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For those unfamiliar,

An impact winter is a hypothesized period of prolonged cold weather due to the impact of a large asteroid or comet on the Earth's surface. (Wikipedia)

In my world I want light and light sources to be a commodity. This means that the atmosphere must be sufficiently dense such that light and heat radiation from the sun does not reach the Earth's surface.

Do we know how large a meteor would have to be to kick up enough debris and dust to crowd the atmosphere to this effect?

For reference: the meteor that is thought to have extinguished the dinosaurs was about six miles wide. But I'm not sure just how dense this impact made the atmosphere.

socrates
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    Um, are you sure you wan to go that route? Everyone will die because the plants have no sun, and if here is no heat everything on earth will freeze. you're pretty much dead. – Xandar The Zenon May 09 '16 at 01:30
  • well, I need to find a balance between luminosity and survivalbility I guess. – socrates May 09 '16 at 01:45
  • Perhaps you could hold off on asking your two questions and make a single question. What is the minimum amount of heat and light for people to survive, then once you have that answer you should know how dark it is. – Xandar The Zenon May 09 '16 at 01:47
  • ok, should i close the other one and merge it into this one? I wasn't sure if it would be incorrect to ask two questions within the single post – socrates May 09 '16 at 01:50
  • @XandarTheZenon It wouldn't have to be cold--you would have a world wrapped in dark clouds--they would absorb the sunlight and heat the air. The no-plants problem would be catastrophic, though. – Loren Pechtel May 09 '16 at 02:36
  • @LorenPechtel would fungi be able to survive? – socrates May 09 '16 at 03:07
  • Fungi would live--but fungi digest dead things. When the supply of dead things to digest is gone the fungi will die also. – Loren Pechtel May 09 '16 at 04:41
  • Take a look at... Ice company, I think the title is. – MakorDal May 09 '16 at 06:29

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Surprisingly, this question has been studied somewhat extensively in academia.

Chapman and Morrison in 1994 did a comprehensive survey of the impact hazard for differently sized comets and meteors on Earth. They have this to say about an impact winter in discussing what constitutes a global catastrophic impact:

A global impact winter will commence when sufficient submicrometre dust is injected into the stratosphere to produce optical depth $> 2$ worldwide for a few months ... This optical depth corresponds to $10^{16}$ g of stratospheric aerosol. ...

Scaling from nuclear weapons tests and the K/T event, Toon and his colleagues find that an optical depth $\approx 2$ would result from a groundburst with a yield of $10^{5}$ - $10^{6}$ MT corresponding to a diameter of 1.2 km for a stony object striking at 20 km per second.

The K/T event here refers to the meteor that killed the dinosaurs.

It should be made clear that it appears the bare minimum size for an impact winter defined this way is the figure above - the authors do cite a range for globally catastrophic meteor sizes in the paper, but they are considering hazards arising from meteors other than just impact winter in providing that range.

1.2 km diameter (or 0.7 miles) would appear to be your best bet as a threshold for impact winter. It should be noted that the authors don't consider an impact winter for a time period longer than a few months, although they do state that:

... a $>5$ km object would create a global firestorm and so much darkness ... that vision would cease.

providing some measure of how bad things can get beyond that threshold.

Long-term impact winters don't seem to be something discussed in the literature despite my best efforts.

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Akshat Mahajan
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  • Great answer. Out of curiosity, do you know how to calculate optical depth, given the specifications of the impactor? It would just require calculating the number density of the particles ejected into the atmosphere by the collision, I think, but I don't know how you could calculate that. – HDE 226868 May 09 '16 at 21:58
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    I think this is usually something either eyeballed or derived experimentally. Calculating the number density from first principles is nuts, haha. – Akshat Mahajan May 09 '16 at 22:01