The modern astronauts can travel only a few thousand miles away from the earth, and therefore their attempt to travel in the sky is something like child's play on the shore of an expansive ocean.
The moon is situated in the third status of the upper planetary system, and in the Fifth Canto of Srimad-Bhagavatam we shall be able to know the distant situation of the various planets scattered over the vast material sky.
There are innumerable universes beyond the one in which we are put, and all these material universes cover only an insignificant portion of the spiritual sky, which is described as sanatana Brahmaloka.
—————–Reference: Srimad Bhagavatam Canto 2, Chapter 5, Purport of Text 40-41
The kingdom of the Supreme Personality of Godhead in the spiritual sky cannot be understood by any process other than hearing from the description of the Vedas. No one can go see it.
In this material world also, one who is unable to pay to go to a far distant place by motorized conveyances can only understand about that place from authentic books. Similarly, the Vaikuntha planets in the spiritual sky are beyond this material sky.
The modern scientists who are trying to travel in space are having difficulty going even to the nearest planet, the moon, to say nothing of the highest planets within the universe. There is no possibility that they can go beyond the material sky, enter the spiritual sky and see for themselves the spiritual planets, Vaikuntha.
—————–Reference: Srimad Bhagavatam Canto 3, Chapter 15, Purport