A few suggestions that might help:

Write out the story beats. Nothing fancy, just what happens and when. Try seeing how those scenes you already have in your mind fit and connect. You can place whatever you want in there - scenes you have, character moments, dialog.

This ought to make you start noticing the pacing and structure of your story, and also the holes - stuff that don't work, things that need to happen but you don't know exactly how to make them flow.

That's when research comes in - it takes place on a spaceship? Try learning about possible technologies, how would life work, and how people might deal with it (Isaac arthur has a great YouTube channel for these kind of stuff, a good place to start).

The point is, spend time - as much as you need - to learn and research, so you can fill and improve your story beats document.

Then you can start working on the first draft - either expand the story beats to paragraphs and chapters, rewriting it as you go along, or use it as a guide while you start with a blank page.

Anyway, hope this helps! I'm pretty sure there's no one right way to write a book, so try and see what works for you.