If a nuclear war occured on Earth, what evidence could a Martian astronaut see?

Question

An astronaut is alone on Mars when, unbeknownst to her, a total nuclear war breaks out on Earth. Other than Houston being unresponsive, is there anything she can do to establish what happened on Earth?

You can presume she's been paying some attention to the news, so the idea of war isn't totally off her mind, but it's not like she's expecting it to happen.

Technology level is modern-day.

What I've considered

• Naked eye - extremely unlikely. Mars is just too far away.
• Radio and TV. I found a Popular Mechanics article which covers interplanetary radio. It only talks briefly about Mars, but it says that you probably couldn't pick out channels from Mars, and that Mars' atmosphere might create issues. I wonder if we're blasting out less high-power UHF than in 1977. But I think this could be believable.
• Spectroscopy?

Answer 1

If we do a little research (hint, hint...) we learn that Fusion bombs:

• Require a fission detonation.
• Primarily emit neutrons.

Fission bombs primarily emit beta and gamma radiation.

• Beta radiation is detected using a Geiger-Müller counter.
• Gamma radiation requires specialized equipment that detects its interaction with other elements.
• Neutrons are detected indirectly via their interaction with charged particles.

So...

1. How easily the war is detected depends on the war, which is a story choice. Do you take the Hollywood route and say that it's a full ballistic exchange that lasts about 30 minutes? Or do you do something more tactical that releases smaller bombs over a period of time with the big bombs only used as a last resort? The longer you make the war, the longer your astronaut has to detect it. However, the longer the war, the smaller the nuclear exchange and the harder it is to detect. That old 1950s nuclear artillery shell was only about 15Kt. Big on the battlefield, small when the emissions get all the way to Mars.

2. We're assuming that (a) the Earth and Mars are on the same side of the sun and (b) that a substantial part of the war occurs on the side of Earth facing Mars and (c) your astronaut and all the equipment are on the side of Mars facing Earth.¹ Radiation travels in straight lines and things in the way (like the sun) will affect the ability to detect the war. It also means that however the equipment is set up on Mars, its location and orientation includes Earth.

3. Nuclear weapons do release an electromagnetic pulse (EMP). That would effect radio waves in that it would create nasty static as far away as Mars with the amount of static determined by the size of an explosion. However, this is something like watching Earth through a telescope, you'd either have to be listening at exactly the right moment or it would need to be recorded to be discovered or analyzed later. Note that if your astronaut is listening to a dedicated connection with, say, NASA, then the link won't be lost at the onset of the war unless NASA (specifically the location of the transmitter) is taken out first (or at least early). But even with a dead channel, that burst of static due to the EMP will occur.

Everything I've mentioned so far isn't worldbuilding, it's just research. Let's get to the worldbuilding

You can rationalize all of the equipment you need to detect the event of the war by making it a part of a scientific study of said radiation/particle types that's valuable due to Mars' thinner atmosphere. This is a perfectly reasonable rationalization as humanity's adventures in space have always included scientific research. That places all the detectors on Mars.

Quick tangent: why not in space? Because it's expensive to put repair people in space and your astronaut is a great repair person, and it allows for larger equipment plus the ability to readily modify or set-up the equipment. This isn't to say there wouldn't be satellite-based equipment — especially communications equipment — and that can be used to rationalize your astronaut almost anywhere on Mars. However, I'm favoring ground-based equipment as it'll give you more to work with when you write the story.

Given all of that, your astronaut sits down to review the daily data, as she usually does, and has a what happened then??!! moment. She then does all the analysis to realize that it's not an astronomical phenomena, but something that happened on Earth. The analysis would be a cool time for her to discover the static bursts on the radio (which was recorded for the purpose of redundancy, aka "story reasons"). While most (if not all) interplanetary communications would be digital, the actual radio transmission is still very much just radio. Modern digital equipment doesn't convey static to the speaker, it just goes dead. But that doesn't mean (and, indeed, wouldn't mean) that you're out of luck. We're on Mars... it's not unusual here on Earth to have signal strength monitoring and it's reasonable to rationalize that your Mars equipment would have that, too. So the analysis might not hear static (it could, with a redundant non-digital communication system), but will register a substantial increase in signal strength during the dead time. That would be another "what on Earth?" moment.

And when all is said and done, she's found staring at the Martian sky, vaguely in the direction of Earth, thinking, "they did it... they actually did it... idiots."

Summary

1. Equipment used to detect beta radiation, gamma radiation and neutrons are on Mars, rationalized as needed for ongoing scientific research.

2. Communication equipment has either an analog backup or signal strength monitoring for digital communications.

3. Everything is recorded, rationalized as a backup should diagnostics be needed.

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_{¹ If you just thought, "geez, it would actually be unlikely for anyone on Mars to know something happened on Earth!" you're right. You can moderate that by putting detection equipment in a polar orbit around Mars so the Earth can always be seen — if it's on the correct side of the sun — but the truth is your story will need a little bit of, "gee, that's convenient" to make this work.}

Answer 2

Personally I think that the first clue the astronauts would get would be "hey, no new email this morning", followed a bit later by "No one's replied to the messages I sent last night either". That'd probably turn into "Why's everyone quiet?" and "Is my equipment broken?" My (unresearched) impression of mission control protocols is that near-constant 2-way comms are standard. In a human-on-Mars scenario, that's going to be emails / direct messages with regular updates to mission plans, reminders to take vitamins & do exercises, feedback on previous day's experiments, health monitoring, etc. With one-way light travel time between 3 - 23 minutes one way, I'd expect the Martians to know something was wrong by the time they finished their morning cups of coffee. Also note that a need for redundancy in all systems means it's unlikely there's only one person on the Red Planet - so someone may well be awake and at least nominally connected to Earth for live updates when comms stop.

As a more-subtle variant, you might have the automated messages continue, but human-contact & discussion channels would cease. Astronauts like to hear from their family back home (citation needed), and will likely be sending in journals/diaries & reports on personal interactions (for psychological monitoring & maintenance). I'd still expect it to become clear that something is very wrong within a day or less of NASA going dark. (And if NASA doesn't go dark, they'll report the problem explicitly: "Moscow nuked Beijing. We're a bit busy down here. Good luck.")

At whatever point the communication issue is detected, typical engineering & scientific training necessary to maintain, diagnose & repair the comms equipment & computers gets top priority for the astronauts. It should be fairly easy to establish that everything on the Mars end is working as intended, especially if they're still getting feeds of ambient radiation, automated messages from Earth, and/or noise spikes when they aim their radio antenna at the Sun, and radio-silence when they aim away. Even without a direct analog comms channel, there are probably logs which will give very precise timing of changes to connection status & power level. (Kibana normally logs at the millisecond level.) With those in hand, the Martians will be able to quickly scan through whatever other instruments they've got on hand for other anomalies that happened about the same time.

For detectable scale, both the radio transmissions from Earth (probably on the order of 20kW or less and the EMP from the nuke(s) will be attenuated by the same factor. Based on a very loose reading of this article's breakdown of nuclear power distribution, I'd guess a 1 MT nuke puts around 50 kT into nuclear radiation from the initial burst and a substantial fraction of that (say around 5 kT) into an EMP - even without trying to make an EMP-focused bomb. That article uses "1 minute" as the duration of the "initial burst", so let's go with that and note that 1 kT = 4.18 * 10^12 J. Multiply by 5 kT and divide by 60 s/min and you've got a power output from your single nuke of around 3 * 10^11 Watts - which is seven orders of magnitude more than your DSN transmitter.

At this point, the Martians know "incoming RF power spike then silence". If they've got decent equipment for looking at the Sun to detect CMEs & flares - which would probably be important to manage their expected radiation exposure - they can probably rule out that RF power spike coming from everyone's favorite ongoing fusion reaction. So they know something went wrong back home and they're now on their own.

Will they know it was a nuke though? 50 kT of radiation = 2 * 10^14 J gets spread out to 2 * 10^-7 J/m^2, or 4*10^-9 W/m^2 by the time it gets as far as Mars. That's not a lot, but 1 GeV particles from cosmic rays show up at around 10^4 /m^2/s. I'd guess most of the direct radiation from a nuke was going to be in the 1-MeV class. Blindly extending that cosmic-ray chart down a few orders of magnitude, we've got a background flux of maybe 10^8 MeV particles per m^2/s, or around 1.6 * 10^-11 W/m^2. So the Martians will see a spike in MeV-class radiation from the direction of Earth, to something like 10 times background levels - assuming they've got basic radiation monitoring which can distinguish solar storms, cosmic rays, and local-to-Mars or Martian-nuclear-power-plant sources on the correct time scales. I don't know that that detail of monitoring is required for a mission to Mars, but I'd certainly call it plausible given how important it is to keep humans alive outside Earth's magnetosphere for extended periods of time in this scenario.

I've made a bunch of assumptions and skipped over some of the details of the math, so don't take my numbers as gospel, but... they should be enough to write some fiction.

Answer 3

Naked eye.

People from Mars with loved ones on Earth will likely tend to gaze at the Earth longingly when it is in the sky. As depicted in this video, firestorms almost immediately make clouds of black smoke. In the case of a major nuclear war affecting much of several continents, the color of the planet should be plausibly "wrong" within hours, and over a longer interval the whole Earth becomes much darker and harder to see.

Answer 4

Someone would tell them...

I know this sounds like a boring answer, but the fact of the matter is that even an all out total nuclear war will not instantly destroy a significant portion of the inhabited parts of the world. There are about 12,000 nukes in the whole world, but only about 4,100-4,500 are kept in a ready to launch state. Furthermore, most of them are so old now that an estimated 25-75% of them are expected to fail to launch/detonate in the case of an actual nuclear war. Major cities are also typically defended by Anti-ballistic missiles. So, it takes 6+ warheads to guarantee a probable hit against a major city meaning that instead of targeting a maximum number of cities, you will in most cases target the same city with multiple warheads hoping one gets through. This puts the probable number of successful strikes against unique targets somewhere in the 200-600 range.

There are 2 major reasons these means someone will be around to tell Mars.

1. Most countries just plain won't get involved. There is no reason to waste nukes on a non-nuclear country in such a conflict; so, even if countries like the USA, Russia, China, France, and North Korea all get bombed to hell, there are a lot of other countries with active space programs that will likely be untouched by such a war such as Brazil or Australia that would have the means to communicate with Mars and let them know what happened.

2. Space Centers are by necessity in mostly unpopulated areas of minimal strategic importance meaning that even the countries that do get wrecked will still have space centers that were untouched and able to relay the message to Mars. Kennedy Space Center is on an Island with a population of just 34,000. Stennis Space Center had a large exclusion zone around it with the closest population center being a little town with a population just over 10,000. etc. These places are simply not "important" enough to nuke; so, they will still be functional even after the initial exchange of nukes.

Now everything that happens AFTER the nuclear exchange is going to be really bad: large scale economic collapse, major environmental damage, famines, wars, and a bunch of 2nd rate countries jostling to fill in the power vacuum left behind after the big guys all alt-ctrl-del themselves, and this turmoil could end a lot of the space programs over time, slowly reducing Martian contact with Earth, but that would all happen after the Martians get the news that the war happened meaning that the communications blackout with Earth, if it ever comes, will be a slow fall off as opposed to a sudden one.

Answer 5

Just to note. Nuclear wars aren't instantaneous events i.e. from the moment one protagonist launches a full scale attack detonations will be disbursed over a time curve depending primarily on distance from the target and type of missiles used.

From memory ground based ICBMS take a maximum of about 45 minutes to reach targets on the opposite side of the globe, submarine launched ICBM's have (or used to have shorter) ranges but can be lunched from closer to the target if the submarine carrying it has remained undetected prior to launch and can get closer to an enemies coast line. Aircraft launched nukes have even longer 'time of flight' that could extend out hours but given the limited number of these that exist their impact would be marginal.

In any event with one side having launched a mass attack the opposing sides alert systems will detect the event almost immediately. Verification might take a couple of minutes but not long. So that leaves many more minutes for national self defense agencies and systems to be alerted and for public warnings to be issued. All in all? NASA or whoever was running the program in question would almost certainly have a enough time to send a last update to Mars before radio silence. That final message wouldn't take more than a few seconds to send.

Answer 6

I am sceptical about neutrons, beta and gamma radiation. The surface of a sphere with the radius between mars and earth is ~6 10^23 m^2. Even at a 100% detection rate and a crossection of 1sqm it would take multiple bombs to go off to make a unique dent in the background. It could be another story if that would be a neutino observatory, but i doublt it.

OTOH even amateur telescopes can get pretty decent pictures of Mars if Earth and Mars are close. So if you would look in the moment of the explosion, i would imagine that it would be visible at least for the bigger nukes.