinetic energy and thus damage from impact.

More realistically, the comet is more likely to deliver more energy to the target simply because comets, coming in from deep space, will be moving far faster as they get closer to the Sun, while asteroids will have much lower velocities. In our own Solar System, the fastest an unpowered object can move and still remain bound by the Sun's gravity is 72 kilometres per second

While the obligatory XKCD comic is concerned with relativistic impact, the initial panels give you a good idea of what the range of impact energies are at interplanetary speeds, which generally are similar to the impact of nuclear weapons. The Atomic Rockets "boom table" should help you calculate just how much damage you can deliver for your story purposes.

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  • $\begingroup$ +1 for the links, this helps visualize things quite a bit better. As I noted in another answer, while I understand the energy transferred doesn't differentiate between asteroid or comet, I think the more porous comet would be more likely to break up, thus spreading out smaller pieces over more landmass (spreading the damage but lessening the individual damages), and the composition of the comet can/will lead to other plot elements. $\endgroup$
    – Rayanth
    Commented Jul 30, 2016 at 2:03
  • $\begingroup$ On entry into the atmosphere at a speed of kilometres per second, the material of the comet or asteroid won't have too much time to break up or disperse. Even an airburst like the 20m Chelyabinsk meteor released something like 500KT or energy, more than a most modern strategic warheads. $\endgroup$
    – Thucydides
    Commented Jul 31, 2016 at 3:31
  • $\begingroup$ a valid point, but I meant more that it would have a better chance to break up as it passed by the moons (as in the original scenario). $\endgroup$
    – Rayanth
    Commented Jul 31, 2016 at 6:33
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A comet and an asteroid are more or less the same thing except a comet has a tail as a result of material ablating in the solar wind, as opposed to the tail a meteor might have as it ablates while entering a planet's atmosphere.

If you're writing a story you can get away with being nonspecific, say an asteroid 1km across has a glancing impact on one of the moons and after that various sized fragments will be coming in at different speeds and on different trajectories. This gives you lots of wiggle room with regard to how much material enters the atmosphere and how quickly its moving. Furthermore it's a better explanation for the fragmentation than gravitational tidal forces tearing the asteroid apart because an asteroid that large probably isn't going to be leaving survivors.

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    $\begingroup$ I figured a comet was more ice-based material, versus an asteroid being more rock-based. Density,a s well as porosity. The ice could come into play in other ways in the story, but I felt it not as important for this particular question. That said, I do like the idea of a glancing blow off a moon. Ejecta from the moon itself could then become part of the incoming strikes. $\endgroup$
    – Rayanth
    Commented Jul 29, 2016 at 3:53
  • $\begingroup$ Since your world has substantial ocean surface, consider that Tsunami factor for any ocean impact... eventually, it would hit both continents, and populations...perhaps refine to smaller fragments, and one land based impact. $\endgroup$
    – Joe
    Commented Jul 29, 2016 at 21:01

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