Thank you! I've been trying to place that since someone mentioned colonies de-orbiting a big rock earlier in the thread, its been driving me mad.
Back on topic, whatever you go with, I think you probably need to go in one of two directions, either the clearly preposterous ala Douglas Adams, or something entirely plausible and within both the laws of physics and the capabilities and motivations of whoever is responsible, sci-fi fans (and I consider myself one) can be quite intense about these things.
One idea I had planned to use for my game was how there was a human-like alien race (well, humans have inhabited most of the universe, as like humans have done to Earth), "brainwashing" Earth's leaders, making them believe them. Another human-like alien thing comes by, opposing that one.
It's been a battle between two alien races for Earth to join their alliance. Long ago, before the game began, the first one rescues Earth from a large meteor (nothing new, folks).
By the time Earth goes against the first aliens, favouring the second one more, the first one recovers that meteor and crashes it into Earth. If they can't use Earth as an ally, it might as well be seen as an enemy. Destroying it prevents it. Very gloom ending for the player, but semi-original...I think.
That reminds me a TV show I saw about the explosion in Tunguska, Russia. One theory was that a meteor was about to fall over Earth and a kind Alien crahsed his spaceship into it, preventing full impact.
I'm no science major, but from a realism standpoint you would be hard pressed to think of a scenario that would make Earth an undesirable place to live (or re-colonize) without destroying the planet physically. No matter what you do to it, it will still be a more desirable place to live / re-colonize / terraform than Mars or Venus - not to mention the other planets in our system. Have your wars, have supervolcanic erruptions, radiations - all that can do is wipe out life on Earth (or only most of life), yet it would probably remain the most hospitable planet in the system.
Aliens destroying it then leaving would center the whole story on aliens themselves - who are they, where did they come from, will they return and when? Tuch an event would sooner cause us to unite and work together to salvage what's left of humanity and ultimately survive, rather than split in mutually hostile (or competing) factions. Earth mysteriously disappearing is also an event that should be properly explained at some point in the game, and would have a similar effect on humanity as aliens wiping it out.
It leaves two things on the list that I see as hypothetically possible in the late 21st century -
- A really, really big asteroid that would throw it out of current orbit, possibly sending it on a collision course with the Sun, or out into the dark, cold space. - A persistent virus spread, lethal to humans (possibly manufactured?).
Both are cliche, I'll give you that. But IMO it is preferable to have something that's been done before than something that plainly makes no sense at all - such as the black hole popping in, swallowing exactly one random planet, and moving along elsewhere. Please don't do that. :)
- Microorganisms in the oceans begin to release massive amounts of hydrogen sulfide into the atmosphere, wrecking the ecology and killing off everything that couldn't take the high sulfide levels, including humans. The result: like the "Pandora" of Avatar, humans and most other mammals can't even breathe on this new Earth because the atmosphere is poisonous.
- A brown dwarf star passes through the solar system. It has an encounter close enough to Earth to alter its orbit. Now Earth's orbit is as close to the sun as Venus at perihelion and as far away from the sun as Mars at aphelion. The result: the massive climate swings completely wreck everything. The mass extinctions that follow include all unprotected humans, and any "re-colonists" need to deal with intensely hot weather in what are now the summers, intensely cold winter in what are now the winters, and just all around crazy weather in what are now the spring and fall.
- An alien interstellar spacecraft of some sort slams into Earth at a significant fraction of c. The result: an explosion as devastating and spectacular as a comet impact.
In the sake of altering the orbit any massive object may do the trick. IMO this is the best way because it will really render Earth unhabitable and people could get off from it, at least some. And for that humans joining together, it can only happen if there is an alien, it won't happen if people start living in colonies weeks away from each other, the inverse process of globalization.
It may not be necessary to make your Earth permanently uninhabitable. Perhaps it is only such during the period just before and/or during your game. See Disney's WALL-E for one example of how such a setup was handled (though I recommend passing on the anti-consumerism motif).
If you decide to go with a World War, consider alternatives to having China, Russia, the US, the EU, and the Middle East annihilate each other. Perhaps a new prophet arose and a religious crusade ensued, or class strife spread like wildfire.
Physics experiments gone bad have a broader range of possibilities than just CERN creating black holes. If your civilization is based primarily in the Solar System, perhaps during the search for "warp speed" space-time folding resulted in any number of the other disasters (moon falling into the Earth, the Earth disappearing, whatever).
Alien lifeforms invading Earth don't necessarily need to be sentients with rayguns. Maybe a comet/ice chunk/rock that crashed into Earth had an alien bacteria or fungus that upset Earth's collective biochemistry.
2087 - Earth has been under Quarantine for more than half a century. At the end of the twenty-thirties, deep sea explorer brought samples of rock and mud from black smokers in the Mariana Trench near Japan to the surface. The recovered muck contained a strain of truly ancient arachae bacteria that escaped the improperly set up laboratories. These bacteria that had not been on the surface world for millions, perhaps even billions of years but found paradise for themselves in the present environment. Spreading though air, water, soil and food, colonies of the bacteria were quickly present in any and all life forms on earth.
At first this didn't seem to be a threat to life itself and was merely studied by curious individuals. Over the next two decades every new generation of plants, animals and humans alike, seemed to house an increased concentration of the bacteria. The arachae accumulated sulfur, cadmium and other toxic materials inside the host, slowly killing them from the inside. Any form of surgery or antibiotics proofed ineffective and only a handful of people were resistant.
Government and nations descended into civil war and crumbled soon after. The few immune lifeforms on earth, including a few thousand humans, still carry the 'yellow plague', as it is now called, in them.
Luckily for humanity, they had already established more-or-less self-sufficient colonies on Luna, Mars and Europa before the plague could leave their cradle. These human exclaves have thrived surprisingly well. Low-gravity and controlled hydroponics did wonders for the reproductive system. Science and automation methods developed by the pioneers of space colonization allowed them to extend their domed homes quickly and multiply like humanity did in the 20th century.
Earth however, is still under quarantine. Yellow clouds of bacteria are visible from orbit and no vessel is allowed to land or leave. SQUID (Space QUarantine Inhibition Department) shoots down every object coming closer than 200,000 klicks of earth.
That's the story of Earth and how it was conquered by it's ancient rulers...
Anyway, I'm all for a story, where Earth just disappears without any explanation. In-game speculation about how that happened would of course always be appreciated.
