i like to think the planet they orbited was reach , totally burned up so you could see the forerunner relics all over its surface. I like to think this because the final sub chapter in halo3 was named full circle. In Halo CE the chief leaves reach and finds the halo so if im right then the trilogy ends where it began hence full circle..... you can read about this theory on halopedia . its all just speculation tho , a buddy of mine says the cryopod he gets into has a independent power source for 1000 years , so 343 could make a halo 4 and set the time period like 950 years later lol and who knows.......