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There are plenty of threats out there that could terminate all life on our little planet in varying amounts of time (Solar flares, meteors and supernovas to name a few).

Are there any disasters that could kill all (intelligent) life on Earth but leave astronauts in low orbit alive for long enough to attempt to come back down to earth? (and restart society?)

Can you describe those scenarios into some detail and explain why exactly everyone on Earth dies, how the astronauts survive and what the state of the Earth is when they return?

For this question you should assume roughly current technology and Earth as the location. Judging from how often food is sent up to the ISS (every 2-4 months and their 45-day food reserve which they have almost needed to start digging into, I'd say 8 months is a reasonable, optimistic guess as to how long the astronauts can wait until they come back to Earth, I'd be happy to be corrected on that number though.

note: The amount of survivors doesn't necessarily need to be 3 of course. In fact, I would personally even prefer more than 3 members of the human species to survive.

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    $\begingroup$ I don't think a population of 3 is enough to restart humanity. $\endgroup$ – ratchet freak Oct 15 '14 at 12:33
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    $\begingroup$ @githubphagocyte judging form how often they send food up there (every 2-4 months and their 45-day food reserve which they have almost needed to start digging into, I'd say 8 months is a reasonable, optimistic guess. $\endgroup$ – overactor Oct 15 '14 at 12:40
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    $\begingroup$ If there were a number of different space stations with humans on board, all equipt with a means of falling out of orbit without burning up, and a means of surviving impact, then there could be a larger population that might be more realistic. $\endgroup$ – trichoplax Oct 15 '14 at 12:40
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    $\begingroup$ This is essentially the plot of the Bond film Moonraker: Drax builds a space station and plots to conquer earth from it with biological weapons. $\endgroup$ – pjc50 Oct 15 '14 at 16:15
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    $\begingroup$ @pjc50 Also the entire premise of the recent show 'The 100' - though in this case looking at the 'time to go back' instead of someone trying to make it happen in the first place $\endgroup$ – Baldrickk Oct 20 '14 at 9:45

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I would say none or close to none. The main problem is that being in space isn't very different from being in some confined places on Earth (with regard to isolation). For example

  • submarines,
  • some very remote areas,
  • shelters,
  • some deep caves, etc.
  • arctic stations.

So, for that (i.e. everyone but those in space die) to happen, it had to either

  • spread over the whole world faster than ~4 months (because of the food transfer), or
  • be selective (target people on Earth and not care about astronauts).

Moreover, any calamity makes people hide and take shelters, so it also has to be sneaky. For that reason atomic apocalypse, asteroid strike, etc. in my opinion won't work. If it was to be a disease, it would have to be dormant at first, so that people wouldn't notice and there would be a possibility to infest all the humans (but not the astronauts) and then transform and kill. Bio-engineering, nano-machines -- all such things seem like a big stretch (with very high probability something would go wrong and the thing would be discovered before the outbreak).

There's a short story Second Variety by Philip K. Dick which considers quite similar scenario, where the genocide is caused by self-modifying robots. However, note that such robots, which aren't able to reach space, aren't able to reach into shelters, submarines, etc. Most of ways dealing with that will also kill your astronauts.

Of course we could make the plague selective, i.e. explicitly target people on the Earth, e.g. an AI which has been programmed to kill people only on the Earth and nowhere else (e.g. a plane would be safe until it lands). Or, you could try to test some new antenna, which (for whatever reason), when turned on in space (on the Earth was fine) generated some yet unknown radiation that killed all the sentient life on the planet (the astronauts survived, because they were on the other side of the antenna).

However, if you don't mind altering your story:

  • the astronauts come from some long mission to find the Earth unresponsive (but for some automatic computer navigation data feeds).
  • I think that having some survivors on the Earth would make the story more interesting, as there would be more tensions and unexpected power-struggles; it close to impossible to destroy all the shelters, but it might be relatively easy to describe how most of them fell (consider reading the Silo novels by Hugh Howey).

I hope this helps ;-)

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    $\begingroup$ There's a book by Margaret Atwood, The Year Of The Flood, in which humanity is wiped out by a disease tailored and released by mad environmentalists. The disease wiped out just about everybody and then went away, so the people in your groups survived and started to rebuild. She forgot astronauts, though. $\endgroup$ – RedSonja Oct 16 '14 at 8:20
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    $\begingroup$ I think it would be a nice addition to the plot that a submarine crew also survives, or deep in a mine or something. The spacecrew certain that they are alone $\endgroup$ – Viktor Mellgren Oct 17 '14 at 12:47
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    $\begingroup$ @Vixen Especially if the astronauts are all males and the submarine crew are supermodels -- or at least that's how Hollywood will demand the storyline goes :-) $\endgroup$ – Carl Witthoft Oct 17 '14 at 12:56
  • $\begingroup$ On a related note, the novel Nightfall, by Isaac Asimov, is, in my opinion, one of the best descriptions of conflict following an apocalypse, though everybody has gone insane in that scenario. $\endgroup$ – Blapor Aug 31 '17 at 3:15
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Plenty.

  • Asteroid

    A sufficiently violent impact at a time where the resulting debris doesn't intercept the stations orbits would wipe out life but leave people in orbit alive.

  • Volcanic Winter

    Volcanic activity obviously would not affect astronauts at all.

  • Plague

    Confined to the biosphere/atmosphere. They should be careful no-one has brought it on board though.

  • Nuclear War

    Let's hope no-one lobs a nuke at the astronauts.

  • Toxic Bloom

    An algae bloom forms covering the oceans and vomiting out a nasty toxic chemical, rendering the air everywhere unbreathable until the bloom kills itself.

  • Rogue AI

    A rogue AI goes Skynet and kills everyone. People in space are not its concern.

  • Ragnarok

    Damnit, the vikings were right all along. The astronauts will be fine until Fenrir devours the sun.

Conclusion

They would all need to be pretty drastic to wipe out absolutely all life on earth but with the possible exception of Nuclear War and Asteroid Impact they would have zero chance to effect bodies in LEO at all.

The astronauts are unlikely to be completely alone though. For example submarine crews and some military bases might survive the toxic bloom. A meteorite killing absolutely everyone would leave the whole planet uninhabitable for quite a while, etc.

The Nuclear War, Volcanic Winter, Rogue AI and Ragnarok are all unlikely to be over in 8 months so we can rule them out.

The Asteroid, Plague and Toxic Bloom are all possible to have the level of impact needed and be over in that time, although 8 months really isn't a long time for the entire process to start and finish. You'd also have to worry about lingering effects such as Impact Winter from the Asteroid, possible infection from the Plague and resurgence of the Toxic Bloom.

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    $\begingroup$ That whole "sun eating" bit from Ragnarok... That might linger a bit past 8 months. $\endgroup$ – Danny Reagan Oct 15 '14 at 12:52
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    $\begingroup$ There are a lot of suggestions here, but I'm not sure if any of them seem viable. An asteroid strike, if violent enough to end humanity, would have effects lingering for (likely) centuries. And how could the planet recover from the nasty chemicals of the toxic bloom in so little time? Plague with 100% mortality, that is highly selective to humans and cannot survive outside of a living host, seems the only viable one of the group. I'm not sure how this could possibly evolve, though, unless it's some sort of superweapon gone rogue. $\endgroup$ – Nicholas Oct 15 '14 at 13:45
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    $\begingroup$ Fenrir eats the sun quite early in Ragnarok. OTOH SOl will have a daughter that continues in her mothers path before this happens. $\endgroup$ – Taemyr Oct 15 '14 at 14:30
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    $\begingroup$ I don't know what I'm more charmed by, the addition of Ragnarok among those options, or the fact that others have commented with critiques of it as a practical option. $\endgroup$ – Nerrolken Oct 15 '14 at 19:18
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    $\begingroup$ I will have to disagree on asteroids. A strike powerful enough to get a 100% kill will not leave a biosphere, period. Anything that leaves a functional biosphere will not take out humanity in the requisite timeframe. There will be survivalists holed up with supplies. $\endgroup$ – Loren Pechtel Oct 16 '14 at 5:55
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One possible scenario is the release of unrestricted self-replicating nanomachines. They're a popular doomsday scenario to illustrate the perceived dangers of scientific progress and are colloquially called "grey goo". You start with a single machine, invisible to the naked eye, that is capable of producing an exact copy of itself by breaking down available resources. Due to the way exponential growth works, the colony will rapidly overrun the earth.

Wikipedia quotes Eric Drexler:

Imagine such a replicator floating in a bottle of chemicals, making copies of itself…the first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined — if the bottle of chemicals hadn't run dry long before.

You can tweak the impact and reach of this goo by adjusting their reproduction process and what resources they consume: metals and biomatter, carbon, molecules or even atoms. So you could limit the goo to landmasses or let them overrun the oceans. Whether they reach the atmosphere likely depends on their granular size and how much of the earth they consume. For your scenario you'd have them ignore bare rock to there's an earth left to return to. You'd also have to tweak the robot's programming to leave plants or certain animals alone if your space explorers want to have any hope of survival after their return.

After the goo has finished converting all available matter, they will run out of energy and eventually deactivate. To quote Wikipedia again:

Gray goo nanobots need a source of energy to drive their replication. For efficiency reasons, the energy would likely come from oxidation and other chemical reactions on the organic matter itself—a process which in organic life is known as digestion—rather than from an external power source. In such a scenario, gray goo replication is self-limiting. The more organic material that the grey goo consumes, the less remains available for further consumption. After exhausting available organic material within a local area, grey goo would experience a population crash in that area, slowing or ending its outward spread.

Note that any robot with a sufficiently efficient mechanism of self-replication that is mobile enough or small enough to propagate throughout the entire planet in waves will work. Even if you scale them up to the size of the "Claws" from Philip K. Dick's Second Variety (as mentioned by dtldarek), the grey goobers would be distinct from those in that they're autonomously self-replicating, instead of produced in factories. They also wouldn't have an instinct for self-preservation beyond an automatic consumption of matter. They'd consume all available life then expire themselves.

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  • $\begingroup$ Wasn't this roughly the concept of Andromeda Strain with the little crystals that replicate in blood? $\endgroup$ – Josiah Oct 29 '14 at 16:48
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The only scenario I see is plague. You need a disease that spreads extremely well but doesn't appear worse than the common cold. It will have to happen during the northern hemisphere winter so there's personnel exchange with the antarctic bases.

It's second--lethal--stage will have to occur after a long enough period that groups like the boomer crews have been exchanged. While it would have to be an airborne vector it would need to be something that can't live very long in the environment so resupply rockets wouldn't bring it to space. Remember, sick crews don't fly--if you've got everyone down with a cold no manned rockets are going up.

All the other nasties that have been mentioned upthread would still have catastrophic effects long past the maximum endurance of the crew.

I can't imagine this as anything but a bioweapon. Perhaps some group thought they had a vaccine... Or maybe they did have a vaccine but the world's intelligence services figured out where they were holed up and someone pushed the launch button.

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  • $\begingroup$ of course anything with that long an incubation time would be carried to the ISS by its replacement crews as well. What you need is something that spreads extremely rapidly, infects the entire planet within a month or so (or you run the risk of a supply rocket being sent up carrying contaminated things), then kill everyone even more rapidly. $\endgroup$ – jwenting Oct 17 '14 at 11:39
  • $\begingroup$ @jwenting I'm thinking of something with two stages, the first is mild, the second is lethal. It won't go to the ISS because astronauts don't fly even with minor illnesses. The world will know about it but they won't realize it's catastrophic until too late. $\endgroup$ – Loren Pechtel Oct 17 '14 at 23:40
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Going a little more fantastical, to add to the excellent answers already present.

I don't know if these are feasible:

  1. Breakdown of Earths protective magnetic Field + Deadly comsic radiation. Stations in Orbit are usually heavily shielded against comsic radiation and Earths magnetic field and the atmosphere usually do the same for us. If this natural shield would somehow turn off for a Month, some kind of cosmic radiation could kill off everyone, who doesn't spend the whole week in some kind of shielded environment like a space station
  2. Some Kind of Electrical Impulse stopping the heart/killing all brain activity If there was some kind of deadly electrical wave-pattern, which could be emitted by lets say Cell-Phone towers, it would kill almost everyone if the cellphone-network-coverage on the planet would be high enough
  3. Devastating solar storm which will roast the whole planet and burn everything to a crisp, if the Spacestation can maneuver somehow and stay on the shadow side of earth long enough, it could maybe hide in earths shadow, while earth is burned - in 24 hours the whole planet would be dead...
  4. Pre-programmed mass-suicide There could be something which is pre-programmed in all humans and activates at a certain time, letting all people burst out into a violent rage for 24 hours killing everyone and themselves in the rage. But some kind of powerfailure/sleeping gas/plot device knocks the people on the space station out for 24 hours, so they cannot commit to it.

I know some of these are pretty far reaching, but maybe they can be refined and used as a plot seed. Otherwise I recommend exitmundi (warning, page is highly addictive)

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  • $\begingroup$ Your ideas #1 and #3 won't work, because you are vastly overestimating the amount of radiation shielding in spacecraft. The ISS is close enough (Low Earth Orbit) to still be partially protected by Earth's magnetic field, but it does not have the equivalent of a couple of kilometers of atmosphere to sufficiently weaken cosmic rays. Naively adding shielding can actually increase the effective radiation dose due to resonance effects. See <en.wikipedia.org/wiki/International_Space_Station#Radiation> for notes on radiation effects on ISS crews. $\endgroup$ – amon Oct 16 '14 at 17:13
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    $\begingroup$ Your solution in #3 has further issues: To stay in the Earth's shadow, we need 1 orbit per day, which requires a very high orbit (the ISS has 15.51 oribits/day, which allows it to fly fairly low). Building manned space stations that high is far too expensive, and spacecraft don't have enough spare delta-v for such a large unplanned orbit change. And then there's the problem of having enough energy for life support: in Earth's shadow, your solar arrays are useless, and batteries are quickly depleted. $\endgroup$ – amon Oct 16 '14 at 17:14
  • $\begingroup$ For the Solar Problem: Maybe they are usually staying in an oribtal position where they still get sunlight and Change the position into the earths shadow when they predict the catastrophe. Then they would not need to change 'altitude', only their 'horizontal' position. Sorry for the layman terms. $\endgroup$ – Falco Oct 17 '14 at 7:53
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A bioweapon spill or war

One of the things that would affect only the planet below and not humans in LEO would be an airbourne virus or plague.

I'm going to take a few liberties with 'Current' technology here, but what I suggest is feasible and essentially boils down to 'incredibly virulent bio weapon'.

As the virus is a weapon, then either it gets out because of an accident or it is used in anger and targeted (in large amounts, all over the globe).

It could be presumed to be natural although for me, this is more of a stretch (ever seen that film with Mark Whalberg where the trees make everyone immediately suicidal? I spent most of it unable to suspend my disbelief).

astronaut safety

Obviously it would not be able to leave the atmosphere unless ferried up to the astronauts with a supply shipment somehow, or the group who unleashed it targeted them. So they could be considered safe (or they dodge the missile somehow).

survivors

Of course hermetically sealed shelters, submarines, etc would not be affected by this, and there are problems with the timeline - unless you take some creative steps with the nature of the plague.

Drawing some inspiration from the idea of a virus bomb similar to the ones depicted in the warhammer 40k books, it could be engineered to destroy all life and also circumvent normal air filters (by being too small or by being able to dissolve rubber on contact, etc..). Being engineered, it burns itself out quickly and becomes harmless, so the astronauts won't get infected when they land.

If it was virulent enough it could explain why normal gas masks and weakly sealed shelters would fall - but it still leaves stronger stuff like millitary bunkers, submarines etc.

This is where the engineered part comes in - This allows you some scope to explain how hardened installations were overcome, they could have been hit directly with warheads containing the virus.

still survivors?

If the virus were to essentially speed up decomposition, and it was a single event in time rather than spread out, you could also play with the idea of setting the atmosphere on fire due to the huge amounts of released gas - plausibly this might take less than 8 months to burn out but as nobody has ever done it before (thankfully!) it may take more or less time.

Also, if it was transferrable through water aswell as air, it could kill all ocean life aswell as take care of those pesky submariners by infecting the water around the sub, which is then taken in and distilled into fresh water (most subs apparently do this, and also can use it for CO2 scrubbing.) That's your vector to kill off the submarine crews.

What would it be like?

If you burn the atmosphere:

The oceans may have boiled? This would take care of the submarines. Otherwise, you essentially have a blackened and charred wasteland with no surviving organic life and no oxygen left in the air. Pretty hellish.

If you don't burn the atmosphere:

Everything organic is dead and decomposed (at an optionally accelerated rate). The oceans are filled with decomposed fish guts and other matter, and the atmosphere may have alot of methane in it.

As no world-ending firestorm happened, the infrastructure, technology and buildings are still intact. Also, canned food might be, depending on how virulent the virus is.

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None. As Martin-Mueller speculated, there are, in fact, a ton of jobs that have to be done planet-side in order for any of our currently existing manned spacecraft to be able to return safely. With everyone on the planet wiped out, the astronauts would be left stranded with no way to make a successful reentry. This could be eliminated in worldbuilding by speculating a near-future spacecraft able to make the return on its own (the modifications needed from current spacecraft really wouldn't be all that extreme), but any such technological changes would change the other assumptions the question relies on as well.

However, if you're willing to allow for just a few planetside survivors, that changes the picture quite a bit. The astronauts could even still be quite isolated, since those who help them down would, mostly, not need to be anywhere near the landing site, and the few who did have to be at the landing site might be contrived to succumb to the residual effects of the disaster shortly thereafter. Even communications with the helpers, which were achievable from orbit overhead, might be cut off once line of sight is lost when they land, depending on what method they're using to communicate (the space shuttle, for example, has a wide variety of communications gear available to it, plenty of which would have this problem if they were what the helpers had available and undestroyed).

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    $\begingroup$ with a functioning Soyuz they can in theory return at any time they want. The journey down might be more bumpy without ground guidance, and they'd have to calculate it to not end up in a sea or large lake, but they could do it. $\endgroup$ – jwenting Oct 17 '14 at 11:40
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    $\begingroup$ @jwenting I was unable to confirm or deny that, do you have a reference i could check? $\endgroup$ – Matthew Najmon Oct 17 '14 at 21:32
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I'm a little worried about what you are planning...

I see a few additional problems other than what has already been mentioned. Even if we could get the spacecraft to return to Earth safely without ground support, after spending 8 months in zero gravity, their muscles would be very weak. It would take some time before they could move around on their own, during which time they would be pretty helpless.

And then if they did survive, rebuilding society would be difficult for such a small group. Especially when it comes to reproduction. If the population is too small, inbreeding would wipe them out in a few generations. And according to http://www.howmanypeopleareinspacerightnow.com/ (which may be my favorite domain name), that population would be a grand total of 6 right now. In fact, that seems to be the total capacity of the ISS. But even that isn't the real number to be concerned with, as the crew does not exactly have an even male/female ratio. Most missions appear to be entirely men, so there goes any chance at future generations. Currently one of them is a woman, but even there, the second generation would all be at least half brothers and sisters.

All in all, I would recommend seeing if you can get a refund on your trip and whatever doomsday devices you have already purchased.

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This scenario is discussed quite a lot: we are running out of fossil fuel, and the governments expect those to end in about 30 - 40 years. People get concerned,
and we will start the Fight for our resources.
Of course, those could lead to some of the causes that Tim-B mentioned.
I think your guess is pretty precise...
By the way, there are constantly six astronauts on-board the ISS.
~Itay

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This exact scenario is the framing situation in the classic Alfred Bester short story "Adam and No Eve". The astronauts new space-drive uses a substance that catalyzes nuclear reaction, and some gets out the exhaust and sears the surface entirely. The astronaut is the only survivor and that's it...or is it?

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Sterilization by boiling the Earth then cooling it down.

The galaxy headquarters have detected some biological activity (us) in one of their 100 billion planets, and a bored operator applies the standard procedure (namely UHT sterilization):

  1. Point a high-energy laser at the planet
  2. Power the laser until the planet boils
  3. Let it boil for 2 seconds
  4. Switch the laser to reverse mode, and cool the planet down to its original temperatures (6000°C core and 10°C crust in this case). This is done to avoid changing the planet's astronomical properties, as that could result in course modification and eventually collisions.

Fortunately for the astronauts, the operator was tired and only let the Earth boil for 1 second. As a consequence low orbit is not affected.

The Earth has changed a lot, in particular the compositions of the crust and atmosphere have changed significantly, but with some luck it could still be livable (it all depends on the specifics of the boiling method, which is up to you). The soon-to-starve astronauts now have 7 months to kickstart biological activity on this planet, hoping it will produce something edible and put some oxygen in the atmosphere. Good thing they have so many plants with them intended for experiments.

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I don't see this answer anywhere, so I'm gonna float it by you.

It happens to the ISS

I don't know what 'it' is, maybe just a boring old wormhole, but basically the ISS gets instantly pushed somewhere or somewhen where the planet below them doesn't have people on it.

I don't want to use details here, the possibilities are endless. The past, the future, alt-Earths, other nearby planets; It fixes the fixed-food problem the astronauts onboard face while still achieving the objective.

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