Complex sentence: After reaching Centaurus and finding it unsuitable to live on, The "Hope of Man" heads towards the next destination, the Alta system; because the ship at this time is unable to attain light speed it takes decades to travel there.
Split into simpler sentences: After reaching Centaurus and finding it unsuitable for human life, "The Hope of Man" heads towards the next destination, the Alta system. Because the ship is unable to attain light speed it takes decades to travel there.