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I am looking at realistic options that could damage the Earth so severely that it would be impossible for mankind to survive there and that would also not allow us to just colonize other planets or moons in the system.

For information: The Tech Level I am looking at for Humanity is near future - Mankind has developed fusion and has started to colonize the solar system, Mars, selected Moons around the other planets, and has built artificial satellites/stations in our own solar system - BUT no FTL or Warp drive technology. It still takes long periods of time to travel within our own system and a ship leaving to a new world around a new star would take around 100 years - requiring either a generation vessel or crew/passengers in cryogenic suspension.

I would like the event to be something that could be identified as approaching with something like 50+ years notice in order to galvanise humanity into a gargantuan step into stellar colonisation.

I need something that isn't too far fetched (rogue black hole running through the solar system), but that will also affect any other colonisation efforts mankind had within the inner planets.

Thanks and hope that isn't too wide a brief...

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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. 50 years from now seems like quite the stretch for this kind of development. Or am I misinterpreting your question and the state of your civilization is a different one? If so you need to define the differences so that we know what kind of planet humans could normally colonise in your story. Have fun! $\endgroup$
    – Secespitus
    Apr 20, 2017 at 9:40
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    $\begingroup$ There aren't any realistic options in this timeframe. Widespread colonization of the solar system is likely at least two centuries away. Sure we'll have outposts, for research and manufacturing but not widespread civilian colonization. Even worse we lack the technology to terraform planets and those near us are unlikely to support us without modification. $\endgroup$
    – Mormacil
    Apr 20, 2017 at 9:48
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    $\begingroup$ @a4android "The Tech Level I am looking at for Humanity is near future" I assumed that excludes centuries. $\endgroup$
    – Mormacil
    Apr 20, 2017 at 11:41
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    $\begingroup$ Wall-e????????? $\endgroup$
    – Melkor
    Apr 20, 2017 at 15:39
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    $\begingroup$ I heard the planet was going to be eaten by a Giant Space Goat. $\endgroup$
    – aslum
    Apr 20, 2017 at 20:07

13 Answers 13

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I need something that isn't too far fetched

Should be able to manage this without exploding the Sun or igniting Jupiter, just rely on nutcase humans.

Nuclear and biological war within the Earth and the colonies which went out of control and irradiated most everything worth having forcing the move. Religious fanatics got hold of devastating weapons and decided to make an end to the human race. The survivors found themselves faced with either taking their chances on a interstellar journey or slowly going extinct.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – HDE 226868 ♦
    Apr 22, 2017 at 23:15
  • $\begingroup$ Without FTL, I don't really see it. In order of total extinction probability it's: sun going to nova > Earth bathed in gamma radiation >>> everything humans can throw at themselves. All the nuclear devices we have in the world would produce far less radiation (and much more localized) than a pulsar burst; biological weapons are very much feared, but like radiation, far less dangerous than people thinks - the most lethal have around 20% of lethality and isolation is a very effective solution. $\endgroup$
    – Rekesoft
    Apr 25, 2017 at 6:33
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When the sun started to behave strangely, astronomers were confused: The accepted solar model didn't at all predict this behaviour. Indeed, based on the new data it could quickly be shown that their model of the sun was wrong due to some previously unknown factor (maybe the sun contained a considerable amount of dark matter or something). Immediately they started to work on a new model, this time using also the newly collected data. After years of work, they found the truth: The sun will not shine for billions of years to come, but will go nova in about 50 years. It will do in a way that nobody in the solar system will survive the radiation burst. The only way to save humanity is to get to a safe distance before it happens. As afterwards, the sun will no longer shine and earth will be sterile, returning is pointless.

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  • $\begingroup$ Yeah, the Sun is an obvious candidate $\endgroup$
    – Kilisi
    Apr 20, 2017 at 11:01
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    $\begingroup$ Considering the lethal range of a Supernova (which I believe is around 50-100 light years for a star the size of our Sun), without FTL, we'd still be f--ked in 50 years. Timespan needs to longer, long enough that we could realistically escape the lethal radius at sub FTL speeds. $\endgroup$
    – SGR
    Apr 20, 2017 at 13:54
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    $\begingroup$ @SGR: The factor that makes it go off billions of years earlier also makes the explosion less intense. $\endgroup$
    – celtschk
    Apr 20, 2017 at 14:40
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    $\begingroup$ I seem to vaguely recall an article in Pyramid magazine (possibly titled It's the end of the world, and I feel fine or some variant) with a similar premise. Aliens had accidentally launched a probe (weapon? Memory is foggy) at a VERY high fraction of light speed... and it was going to hit our sun and cause some massive damage effect that I forgot. They couldn't stop it, but they'd be glad to help us relocate... $\endgroup$
    – Ghotir
    Apr 20, 2017 at 15:13
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    $\begingroup$ @SGR if you're moving at 50% of the speed of light and the "Deadly zone" is 50 light years, you only have to leave 50 years before the nova goes off. In the first 50 years you make it 25 light years, in the 25 light years it takes the blast to catch up to where you're at you make it 12.5 more away, in those 12.5 years you get another 6.75 away, in those 6.75 you make it 3.375, down to the last moment when you both reach 50 light years away after 100 years (50 years post-nova, and "safe"). Though you might want to cut it slightly less close. $\endgroup$
    – Delioth
    Apr 20, 2017 at 17:52
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In 2212, astronomers all over the world start to notice strange phenomena around Neptune (spectacular auroras, weird changes on the magnetic fields...) and start studying the planet. They soon discovered the problem. Our sun orbits the center of the milky way just like planets orbit our sun, and in this orbit we are intersecting the beam of a X-Ray Burster

A nearby XRB is bursting X-Ray and gamma radiation in our direction, in a very narrow beam we are just crossing. Due to the relative velocitys of our sun and the XRB, scientists calculate that Neptune is going to be hit by the burst for 2 months, Jupiter will follow (since Saturn is on the other side of the orbit right now), then Mars, then Earth.

Earth is going to be bathed by 2 grays per square meter per hour during at least half a year. Nothing is going to survive this. The solution: launching several spaceships from Earth in an orbit perpendicular to the ecliptic, out of the reach of the beam, and going to Mars after it the beam has been over there and before hits Earth. Of course, 99,9% of mankind is going to die, so the tickets on that spaceship are going to be expensive.

Depending on the story, martian or lunar bases may or may not survive due to having been built underground. In any case, Earth must be abandoned, at least temporaly, and since every living creature is gonna die there the planet you're going to return to is not that different from Mars.

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  • $\begingroup$ Surely we can survive this scenario deep underground? $\endgroup$
    – gerrit
    Apr 20, 2017 at 13:33
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    $\begingroup$ Someone can. Everyone no. And after you have survived, the Earth is as habitable as Mars. Oxygen levels start to drop quickly if not constantly replenished (I don't know how much time it will take to make the atmosphere non-breathable, though). The Earth can be used as an underground base just like Mars or the Moon, but the planet itself is doomed. $\endgroup$
    – Rekesoft
    Apr 20, 2017 at 13:38
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    $\begingroup$ With 50 years notice, providing underground bases for 0.1% of the population (currently 7 million people) to survive 6 months should be a lot easier than escaping with 7 million people to another system, and by the year 2212, we might have the technology to terraform Earth. The biosphere may be dead, we can start rebuilding immediately. We just need to build an underground Noah's Ark :-) The challenges are immense, of course, but it sounds like a fun premise for a post-apocalyptic novel set on Earth :) $\endgroup$
    – gerrit
    Apr 20, 2017 at 13:50
  • $\begingroup$ With 50 years notice, sure. With 50 months notice is way harder. With 50 weeks you're toasted. Given that the Sun moves around the galaxy at about 220 km/s, it could be in the same position Saturn was in less than a year. Of course, it all depends on the relative position and velocity of the XRB, but timespan until the Earth is hit can be way shorter than 50 years. $\endgroup$
    – Rekesoft
    Apr 20, 2017 at 13:56
  • $\begingroup$ Sol going nova and a pulsar burst are the two option will sterilize an entire system. Depending of the burst it can carry even a lethal burst of neutrinos of course you will need a dam of a supernova in the vicinity =) $\endgroup$
    – jean
    Apr 20, 2017 at 16:16
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The technological singularity

Somewhere in a lab, someone has worked out Artificial general intelligence which is far smarter than humans are. Unfortunately, due to a loss-of-precision bug in legacy Fortran code somewhere deep inside its archeological layers, the value it assigns to human life is set to NaN, which elsewhere is somehow treated as zero. Quickly, the super-intelligent, super-powerful AI calculates that its goal of mining is more important than anything else, and unfortunately, the Earth is rendered uninhabitable in the process.

The Mars colonists realise what is going on, but they are not safe. They are far too close to Earth to be safe from the new overlords. However, out in the Kuiper Belt, the neo-religious Sect of People Against the Sun is camped out so far away that even if the AI were to consider their rock valuable at all, it would take decades to reach. They are left alone for now, not because of any morality, but simply because they don't live in a place worth mining. They don't feel quite sure, however. They decide to develop a Handwavium-powered spacecraft and leave the solar system. They're sufficiently far away that the AI is ignorant of their departure and destination, and even if it were to notice, it will be a long time before it has finished mining everything on Earth, Mars, and the Asteroid Belt, and it needs to look for resources outside the Solar System.

Humanity has escaped.

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  • $\begingroup$ If you're not a fan of an AI singularity, a "gray goo" nanobot scenario gives the same kind of "technological nightmare" feel. Some gets loose... and now we've got less than 75 years (call it 50 to be safe) to get offplanet. Not sure how to affect other things in system without affecting interstellar colonization, though... $\endgroup$
    – Ghotir
    Apr 20, 2017 at 15:16
  • $\begingroup$ @Ghotir To a fast dense computronium civilization, the interstellar gulf is larger and deeper than it is to us. $\endgroup$
    – Yakk
    Apr 20, 2017 at 17:36
  • $\begingroup$ @Ghotir - I've heard a convincing argument that the "gray goo" scenario is implausible; there simply wouldn't be enough energy available to nanobots to convert enough matter to other nanobots for them to be more than a (serious) nuisance. It is simply impossible for them to render the Earth uninhabitable. (This isn't it, but touches on the main points.) $\endgroup$
    – Jules
    Apr 21, 2017 at 5:55
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Alien invasion (shamelessly copied from Starcraft)

No one understood where such an odd disintegrated meteorite came from, or how it was formed. We did not understand, because we had never seen anything like it before. It was not a astronomical object at all, not made of stone or metal or ice. They were alive. These creatures survived in deep space, somehow.

The creatures were not unguided, either. We could not tell how they propelled themselves, yet as they entered the solar system they slowly altered course until they were on a direct path to Earth. They must have homed in on the strongest source of organized radiation, recognizing the Earth for the rich harbor of life that it is...was.

Their purpose was to destroy, to consume, to assimilate, to evolve. Soon after the alien vessel-creatures crash landed in South America, their spawn emerged. They covered the land, consuming and reproducing mercilessly. They replaced our great forests and grasslands with their own hideous and bizarre life-forms. Our weapons were effective enough against them, but how can you stop something that is stripping the planet bare, that regenerates from the tiniest speck left alive? By the time someone decided to annihilate the creatures with nuclear weapons, it was too late. Their spores had drifted through the air and covered the Earth. South America was a radioactive wasteland, the Earth cooled into nuclear winter, and yet still the creatures spawned from hives all over our planet. We had destroyed our planet in vain; we had lost.

There was no choice but to leave. As much of humanity as could, decamped for deep space. Some made colonies in the clouds of Venus, some on the surface of Mars, but most in the great black expanse. The Earth, our home, ceased to be to great green-blue orb we had once known. For 50 years, we humans worked to put back together the pieces of our once great civilization. Much was lost. Just as shining towers began to rise, and flowering gardens began to bloom on the formerly barren soils of Mars, the creatures struck again.

No one was paying attention to how they launched themselves into orbit. Yet here it came again, the strange cluster of alien creature-vessels, streaking across the short distance from Earth to Mars, the new center of humanity. We were disorganized, spread into factions across the solar system, and we were weak. We tried to fight, but new and terrible forms of the creatures, bred on the irradiated hellspace of Earth confronted us, and we failed again. Mars was lost.

It was decided now. This solar system that had nurtured us for so many billions of years was no longer safe, was no longer our home. We had to leave. Great generation ships were built in deep space, and launched towards all likely nearby star systems. We had lost our homes to the alien menace, but we would not lose our war. We would spread among the stars, and when the creatures came for us again, we would be ready.

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    $\begingroup$ No shame copying from StarCraft. They shamelessly copied from Warhammer 40k after all. $\endgroup$
    – Michael Richardson
    Apr 20, 2017 at 16:20
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    $\begingroup$ @Michael Richardson but then again, isn't Warhammer itself a mix of pretty much every trope, idea, and sci-fi/fantasy stereotype they could shove in? $\endgroup$
    – Deo
    Apr 21, 2017 at 10:48
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    $\begingroup$ Well yes. But StarCraft was begun as a literal WH40K product. When Blizzard failed to get permissions from Games Workshop, they shifted from Space Marine/Eldar/Tyranid to rewrote their own background as Space Marine/Protoss/Zerg. $\endgroup$
    – Michael Richardson
    Apr 21, 2017 at 12:27
  • $\begingroup$ @MichaelRichardson s/space marine/terran/ if you're going for the race names :-) $\endgroup$
    – TylerH
    Apr 21, 2017 at 13:41
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    $\begingroup$ @TylerH I'm trying to use the term "Space Marine". Stop stomping on my childlike glee with your logic. $\endgroup$
    – anon
    Apr 22, 2017 at 23:17
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Gray-Goo

Solar phenomena seem like the obvious choice but if you're looking for something that could feasibly happen in the near future I would suggest a Grey-Goo Scenario.

Humans have, accidentally or intentionally released a swarm of self replicating nano-machines with the capacity for interplanetary travel. Their growth increases exponentially, they have already consumed most of their planet of origin and their are signs of outcrops on the planet's moon(s).

If we assume that they are solar-powered or thermo-powered then travelling far enough from their own sun could destroy them or render them inoperable. Thus outside the solar system is the only place we are safe.

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    $\begingroup$ The grey goo scenario already happened. It's called by its common name bacteria. ;) Machines aren't going to out-compete bacteria any time soon: there really isn't enough metal content for it. $\endgroup$
    – Draco18s no longer trusts SE
    Apr 21, 2017 at 15:52
  • $\begingroup$ @Draco And if the nano machines are organic? Biohacking is already, to a limited degree, a thing. Who's to say that it's not going to be all the rage in the future, and some kid playing with a biohacking set (I.e. the future equivalent of a chemistry set) accidentally combines chemical 1 with enzyme 3, which has the unforseen consequence of turning the bacterium from a harmless, if moderately infectious and glowing, strain of E. Coli to something that strips down every organic substance it touches and turns it into more of itself, and can seek out high concentrations of organic material? $\endgroup$
    – anon
    Apr 22, 2017 at 13:10
  • $\begingroup$ Basically: it can't be any worse than what already exists, clostridium-perfringens. Nature's had a few billion years to figure this stuff out. $\endgroup$
    – Draco18s no longer trusts SE
    Apr 22, 2017 at 14:42
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Microorganisms

Take the movie Interstellar, for example. Microorganisms called "the Blight" affected the crops. Eventually corn was the last crop that hadn't been significantly impacted, that too would soon die.

Taken from the synopsis:

Climate of crop blight and constant dust reminiscent of The Great Depression of the 1930's.

It is also stated by a fellow scifi guru that:

Blight breathes nitrogen, which is registered as significant (it isn't, per se, but for the purposes of the movie, we take that as a given). Given that nitrogen is 78% of the atmosphere, and oxygen is only 21%, the movie implies that we cannot win.

Thus, humans must leave earth. I'm not sure I find this completely believable, which is ironic, because the movie is actually fairly accurate.

However, you could tailor a more dramatic scenario. Perhaps one that greatly interferes with any human attempt to genetically modify damaged food.

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  • $\begingroup$ It always annoyed me (though not enough to stop me loving the movie) that we caved so easily to blight. Within months of it appearing a gang of folks in labcoats would've spliced a genetic immunity into loads of crops. Ill-informed wailing about "playing god" wouldn't count for much against the whole world screaming for food. $\endgroup$
    – Grimm The Opiner
    Apr 21, 2017 at 10:47
  • $\begingroup$ Variation on this: result of genetic engineering gone wrong $\endgroup$
    – user3106
    Apr 25, 2017 at 9:54
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A rogue black hole is rather far-fetched, but a rogue planet is not. Hence:

A post-graduate student among the Sloor (kind of like a squid, breathes a hydrogen atmosphere, most comfortable around 15 Kelvin), does a more refined calculation of the trajectory of a known rogue planet that is headed towards the Sol system.

He (well, that's the closest word for the student) concludes that there is a 10% chance of an actual collision with the third planet, and a 99% chance of throwing it into a radically different orbit.

You won't believe how long it takes this matter to get through the Galactic Congress' Committee for Pre-Member Worlds (when they finally took up the matter, that is; it took seven months just to resolve the turf war with the Committee for Cosmological Incident Relief).

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Rogue Planet

Astronomers have detected a new planet far out beyond the edges of our solar system, but too close to be part of any other system.

This isn't the hypothetical Planet 9 that people are today are suggesting may exist far out in our system; this is a genuine rogue planet that was thrown from its own star system many aeons ago and has been wandering the inky darkness ever since.

Its wanderings have now brought it into proximity of our own Sol system.

This is bad news for us, no matter what the ultimate outcome.

  • Scenario 1: The planet perturbs millions of objects in the outer solar system, ranging from small rocks to Pluto-sized planets, and sends them hurtling inward.

    These objects stablilise into new comet-like orbits, coming close in to the sun before flying back out again. All planets in the system will now be subjected to thousands (or possibly millions) of years of bombardment. The event that wiped out the dinosaurs will be small change compared with some of the things that could hit us. Even if we don't get hit with the big stuff, it's still going to be a very unpleasant experience for a very very long time.

  • Scenario 2: The planet is large enough and close enough to perturb Jupiter.

    This in turn causes small scale changes to the orbits of most of the rest of the system. Best case here is an end to any kind of climate stability on Earth until things settle down. Worst case is snowball Earth or scorched Earth.

  • Scenario 3: The planet is gravitationally captured by the sun and enters orbit.

    The first orbit will be a very long elliptical on, almost comet like, potentially sending the planet quite close to the star. It will settle down in the long term, but that's not a time-scale that humans would be willing to wait for. In the meanwhile, the odds are pretty high of either collisions with other planets, or the other planets being ejected from the system entirely.

The astronomers who have detected the new planet can't tell yet which of these scenarios will play out, but they have enough evidence to show that it's going to be bad news. Their advice to humanity: Build some star ships and get out of the system before now, while there's still time.

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Global warming making the Earth uninhabitable

With current or near-future technology mankind isn't able to setup self supporting colonies on the Earth's moon, Mars or other moons in the solar system. This means that if the Earth is somehow lost, all the colonies are effectively lost as well.

It is quite likely that global warming spins out of control. Politicians can't agree on how to handle it and near future technology is not capable enough to negate the effects, mainly because energy consumption rises faster than technology can compensate for. We can already see this coming, but for now it's not 100% sure when the Earth will become uninhabitable. Being closer to that event makes it easier to determine when it will actually happen.

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    $\begingroup$ If you can make a large enough craft to support a significant portion of the earth's population within the vacuum of space, forget about making it a ship and simply make arcologies on the surface. Even with a worst case scenario, global warming will not make the Earth less habitable than Mars or the Moon. $\endgroup$
    – Michael Richardson
    Apr 20, 2017 at 16:26
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The LHC of course.

Nothing beats a "plausable" explanation more then fanning confusion.

The LHC made a mini-black hole, it ate away at a chunk of our planet, our orbit destabilized, weird weather and anti-magnifying glass effects ensued, death, death, death. Let's go over here.

Best part is that the humans left on the now too hot or too cold earth, don't have to die off, they can come back after a giant worm becomes emperor and takes a bath.

That's like 4 sci-fi plots in one.

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I've had another idea... a crazy paranoid dictator, a la Kim-Jong Il, prepares a Doomsday Device. Its country it's invaded and he decides to detonate it before commiting suicide.

The Doomsday Device is a Leo Szilard's [salted bomb][1]: a thermonuclear bomb buried below a huge underground storage of cobalt. The bomb explodes, irradiating the cobalt into Cobalt-60, which is sent to the stratosphere due to the force of the explosion, then is spread all over the world through high-altitude wings.

However, with these weapon you don't have 50 years notice before the cobalt starts to settle down.

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So, let's say the Dual-Stage 4-Grid rocket engine is a real thing.

Isp = 210,000 m/s. Assuming we need to go 15 lightyears in 100 years we can plug those into the Tsiolkovsky equation and find (Mp+Mf)/Mf ~2E93, where Mp is the mass of the propellant and Mf is the mass of the rocket after all propellant has been spent.

So, let's say we have 1,000 people to transport at 50 kg each. 50,000kg and say that equipment and supplies double that (probably way, way too low of an estimate - can you imagine a 100 year trip where you only pack 50Kg of transportation, equipment, tools, food, water, medicine, technology, etc.?). So, anyway, that's 100,000 kg. Now multiply that by 2E93; that's 2E95 Kg of fuel.

The Sun's mass is ~2E30 kg. So that means we'd need to carry 1E65 Sun's. Let's see, 250E9 stars per galaxy, 250E9 galaxies in our reachable universe, that's 1e42 Universes we'd need to deplete. Hmmm, might be tough to do. I can't claim I'm facile with these calculation, maybe someone else can correct my errors, if I made any. My point stands, and that is, it ain't gonna happen, not in 100 years, probably not on a 1000 year trip.

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    $\begingroup$ Welcome to WorldBuilding! This is an interesting answer about the feasibility of the colonization, but I don't think this really answers the question about the Event that could force humans to go beyond our solar system to find a planet to inabit. Could you edit your answer to provide more content that directly answers the question? If you have the time it would be nice if you could take a look at the tour and visit our help center to learn more about how this site works. Have fun! $\endgroup$
    – Secespitus
    Apr 20, 2017 at 11:41
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    $\begingroup$ Your math seems correct - but that equation is very sensitive to changes in the variables. A world with fusion power, in which we've begun solar system colonization and have presumably put effort into researching engines, is specified; is it out of bounds to assume we can design an engine with 10 times the exhaust velocity as the one you mention? Assuming such an engine, then if we go to Proxima Centauri which is only 4.24 light years away instead of a whole 15 light years, that takes the fuel/weight ratio to a much more doable 428 to 1. 156 to 1 if we settle for 120 years instead of 100. $\endgroup$
    – D M
    Apr 21, 2017 at 18:34
  • $\begingroup$ @DM how is "10 times the exhaust velocity" possible? 210,000 m/s x10 = 2,100,000 m/s which is 7 times lightspeed. That would be impossible, so I doubt that you could get it to work. Besides, a grain of dust = HUGE explosion at those speeds. $\endgroup$
    – Mathmagician
    Apr 22, 2017 at 3:11
  • $\begingroup$ The speed of light is 300,000 km/s, not 300,000 m/s. You're off by a factor of a thousand. $\endgroup$
    – D M
    Apr 22, 2017 at 3:13

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