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If a 50 megaton nuke was dropped in Lower Manhattan, New York, how long would it take for people to be able to live there again?

The bomb detonated on the ground. My definition of live is to breathe comfortably, drink the nearby water with modern technology, and properly grow crops that are safe to eat.

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    $\begingroup$ 1,000 as "one thousand megaton"? Depends on a type of nuke (and 1000 is beyond anything that we have today). $\endgroup$ – Alexander Sep 20 '18 at 16:23
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    $\begingroup$ Considering the fact that the largest bomb ever tested was 50 megatons, a better question would be how long until a floating town could be built in the middle of the Gulf of Manhattan... $\endgroup$ – Giter Sep 20 '18 at 16:30
  • $\begingroup$ alright, i'll set it to about 50 instead. i didn't know whether or not it was possible to estimate one that large $\endgroup$ – AzaleaGarden Sep 20 '18 at 16:45
  • $\begingroup$ Even 50 megatons might not leave much land if it's detonated on the ground, though I'm not sure how to calculate crater sizes. As a reference, the bombs dropped on Hiroshima and Nagasaki were 0.015 and 0.021 megatons. Here's a neat tool to estimate the rough size of a blast. $\endgroup$ – Giter Sep 20 '18 at 17:02
  • $\begingroup$ i am currently using nuclearsecrecy for finding dimensions and such $\endgroup$ – AzaleaGarden Sep 20 '18 at 17:05
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Edit to note, the original question asked for the detonation of a 1000MT weapon, but did not specify the burst height.

The correct answer is "never".

Consider the effects of the Tsar Bomba, had the Soviets ever added the Uranium tamper and airburst it over NYC: a 100 Megaton detonation

enter image description here

100 MT airburst over NYC from NUKEMAP

The circles in the diagram are (in order) Fireball radius, Air blast radius (5 psi), Thermal radiation radius (third degree burns), Air blast radius (1 psi).

The airburst needs to be at least 5km above the surface to prevent the fireball from touching the ground, and the ring of collapsed buildings will reach almost to Philadelphia. The firestorm ignited by the thermal pulse will engulf the entire city and a great deal of New Jersey and Long Island.

This is calculated from the largest bomb ever built (the real Tsar Bomba used a lead tamper to moderate the yield to 50 MT). What you are asking for is a Gigaton level blast, which is now equal to the impact of a small asteroid. The fireball alone will likely touch the ground no matter how high in the atmosphere it is detonated, melting the ground and creating a massive radioactive fallout cloud. The areas of thermal and airblast damage are also correspondingly larger (Nukemap can only calculate up to 100MT).

Trying out an asteroid impact calculator (Earth Impact Effects Program), with a 1.3 km rock asteroid impacting at 17km/s, we get an impact energy of 5.46 x 1020 Joules = 1.30 x 105 MegaTons TNT, a crater diameter of 18km and a depth of 707m. This will create New York Lake, a popular tourist and diving site about 1000 years from now when the ambient temperature of the rock has dropped sufficiently. If a nuclear weapon dug out that crater, it will likely be radioactive and hostile to life for many thousands of years.

So, using Gigaton level weaponry, you effectively erase a site from the Earth for all time.

As an aside, the largest semi plausible weapon ever described is a 5 gigaton device designed as a thought experiment to take out the Western Siberian Missile fields in a matter of minutes from launch from the United States in order to prevent a Soviet launch. The weapon was so huge it was to be carried on an ORION nuclear pulse rocket. This implausible device was apparently a project of Anthony Zuppero in his early career.

enter image description here

The Doomsday Orion, by William Black: https://www.deviantart.com/william-black/journal/Hard-SF-Feature-04-Scott-Lowther-504258455

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  • $\begingroup$ "1.30 x 105 MegaTons TNT" - that is 130,000 Mt. Much bigger than the original question's 1,000 Mt and enormous compared to the revised 50 Mt. $\endgroup$ – Alexander Sep 20 '18 at 21:04
  • $\begingroup$ I was in a bit of a hurry so didn't experiment with all the possible combinations in the program. Imagine if I had plugged in a comet moving at 70km/sec! $\endgroup$ – Thucydides Sep 21 '18 at 3:49
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    $\begingroup$ I am hereby renaming that weapon at the end of the post the condom bomb. $\endgroup$ – Renan Sep 21 '18 at 12:39
  • $\begingroup$ @Renan can i just say i love you for that $\endgroup$ – AzaleaGarden Sep 22 '18 at 0:13
  • $\begingroup$ You're way overestimating how hot the crater will be, both in a physical and nuclear sense. $\endgroup$ – Loren Pechtel Sep 22 '18 at 2:47
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About 50 years for modern safety standards, 5-10 years if high mortality is acceptable.

Bikini Atoll can serve as a model for this estimate. Between 1946 and 1958 it was a test site for thermonuclear weapons with combined yield of 42.2 Mt (nearly all of it were surface detonations). By June 1968, radiation levels have substantially receded, however, a more detailed study revealed unsafe levels of strontium-90 and caesium-137. By that time, wildlife on the island had fully bounced back and there were no immediately visible effects.

A 2012 assessment had found that levels of caesium-137 had dropped considerably and the island could be resettled.

50 megaton detonation will create a crater, which will change topology of Manhattan, but other boroughs would not likely be affected. For comparison, Meteor Crater in Arizona is 1.186 kilometers wide and 170 meters deep. The estimate for its impact energy is 20 megatons. So, for 50 megatons we should get some 2-km wide lagoon in Lower Manhattan.

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    $\begingroup$ Manhattan is standing on solid granite, the hole probably wouldn't be quite as big. The surroundings of Meteor crater have a sediment geology. $\endgroup$ – Karl Sep 20 '18 at 22:15
  • $\begingroup$ Tsar Bomba in 50mt mode was a very clean detonation. Thus it would be safe even sooner than extrapolating from Bikini Atoll would suggest. (Now, if it was in 100mt mode that would be another matter, but it was so hot that an awful lot of the crud would be deposited much farther away.) $\endgroup$ – Loren Pechtel Sep 22 '18 at 2:50

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