Time. We dont have the technology to survive the time it would take to get to the nearest star to our sun. Alpha-Centauri is the closest star to us. It is: 4.37 light years or 25.8 trillion miles or 41.5 trillion km or 277,600 AU away. It would take 30,000 years at our fastest speed to get there. The average lifespan is 75-80 years, it would take almost 1200 generations to get there.