The 'Star Trek' OG has a new book coming out -- titled "Boldly Go: Reflections on a Life of Awe and Wonder" -- and Variety obtained an excerpt this week ... which sheds light on how WS actually felt as he was shooting up into the great beyond above our stratosphere.

Here's the quote everyone's mentioning -- it starts off with Will saying he thought rocketing into space would be a huge catharsis and illuminating ... thanks to Hollywood's portrayals.

Instead, however, Captain Kirk had the exact opposite feeling -- saying, "I discovered that the beauty isn't out there, it's down here, with all of us. Leaving that behind made my connection to our tiny planet even more profound."

There's more ... William says his space flight made him incredibly distraught with sorrow. He says, "It was among the strongest feelings of grief I have ever encountered. The contrast between the vicious coldness of space and the warm nurturing of Earth below filled me with overwhelming sadness." He says the idea of us humans destroying that filled him with dread.

Will adds, "My trip to space was supposed to be a celebration; instead, it felt like a funeral."

Remember, Will was visibly emotional when they touched down ... but as he was trying to relay something to JB, the dude sorta interrupted his thought by popping some champagne.

Later, Will certainly made it seem like it was a positive moment -- calling the Blue Origin flight "profound" and "extraordinary."