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I am considering writing an alternate history story in which 1950s nuclear testing attracts the attention of benevolent extraterrestrials who do not want to see us destroy ourselves. However a misunderstanding ensues between the ambassadorial parties sent by the superpowers to meet the aliens; the whole affair goes south in a big way.

I looked up the various tests done by both superpowers in this era and it seems that the Castle Bravo test represented a major upward spike in blast force and radioactive dispersion. How would an alien probe stationed somewhere in our Solar System (you can decide where) be able to detect this event? Would smaller blasts also be detectable? Let's assume the aliens have a means of FTL which permits their probe to communicate to their homeworld "Hey guys. The monkeys from Sol III are testing megaton-level nukes now. Maybe we should have The Talk?"

Just to clarify, I am not asking about a global nuclear holocaust scenario. I am specifically referring to the "controlled" atmospheric tests conducted in the early years of the Cold War.

Edit a few hours after post: Thank you for the answers, everyone. Keep them coming, I'm all ears!

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    $\begingroup$ There was an early Arthur C. Clarke story about that back in the collection "Expedition to Earth": loophole. (first published in 1946 in the magazine Astounding Science Fiction.) After getting rattled by some nuking on Earth, Martians impose a blocade and Martian Himmler gears up for a bit of xenocide ... but there is a loophole! $\endgroup$
    – David Tonhofer
    Commented Mar 12, 2020 at 11:28
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    $\begingroup$ If they have the technology to get to our Solar system then they easily have the technology to detect nuclear explosions from anywhere in the Solar system. Even if for some reason they missed it, they'd hear about it on the radio and TV. $\endgroup$
    – N. Virgo
    Commented Mar 12, 2020 at 15:26
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    $\begingroup$ Can "somewhere in the solar system" be in Earth orbit? Can the probe be comprised of multiple separate components that can be in different orbits? $\endgroup$
    – asgallant
    Commented Mar 12, 2020 at 15:29
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    $\begingroup$ We saw it from right here on Earth. We walk among you. $\endgroup$
    – Frank
    Commented Mar 12, 2020 at 21:11
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    $\begingroup$ FWIW, in terms of "upward spikes", Ivy Mike in 1952 was the first two-stage thermonuclear explosion, with a 10.4 MT yield. The largest explosion before then was Greenhouse George at 225 kt. Then the Castle tests involved five MT-range explosions, three of them larger than Ivy Mike, in about three months in early 1954. $\endgroup$
    – Salda007
    Commented Mar 13, 2020 at 5:59

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They don't check for the event, but for the expected consequences.

Having a single satellite check for events that are only directly visible for a few seconds brings with it one huge problem. If the probe is on the other side of the planet, you will miss the flash.

But if you monitor the atmosphere (which seems reasonable if you're interested in a habitated planet), you'll notice a huge increase in several radioactive isotopes (C14, for example) [1]. Since we can analyze isotopic ratio in Jupiter's atmosphere even now [2], your aliens would surely be able to build a probe that can do exactly that. Since it takes time for the particles to disperse, it would take a couple of weeks or months for them to notice, but they would notice.

The atmospheric analysis also gives your aliens a reason to have another Talk a couple of decades later, when a rise in greenhouse gasses and subsequent temperature increase are detected.

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    $\begingroup$ The greenhouse gasses talk would hypothetically be on the table several decades earlier (not later), though... unless their systems have a high greenhouse gas threshold to allow for the possibility of a century of drastically increasing volcanic activity. $\endgroup$
    – Sam
    Commented Mar 12, 2020 at 16:48
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    $\begingroup$ Given how many nuclear tests were done in the 50s, would it really matter if it misses the flash for any specific one? I'd imagine it'll eventually see one either way. $\endgroup$
    – CPomerantz
    Commented Mar 12, 2020 at 20:09
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    $\begingroup$ besides non-CO2 toxic greenhouse gases, there needs to be a talk about toxic chemicals, plastic and trash dumped into rivers and oceans $\endgroup$
    – user54989
    Commented Mar 13, 2020 at 12:18
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How would an alien probe stationed somewhere in our Solar System (you can decide where) be able to detect this event?

We are able to detect the signal emitted by the Pioneer 11 and Voyager 1 and 2, which are way past Pluto. Those devices emit way less power than a normal bomb, not even a nuclear bomb.

Therefore by having a suitable antenna looking in the right direction it would be surely possible to detect the signal of an atomic explosion.

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    $\begingroup$ The antennas looking for these spacecraft know exactly which frequencies and exactly where to look for them. So a probe that's watching earth, and looking for specific signatures, should be able to detect it. Something that's looking for something else, or not observing Earth, probably won't. $\endgroup$
    – Andon
    Commented Mar 11, 2020 at 22:07
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    $\begingroup$ If the detonation happened on the other side of the planet to the probe, could it stil detect it? Or would the aliens need a network of probes to keep Earth covered from all angles? $\endgroup$
    – Kyyshak
    Commented Mar 12, 2020 at 7:35
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    $\begingroup$ @Kyyshak the explosion itself would likely be undetected: even as close as from our Moon, we can't observe what is on the other side of Earth. $\endgroup$
    – Cœur
    Commented Mar 12, 2020 at 17:34
  • $\begingroup$ LIGO detects events billions of light-years from earth, and that's lowly human-tech. Could an alien probe in the solar system detect a bomb on earth? Probably. $\endgroup$
    – Doug Morgan
    Commented Mar 17, 2020 at 1:42
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Just have a Manhole cover wipe out an alien satellite after it is almost accidentally sent to space (it's not believed it made it in our world, but instead melted but maybe a alternative history can hand-wave that away )

The Aliens turn up thinking you have monkeys with ground to orbital weapons and arrive ready for the talk about not blowing each-other up from space.

Starts you off with a big misunderstanding on the technological weapons levels of everyone involved which you can add to the confusion of the ambassadors.

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    $\begingroup$ This has the feeling of the sort of astronomical odds from Hitchhiker's Guide to the Galaxy, it could be quite fun $\endgroup$
    – bendl
    Commented Mar 13, 2020 at 12:52
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You know that nuclear detonation detection system? Tell ye a sicrit: 't's theirs.

Point

"Hey guys. The monkeys from Sol III are testing megaton-level nukes now.

That now implies they got to know about the other smaller nuke blasts in the past too - which is more than plausible, it's probable from their mission of observers (it's not very hard to learn about them anyway if they listened radio/TV transmissions anyway).

If nuke explosions were a sign warranting The Talk in some future, is only natural to keep an eye on it. I mean, you don't wait for a policeperson to drop by and inform you that your kid has been busted for indecent behaviour or sexual assault in order to decide that telling them about flowers, bees and storks is necessary.

So, what better way to keep an eye on it than to drop some baby monitor like devices on Earth? Devices which could even use FTL comms for a 'some slow signals coming your way, you may want to have a look' heads-up.

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  • $\begingroup$ About how far out into space did Earth's radio transmissions reach in 1954, the time of the Castle Bravo test? $\endgroup$
    – Jem
    Commented Mar 12, 2020 at 0:16
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    $\begingroup$ @jem How is this relevant in the context of How would an alien probe **stationed somewhere in our Solar System** specified in the question? $\endgroup$
    – Adrian Colomitchi
    Commented Mar 12, 2020 at 0:20
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    $\begingroup$ @Jem to answer your question on how far, a radio wave sent out now would reach pluto in 5 hours, a radio wave sent out in 1950 could of reached alpha centauri by 1954. and the 1900 transmissions (if you ignore how weak the signal would be) could of reached the nearest 100 or so stars by 1954 easily. So pretty far. $\endgroup$
    – J.Doe
    Commented Mar 12, 2020 at 16:44
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It's unlikely. For one thing, if they're not looking in the right direction at the right time, they'll miss it. The "right time" is dependent on their distance from the Earth while looking, and if it's in the 1950s, they can't be very far away.

They only have a strong signal for a few seconds at most. A long blink and it's all over. With FTL, they can zoom out further and wait for the signal to pass them again, but that presupposes that they know there's something to watch for (making this a useless point).

Furthermore, they have to be in the right spot. If they're on the other side of the planet, or if another large body occludes the Earth at that point (the moon is the most likely candidate, though there's only a small chance of it occluding, too lazy to math it), then they can't see it either.

Other than by utter chance, the only way they'd notice is if they were already watching and even then they wouldn't likely catch every nuclear test.

They're much more likely to notice our persistent signals then. Some of the old radio broadcasts had insane wattage, and went on 12 or even 24 hours a day. Even with those, consider how directional they'd be in space.

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    $\begingroup$ When standing at infinity, the moon covers ~7.4% of earth's facing surface (it's actually very slightly larger because earth is a sphere not a disc, sorry flat earthers) $\endgroup$
    – bendl
    Commented Mar 13, 2020 at 12:53
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    $\begingroup$ @bendl Yeh, but we could also use its orbital period to give some percentage of the time it would actually occlude from any particular position, and I'm guessing that's way less than 1% of the time. Only pointed it out to be thorough. $\endgroup$
    – John O
    Commented Mar 13, 2020 at 15:50
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GPS Satellites have a nuclear detection system that not only detects those explosions, but gives the operator exactly where it was.

Besides this, nukes cause a specific "double flash" in the visible light spectrum, the Vela Incident was detected by that. If a random probe wasn't specificly designed to detect nukes going off, but to look around in the light produced by the local star, it would detect this.

If I was asked to choose a probe's location in a solar system with 1 star, 8 planets, some interesting moons and dwarf planets, and only 1 of them had life on it, I'd put that probe close to the one with life on. Because that's the most interesting part of that system. So my probe would detect that blast.

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    $\begingroup$ Excuse me, but how would GPS satellites detect nuclear explosions? I am pretty sure you have misunderstood something. (I have been working with radiation detection technology most of my life) $\endgroup$
    – MortenSickel
    Commented Mar 13, 2020 at 5:43
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    $\begingroup$ I think when only one nuke goes off in a short period of time, the difference of time it takes neutrons/gamma rays to travel to well known positions (GPS satellites) gives out the source. But I didn't do any research. Some source in case you want to read more: osti.gov/biblio/10185731 $\endgroup$
    – Nyos
    Commented Mar 13, 2020 at 5:55
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    $\begingroup$ @MortenSickel - GPS satellites carry nuclear detonation detectors as part of their equipment. en.wikipedia.org/wiki/Global_Positioning_System#Military $\endgroup$
    – Salda007
    Commented Mar 13, 2020 at 6:10
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    $\begingroup$ Cool, thanks both of you! Learned something new today! $\endgroup$
    – MortenSickel
    Commented Mar 13, 2020 at 6:55
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If the aliens aren't concerned that the primitive hairless apes on the little blue marble colloquially known as Earth would detect their probe, a set of Earth observation satellites in polar orbits could monitor 100% of Earth's surface continuously. They would be capable of monitoring a great deal more than just nuclear tests: land use, deforestation, ocean acidification, pollution, atmospheric composition, surface temperatures, population levels, conflicts, radio/TV broadcasts, etc. That telemetry would provide any number of "these idiots are about to destroy themselves" alerts.

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There were plans to detonate a nuclear weapon on the moon as a demonstration of both our weapons technology and our rocket technology. The reason to do it on the moon was for the visibility and too prevent more damage to the earth.

https://en.wikipedia.org/wiki/Project_A119

A alien traveling through the solar system would easily see it. An Alien in another solar system would have a more difficult time.

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Good points above about looking at the right spot at the right time, but remember during that time, there were quite a few nuclear tests going on, so somebody monitoring us with a sufficient good optical resolution for some time would probably pick up one or more suspicious flashes - especially as, as far as I remember, quite a few of the tests were done just before dawn. And then, as said above, that could give them an idea of what was going on and they could do various other types of measurements to prove what it really was.

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Neutrino detectors in outer space.

These are marginally feasible at short range with current Earth technology. It stands to reason that a more developed technology and a longer baseline would allow to reliably detect not only nuclear explosion events, but also the increase in steady fission flux.

To a neutrino telescope, the Earth would appear as a diffuse spherical luminosity growing brighter towards the center (not all the way), due to the presence of radioactive uranium and thorium isotopes in the core.

Then, on the periphery, bright spots develop. Some comparatively very large and really short lived - neutrino flashes, so to speak - others dimmer but steady, and visibly fixed on a rotating surface, which positions them on the crust.

And one day, they receive the signature of a nuclear fission plus hydrogen fusion event, and Naron of the long-lived Rigellian race, Keeper of the Galactic Records, gets informed that Earth people have gone thermonuclear.

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