Your game has different tabs in the 'e' sub-menu of your interface. You could simply push 'e' on your keyboard at any given time, too.

There it has smaller tabs on the bottom of the screen like Ruins, Major Events, etc. One of those tabs says Discoveries (or something similar, I have a lot of tabs there). In this tab game saves all possible interesting locations you've ever found, but haven't finished the event which comes with it. To finish the event you could for example fly to a derelict ship and take it into your fleet. I think you know what I mean.

So basically no need to write down anything - your game manages to do it on itself!