From what I understand, the space missions are any requests you will get out in the space field. Most of the time, it's going to be vehicle requests from other spaceships in the systems (towing, escorting, fighting ships, the like). There are some that will require you to land on the depots to pick up requests as well. Best suggestion would be to use the Galaxy Map and your radar to finish the systems off systematically. However, you won't get this until after you beat the game, when you unlock the final system (where Lord Vorselon's ship awaits for a third and final fight).
Supplemented by NickMon68:
If you have completed all the depot landing missions and also all yellow triangle missions and this still isn't popping, then you may have missed one, and its easy to miss.
On Korthos Alpha once you land and explore you will find a Vullard there. You want the one by the shack, not the one walking around on the planet.
Jump back in your plane and go to space and complete the mission for him.
This does not show up on the mini map at all ! This may be the one you are missing.
Once complete two new green items will show up on the mini map. Go back to the depot and speak to him again, complete that task then go to the other one.