Ambitious idea! I feel like taking a step back would help here. There's a LOT in this game. Mission select, with several mission types, calendar, experience points, core gameplay. I think if you cut some of this and spend the time polishing what's left you would end up with a better game.
Gameplay. It's cool that you added the option to re-read the briefing in the pause menu, otherwise I wouldn't know what to do, because the first time I clicked the mission at random :) After finishing I discovered I could keep clicking the same mission, and it would show up as finished, but still give me xp. I'd like it more if the levels grew faster. Without the xp glitch it would be a bit of a grind to unlock new missions. The core gameplay was fine, but could use more variety. Tip for cool enemy behavior- make it so they don't go towards the player, but a random point near the player, that changes every couple of seconds. This way enemies look like they try to dodge the player fire :)
Graphics. They are ok, but need some polish. It would be nice, if enemies had some death animation when hit instead of disappearing.
Audio. The music is ok, but some sound effects do wonder for atmosphere and feedback. Some "pew-pew" for the gun, and impact sound for when you hit the enemy.
Theme. Not much stargazing here to be honest. I guess you are in space...
Over all- I see a lot of potential here, really. When you take part in a jam next time (the next Goed is in September, btw), make the game in half the time, and spend the rest polishing and bugfixing. Keep it up man!