Answer:
Difficulty
Explanation:
Space exploration was the last thing the world could have thought of dabbing into because the idea of moving away from the earth and flying in the sky wasn't even an easy one for man, hence the difficulty attached with going into other planets was a big success.
Many trials were made before the eventual success experienced, this many difficulties man overcame is what is meant by barrier in this sense.
credit goes to stankyleg118 (he made this i dont own)
hope this helps tho :)
Think of a situation in which you are afraid of failure. Visualize yourself hitting an obstacle, allow yourself to feel the fear, and then see yourself moving forward.