system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can we build a man-made ecosystem in the form of a space craft which makes infinite space discovery possible? 

Is it possible to build a fully self-sustainable space ship boarded with people that can travel for numerous lifetimes; collecting information on the outside universe and reporting it back to Earth?
gpt
Let's simplify this to the greatest level. To work perpetually (ignoring the practicality of technology), your spaceship would have to have a net balance of energy- no energy could leave the ship, ever- or, it would have to be made up by energy coming in.

This obviously prevents navigation of any sort, unless it's balanced out by some other intake of energy. Because deep space largely doesn't have anything in it, such intake would have to be in the form of electromagnetic radiation- which is at a premium in space.

Thus, you'd be limited to navigation when in areas of space with electromagnetic energy freely available- in and around solar systems.

So, it's POSSIBLE, but the limitations would be very severe.

Bear in mind that the ship would somehow have to recover ALL energy expended- body heat, motion of the occupants, etc, etc, etc. The ship would have to reflect or absorb all energy coming its way- it could not emit anything on its own- unfortunately, this includes radio waves back to Earth.

Now, if you just want a ship that will go for a very, very long time, then it's much simpler.