In 2109, an asteroid was detected, cataloged and promptly forgotten. Odds are it would never hit. The looming fragmentation of the United States of North America and the ongoing African Wars of Unification took up most of planet's resources and attention. Yet another rock wandering around the solar system was hardly worth mention. Mars colony was stable, habitats had been set up in the asteroid belt and Neptune Base was well on it's way to becoming the next stepping stone on mankind's march to the stars.

Six months later it became apparent that it was going to pass close to the Earth. Too close. Every available space craft was set on a collision course in an attempt to destroy or, if it was nudged just a teensy bit, it could deflected and miss the Earth entirely. It was not enough.

The nations of the world launched every nuke available at the rock. Well, not EVERY nuke. Some where held in reserve. Just in case, you understand. The nuclear firestorm in space met with a margin of success. A very limited margin of success. Instead of one big rock, it became a not quite as big as before rock and oodles of smaller rocks. All of which were now radioactive. And really, really angry.

Then there were the END OF THE WORLD spoiled sport last licks and paybacks. Some of the warheads reentered and vented a little atomic fury on predetermined terrestrial targets. And some chemical fury. And some bioweapons. And some nanoweapons. And lots of dataweapons. Then the half-assed doomsday weapons that most folks completely forgot about because they didn't really work all that well automatically detonated.

The big rock missed. Later analysis proved that it would have passed close enough to effect the tides if it was left alone. But just enough of the smaller fragments hit to spread all the contaminants about, mixing most of them together with earthquakes, tidal waves, volcanoes, superstorms, and, of course, the psychic maelstrom of a several billion simultaneous deaths. 

At least one of the asteroid fragments proved to be a colony ark for an alien race, the last vestiges of a once proud and advanced civilization. Then there was the rip in spacetime and possible doorway to another dimension hovering over where was once the Amazon rainforest.

Lemme see... We've got all sorts of possibilities here. The earth is in ruins, sea level fell 3000 meters and all sorts of mutation-causing goodness -don't forget all that illegally dumped toxic and radioactive waste- has forever altered the amazingly still-habitable biosphere. There are pure humans left in secret, underground bases scattered around the globe who will eventually build huge, domed cities from which to lord over their particular patch of hell. There is the possibility of lost alien technology and, of course, the aliens themselves. The human colonists on the Moon, Mars, Neptune and the asteroid belt are screwed because most of their ships have been trashed trying to save the homeworld and, as all the major ship building facilities that were orbiting Earth were likewise obliterated, it will take, oh, 200 to 300 years minimum to build up enough industry and population to even think about spending months traveling to Earth in any serious manner. And even then they're only going to use small scouting forces to see what happened. Considering all the wreckage now orbiting the Earth, it will be difficult to get there, Aliens, mutants, mid-tech & high-tech humans, lost civilizations, extraterrestrial and extradimensional invaders, etc. Now you can use whatever figure you want in your games AND the original force lists from whatever rules you like. Happy?  ;)