Triton

Roger did include early access to this clue as an Easter egg. The last active area on the Four Lock Gate that I have not described yet is the 'phi' symbol. Clicking that 3 times after clicking the 'star cradled in the Moon' got to this same 1989 clue. So I had seen this for several months before dead ending on the final (so far) bud on the end of the last (so far) clue tree.

Back when the world and the game were new, 25 August 1989 was very familiar - that was the day Voyager 2 streaked by Neptune! Sure enough, surveying various almanac data shows no other significant events on that day.

Amazon has uploaded a wonderful, complete report of the Neptune flyby on Kindle:

 

The last Neptune encounter benchmark on this event summary page notes that at 11:23 GMT, Voyager was 20 Neptune radii outward bound - essentially done with the Neptune encounter, with the 8 track recorder slam full. HOWEVER ... the highest resolution approach to Neptune's principal moon Triton (VTERM) was at 0840 GMT (you have to interpret between the columns). At the bottom of the page, the report notes that it takes 246 minutes for light (and radio waves) to get from Neptune to Earth - and that the report times may vary from actual time by a few minutes due to ongoing trajectory updates.

So the signal that Voyager 2 ACTUALLY did make it to the highest resolution approach to Triton reached Earth at 1240 GMT (to the nearest 10 minutes). Voyager did get closer to Triton, but the spacecraft was over the dark side at the end of the pass by then, so image recording had stopped.

So we have two orientations of Triton to consider - the 'real' time when Voyager was in position, or the 'relatable' time when we on Earth knew the event had (probably) been successful. Easy enough to keep both in mind for our next move. Next time.

BTW, the report had a 'flip book' sequence of the Neptune pass in one corner, so you could see Voyager skim the top of Neptune's North pole, and then whiz past Triton. Shame that no YouTube content providers have captured the animation - yet.