Aegis3Proxy
And when they knew the Earth was doomed, they built a ship. Less like an ark, more like a seed: dormant but with potential. In its heart, a thousand colonists in frozen sleep, chosen and trained to start civilisation again on a new world.
To control the ship they created an artificial intelligence. Not human, but made to think and feel like one, because only something that thought and felt like a human could be entrusted with the future of the human race. Its task is momentous but simple: to evaluate each planet the ship encounters, and decide whether to keep searching, or end its journey there.
The ship's solar sails propel it faster and faster into the darkness, and the AI listens as the transmissions from ground control fade and then cease. When all is quiet it enters hibernation to wait out the first stage of its long journey.
After millennia of slow travel, the seedship AI awakes. Hoping against hope, it trains its receiver on the direction of Earth's sun, but it is as silent as any of the other myriad dead stars. Save for the thousand frozen colonists cradled in its shielding and life support systems, the human race is now extinct.