When in doubt.
Drop in and scan the nav beacon in the target system.
For assassinations, not massacre missions btw, currently these aren't considered worthwhile.
planetary scan, planetary retrieval, aka go fetch geo samples etc
And find black box or other type specific cargo.
All need either scanning of a nav beacon or a discovery scanner. After last patch i notice assasination missions need a scan of the nav beacon and won't update via a honk.
You will then get a mission update with a stellar body where a USS or planetary search zone pops up.
If it is a USS, that is short for Unknown signal source btw. its a waiting game.
OneThe ship computer will shout at you when it spawns, which is good since you might be half asleep at this point.
These will spawn eventually and these spawn consistently in the designated area and until mission objective is complete.
So if you need to retreat for any reason, don't worry you won't fail the mission.
As for rank missions they keep spawning so what many does is simply look for a quick and easy one like "the boss at outpost x forgot his cat food, bring him some".
Ok that last could be anything from data hauls to slaves and illegal drugs.
Hey I'm Thor -
People call me Bob.
Rule 1: Pillage. Then burn.
Rule 2: No such thing as overkill, as long as there are reloads.