How do you explain it?

[dogscoff]

Only the energy of the earth will be far from suficient in a distant futur. I gave as an example the Star Trek Enterprise. In theory, a ship like that one would deplete the earth energy in a very few time.

Exactly, so why would you ever do that? By the time you have that kind of technology, you'd (hopefully) have sources of energy that don't require you to bleed your homeworld completely dry in a matter of years for the sake of delivering a few red-shirts to their grisly deaths around the galaxy.