I found that space missions served better as a supplement to what you are doing on planets for xp.
What I did on my first couple characters was to do the daily quests for space missions that were not grayed out. The space missions themselves aren't great xp for the time invested, but when you couple the xp you get during the mission with the daily quest turn in xp reward, it's worth it.
Just don't bother with the quests that are grayed out. You should have between 2 and 5 quests to do every day, which is a nice chunk of daily xp if you want to do it.
This. If I am close to leveling a character I will do space combat missions.