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So specifically I'm looking at three asteroid impacts at once.

Little backstory: It takes place over 100 years in the future. Population steady grew. For unimportant reasons aliens show up. Three separate ships each fire a mass driver on a large urban center. They're fire 'small' asteroids mined beforehand.

They impact Delphi, Shanghai and New York. They need to be large enough the utterly destroy those cities. If the New York one can also destroy DC that would be great. If that would be to big an impact that's fine too.

So I'm looking at the impact on Earth in three time periods. The next few hours/days to deal with the immediate aftermath. Followed by the following weeks/few months. And lastly the story takes place over a century so some long term effects would be nice.

I hope this is concrete enough. So impact on three specific areas, roughly at the same time. Looking at a way that would not utterly annihilate our species.

edit: As the above is to vague I guess we go with each asteroid being about 2km in diameter. They travel about 20km/s. They hit within 30 minutes of each other. They're fired about 150.000km from Earth.

I'm looking at the effects of the impact on the climate of Earth and the how long it should take of the effects to settle. If effects is to vague, severity of the impact winter, temperature drop, earthquakes away from the impact areas.

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closed as too broad by L.Dutch, Mołot, Hohmannfan, kingledion, Josh King Mar 9 '17 at 15:04

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ There's an amusing video here that demonstrates the effects of various-sized asteroids striking New York City. And there is, in fact, an asteroid like that that will have a near miss in 2028. (Also, welcome to the site.) $\endgroup$ – Frostfyre Mar 9 '17 at 13:40
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    $\begingroup$ Internet has a lot of asteroid impact calculators, like the one here impact.ese.ic.ac.uk/ImpactEffects What exactly you are not able to find there and want us to help you with? $\endgroup$ – L.Dutch Mar 9 '17 at 13:43
  • $\begingroup$ The effect of multiple impacts at once, kind of earthquakes that would create. The long(er) term effects on climate because of dust buildup in the atmosphere. At least I assume there will be a dust build up. $\endgroup$ – Mormacil Mar 9 '17 at 13:45
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    $\begingroup$ I am afraid that the 'what are the effects' question is just too broad. Perhaps you should retool the question as: 'Asteroids strike NYC, Delhi and Shanghai simultaneously, what is the largest size they can be to not wipe out humanity?' Then based on the answers to that question, you could ask more follow on question. As it is, I am voting to close this question as too broad. If you edit the question narrower, I will nominate to re-open it. $\endgroup$ – kingledion Mar 9 '17 at 14:11
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    $\begingroup$ @Mormacil It's always a walk on a tightrope. Normally you should try to post a specific question and then use the input you get over the course of a few days to make another question. We have series of questions quite often. I think your question is better now. $\endgroup$ – Secespitus Mar 9 '17 at 15:08