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.
The word 'barriers' suggests the difficulty and the vast challenge and mystery that is space exploration.
Space is sort of like a limitless and expanding entity and the conditions in space is not particularly favourable to human existence and activities. On that note, this poses a huge challenge for astronauts and other space scientists trying to do their jobs. It therefore requires more efforts, research, funds and creativity to be able to explore space and profer solutions to current and future problems that might arise from space. Such solutions being meteorological predictions of weather conditions, satellite launching and the study of changes in space that might affect our planet.
