Okay while approaching Earth, aliens have bombarded three of our largest cities into smoking craters. Obviously they mean us harm. So when their large motherships start to enter our atmosphere our leaders make a desperate choice. They fire the majority of Earth's nuclear arsenal at those ships.

Now my question is, what would happen to the planet. High atmosphere detonations (500-600km) don't produce enough (dust) particles to create a nuclear winter. Fallout will be widely dispersed but much lower because it doesn't bind with said particles I think? Further more the ionized radiation will react with the magnetic field of the planet. Creating a radiation belt.

What I've found difficult is estimating the strength of the radiation belt, how long the radiation particles remain trapped in the magnetic field and the amount of fallout. Will the fallout be strong enough to kill most life on Earth?

So with the detonation of the majority of Earth's nuclear arsenal at high altitude, how would that effect the Earth and life on it?

Obviously pretty much the entire planet will be enveloped by EMPs. On that I can find data, it's the rest I struggle with.

Some additional data that might help. There 9 motherships landing. They're en route to: - Houston - Orlando - Beijing - Mumbai - Moscow - Johannesburg - São Paulo - Canberra - Frankfurt

  • $\begingroup$ What altitude are you thinking, and which estimate of nuclear arsenal are you using? $\endgroup$
    – apaul
    Mar 19, 2017 at 21:09
  • $\begingroup$ Mainly working out a scenario that won't kill the entire planet. As for the altitude, around 600-700 km. The arsenal would be the former US and Russia republics. It's little over a century in the future. The international landscape has changed a bit. But the big two kept their arsenal around, never truly disbanded. $\endgroup$
    – Mormacil
    Mar 19, 2017 at 21:12
  • $\begingroup$ Nuclear weapons rely on fission as a method of operation, either as pure a fission weapon or a fission bomb to light a fusion reaction. In either case, the weapon's effectiveness degrades over time and will require the creation of new ones. Meaning that a century from now the existing stockpile would see heavy refurbishment and thus much few numbers or terrifyingly, replaced with a new generation. So my question for you is what level of technological advancement have you allowed for the weapons? $\endgroup$ Mar 19, 2017 at 21:35
  • $\begingroup$ That's something I forgot to take in account. The last century has mostly been marked by civil war and insurrections. Plenty of smaller nations have been annexed. So I figure we went the worse way, improved more precise nuclear weapons. $\endgroup$
    – Mormacil
    Mar 19, 2017 at 21:40
  • $\begingroup$ A single large hydrogen bomb, detonated 600km above you, will illuminate you with 11 Kj per square meter. You plan to detonate some 10000+ of these, at the same time, split between 9 targets. So 1000 * 11Kj of ground heating. Hello global firestorm! $\endgroup$
    – user79911
    Nov 5, 2020 at 20:36

3 Answers 3


Effectively all out nuclear war in space a century from now, here is how it plays out.

The Weapons

Improving the weapons means we have upped the killitivity coefficient by switching to purely fusion driven weapons, maybe a miniaturized Z-machine to light fusion of hydrogen. How really doesn't matter, the fact is they are fusion weapons and will likely even have a dialable yield. So the world leaders will go for broke and turn that dial to 11 so the maximum yield may very well be multiples of the Tzar Bomb. Which actually will have little effect on the EMP production because you can only ionize the ionosphere so much, according to my EM professor 100kt or 1Mt makes little difference.

On Earth

As stated, massive global HEMP's could be disastrous in its own right for sure. But the world of 2100 is a little different than our own. Computers have gotten smaller and are thus more susceptible to electronic noise so they are also better shielded and have greater redundancy built in, as a result, may fair better in an EMP. Also, (hopefully) advancements in nanotech would allow for SOC devices to have their own repair mechanisms and/or naturally resilient circuits. For larger infrastructure we may have gotten around to actually protecting against Solar Storms which will serve to protect from the E3 Pulse. Additionally, seeing that it is somewhat of a troublesome time it's fair to assume that EMP resistant infrastructure may be mandated. So it may very well be reasonable to assume that the EMP's would not do much damage.

Purely fusion weapons would have little to no fallout, unless they were designed to. At the altitude stated little radiation from the blasts, if any at all, would make it to the ground because of the atmosphere. Without knowing the yields of the weapons other effects may include: blindness, extreme weather from localized heating (which at the altitude stated would take enormous weapons or high concentrations of detonations).

In Space

All satellites/space stations in LEO are probably toast as a direct result of the blast effects. Stations with polar orbits may fair better, but if the actual engagement is occurring in LEO that's a very bad place to be. As a result of the distance and improvements in EMP resistance things in higher orbits should actually be okay during the nuclear strikes.

After the action the Van Allen Belts (especially the inner) would see an enormous increse in radioactivity because the fusion bombs liberate plenty of particles. Things/people not extra rad hard would not do well against that. There is also what is left of the fleet itself; presumably many thousands, if not millions, of tons of vaporized ship that would be blasted into all kinds of orbits. Effectively sand blasting anything it encounters. It would also help deorbit things by increasing drag and probably interfere with radio signals between earth and space. For items not vaporized, you now have hyper velocity shrapnel to deal with and of course, the remaining wreckage of the ships.


Most people on Earth, probably okay. LEO will be hazardous for years and require extensive clean up before it is safe. Beautiful night skyies because of nuclear detonations, auroras, and falling spacecraft.

  • $\begingroup$ Interesting range in answers so far. So lowering the height of detonations would both decrease the effect of the EMPs and increase the fallout that would reach the ground correct? But given the amount of nuclear weapons going off it's unlikely to matter EMP wise? $\endgroup$
    – Mormacil
    Mar 19, 2017 at 23:48
  • $\begingroup$ With fusion weapons, it's not the actual fallout of radioactive particles, but it's the pulse of radiation from the reaction itself; in either case yes, lower to the ground more radioactivity less EMP. But then you start getting the actual blast effects and extreme weather from heating. $\endgroup$ Mar 19, 2017 at 23:55
  • $\begingroup$ Well your answer made it sound like Earth itself would be pretty well off afterwards/ Sure it will take a while to get back into space effectively but beyond that. See story-wise I aimed at not an extinction event but still a clear worldwide disaster. $\endgroup$
    – Mormacil
    Mar 19, 2017 at 23:58
  • $\begingroup$ I focused on the Nukes themselves. I should have gone into detail about the destroyed shops falling back to earth. How large is the fleet and how effective are the nukes so I may edit my answer? There is plenty of room for a global cataclysm. $\endgroup$ Mar 20, 2017 at 0:01
  • $\begingroup$ Right I should totally add the fleet number, it's 9 ships. The nukes are probably about the same as we got nowadays. The larger factions will have less but better ones and I figure some other emergent factions acquired some as well. $\endgroup$
    – Mormacil
    Mar 20, 2017 at 0:04

In short, most everybody dies.

The Ploughshare's Fund estimates the world's nuclear arsenal at 14,900 nuclear weapons: http://www.ploughshares.org/world-nuclear-stockpile-report

Let's be generous and say 25%, of these were simultaneously detonated in the atmosphere you're still talking about 3,725 nuclear blasts. Also keep in mind that some of these weapons will likely be quite large...

You've already mentioned the emp and radiation belt issues. Let's explore that. In early high altitude tests they noted that the resulting radiation belt knocked out 3 satellites and the emp affected street lights and communications some 900 miles away.

Now multiply those effects by 3,725...

You're likely looking at a global blackout. All communications, and satellites down. That alone will likely kill a ridiculous number of people. Cities need power to survive, without a steady stream of trucks and trains bringing in food and supplies they're dead zones.

Also when the power goes out you eventually loose modern sanitation, the water stops running. Pump stations stop pumping. Expect massive outbreaks of cholora and the like.

At best you're looking at an apocalyptic scenario. People starve, loot and panic till only a handful are left and the Earth is left defenseless when the aliens send their second wave...

  • 1
    $\begingroup$ Luckily for us this unprovoked attack draws the ire of the larger galactic community and given a few months foreign aid should arrive. I agree the devastation would be enormous, even with the warning we'll get as it's a strike we initiate ourselves. Also, those 3725 will only create all EMPs if they detonate at the exact same moment. As EMPs prevent the creation of more EMPs in their area for minutes to hours. $\endgroup$
    – Mormacil
    Mar 19, 2017 at 22:35

This was tried in the 1960s. The US detonated an atomic bomb in the Van Allen belt with the thought that this might be a sort of anti satellite shotgun. The experiment was too successful and the radiation was trapped in the belt for over a year.


It also had the effect of producing a tremendous light show. Which I had read about, but in researching this question got to see!


But as far as I can tell, the effects of this craziness were confined to near space. Your 600 km is 370 miles; the linked article says the VanAllen belt is 400 to 600 miles. So this would be freakily beautiful for those watching from the ground, possibly effective for the purpose against the aliens. Although I would think any craft capable of interstellar travel would be more than a little shielded against radiation.

  • $\begingroup$ Yeah, my loophole being that they're more like sloops while the large interstellar part isn't trying to land. That US experiment is interesting, all I'd found was that Soviet experiment. Over a year is also a great concrete number compared to what I had, thanks. $\endgroup$
    – Mormacil
    Mar 19, 2017 at 21:55

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