The 6th Mission for Terrans is this...
6. SECRETS OF THE SPACEBRIEFING: "BLURRED OFFICIAL ATTITUDE".
Destroy any terran pirate fleet that is worth at least 100 stat points.
It will open once you kill Pirate Fleets worth at least 100 points or more of ALL 12 pirates on the list in mission 5.(that completion awards a level 1 Pirate Officer once completed)
I know this because I just completed the 12th kill today and opened the mission 6. I am still looking for a Terran pirate fleet for it lol...
Good luck! :)