What is the theme of the poem?

By and by Man will try
To get out into the sky,
Sailing far beyond the air
From Down and Here to Up and There.
Stars and sky, sky and stars
Make us feel the prison bars

Suppose it done. Now we ride
Closed in steel, up there, outside
Through our port-holes see the vast
Heaven-scape go rushing past.
Shall we? All that meets the eye
Is sky and stars, stars and sky.

Points of light with black between
Hang like a painted scene
Motionless, no nearer there
Than on Earth, everywhere
Equidistant from our ship.
Heaven has given us the slip.

Hush, be still. Outer space
Is a concept, not a place.
Try no more. Where we are
Never can be sky or star.
From prison, in a prison, we fly;
There's no way into the sky.