Quests (Missions with Space game) are designed to add sub-plots to the gameplay.  They might be escorting merchants to another city or exploring a mysterious planet.  When designing mission cards try to limit the scope of the task; you do not want a simple side mission to dominate over the broader plot of the game. Quests are help in letting players build up their characters quickly (which means acquiring supplies, enhancements or allies) Quests can be linked to a broader plot or completely stand-alone. Typically, they offer substantial rewards in items or money and take multiple turns to complete but do not require the player to deviate too greatly from main objectives. 

You cannot have two missions at once except when you time travel.

You can only get a mission by going to a space station/civilization. 

Players can work together on the same mission for example, find all of the anomalies 20 gold, bounty find villains 1D20 gold, when two players work together they split the credits, if it was 20 gold then each player would get 10 gold. You can only work together if you are close to each other.