I like the interesting retro style, but I do have a few pointers.
I would remake the asteroids. They have a kind of blur around them that makes them feel different from the other assets. Maybe try using the pencil tool in Photoshop instead of the brush to get a more pixelated look. (That is, if you're using Photoshop)
Try using higher contrast shading on some of the smaller sprites. Looking at the large, red planet, I think that has the best contrast. If you apply the same hard edge shading on the little red ship that faces right, then it will look more dynamic. Also, the grey ship at the top could also use some more dramatic shadow to bring it to life.
As for the designs themselves, I think they're a little plain. There have been a lot of space games over the years, so it's difficult to create something that people don't reference back to something else, so I don't think it's that big of a deal but maybe start playing around with some of your ideas and expand them. It not that big of a deal, but that's just my view on that part.
And finally, I feel you could use some fixes to scaling. Notice how some of your assets have large, easy to see pixels where as others are scaled down pretty far, making the pixels much smaller. It creates some conflicts in continuity in the over-all aesthetic. By finding a good balance in the scale of your assets, they will fit together better and draw the player even further into your game.
But aside from all of that, I think they're pretty good!