"The spaceship is flying through space."
Translation:Das Raumschiff fliegt durch den Weltraum.
The German doesn't omit the article? Is there a general rule for this difference in convention?
Yes, German uses der Weltraum with article.
Perhaps the question is why English omits the article... we say "the air" and "the environment" but we say "space" and "nature".
Probably not either I'm afraid :)
The rules are mostly the same between the languages but not completely, and I'm not sure whether it's always possible to explain logically why the article is / is not used in certain cases.
Durch is the preposition. Durchs is a compound of this preposition and the article das. Das All but den Weltraum. There is no compound for the article den. So you can always say durch. Ich fliege durch das All, ich fliege durch den Weltraum, ich fliege durch die Küche.
Depending on regional circumstances, people might even mark your "durchs" as a complete mistake and want you to use durch + article das. Its a rare used word but is "more alive" in some regions/dialects.
As comparision: in das and ins. Ich gehe ins Haus, ich gehe in das Haus. Both those sentence mean the same and have the same type of construction like durch/durchs. Only that ins is a far more common word.