The real (and never stated) objectives of the game are:
      1. Make money.
      2. Improve your ship.
      3. Explore the universe.
      4. Kill Xenons.
Making money comes first, because you need money to improve your ship, and you need to improve your ship to explore and to kill Xenons.  You won't be able to explore very far until you are strong
enough to kill Xenons.
As your exploring takes you to the end reaches of the universe, specific "missions" will present themselves.  But they'll do you no good unless/until you've got money and a strong ship.
Don't worry about returning to Earth.  You've got a long way to go before you can even think about it.