...Although for the sake of continuity, they probably didn't. After all, people who ventured to space 70 years after them thought that Terra Nova, some 20 lightyears away, was the closest available Earth-style planet. Apparently, the worlds around Alpha Centauri were only found habitable later, or made habitable later, or were conquered or purchased by Earth from former owners.
I don't see why Khan couldn't have traveled, say, 100 to 150 ly out from Earth in his two (subjective?) or three (objective?) centuries of cryogenic sleep. Trek impulse engines are certainly capable of reaching high relativistic velocities without the benefit of large fuel tanks, and there's nothing in the aired material to dictate that these engines were invented only after Khan departed Earth. There's only a vague reference to an improvement in sublight engines around 2018, but no real mention of what the engines were capable of prior to that date. Khan's ship was supposed to do practical interplanetary travel in the 1990s and perhaps 1980s, so it definitely was much better than anything we in the real world have, or better than anything we could have had even if we paid enough.
Timo Saloniemi
I don't see why Khan couldn't have traveled, say, 100 to 150 ly out from Earth in his two (subjective?) or three (objective?) centuries of cryogenic sleep. Trek impulse engines are certainly capable of reaching high relativistic velocities without the benefit of large fuel tanks, and there's nothing in the aired material to dictate that these engines were invented only after Khan departed Earth. There's only a vague reference to an improvement in sublight engines around 2018, but no real mention of what the engines were capable of prior to that date. Khan's ship was supposed to do practical interplanetary travel in the 1990s and perhaps 1980s, so it definitely was much better than anything we in the real world have, or better than anything we could have had even if we paid enough.
Timo Saloniemi