Nice job, good choice of palette, really fits space well.
Two things about the graphics.
First, I love the design of them. The simplicity of the rocks are just right and provide a really good atmosphere. The planets as well. The explosions are very well done, and that floating head is truly creepy haha.
Second, I think the ship could use some work with the 'transitions' between the sprites. What I mean is, the design of the ship is fine, but it might look better if it returned to the side on view after letting go of a key rather than staying in it's 'movement' look. Also, it's a little weird that the thrusters would appear when you're moving backwards [ slowing down ], but turn off when moving forwards [ speeding up ]. This one is a total nitpick, but I think it'd help to flip that.
Overall, great job. Simple but fun to play, nice work :)