Note: Game's quite long and has a lot of levels, so I'm rating this without finishing it for now. I'll update my rating when I have time to finish it. At the time of this comment, I've reached up to level 18.
Gameplay: I liked how yo used the gravity aspect and decided to divide the game into to phases. The general vibe is calm and it's a game you can get immersed in for a long time. The levels have the right level of challenge: not too easy but not incredibly hard. It's nice that you introduce the new mechanics slowly as to not overwhelm the player. Perhaps sometimes it's hard to guess where your rocket will end up and having to reset the simulation again and again can feel like guesswork. Using D+Right Click feels a bit weird, but overall the controls are ok. There are some bugs, like not being able to place moons once you deleted them on some levels. I haven't tried the hint system yet.
Graphics: Graphics are a bit barebones. Maybe some particles for the rocket when it's moving would help.
Audio: Music really adds to the experience. It's calming and not annoyingly repetitive. Sound effects are a bit harsh on the ears.
Innovation: I've seen a lot of games using moons and gravity, but I don't know why this doesn't feel unoriginal to me.
Theme: It's a basic interpretation of the theme, but it works.
Overall: A calming puzzle game with a lot of levels! I feel like the only major improvements you could make are the graphics and fixing some of the bugs. So good job :)