I'm so mixed on this. I love the core of the game and its presentation is lovely. It immediately gives the player an intuition of what an orbit is and how the initial velocity of a body determines its orbit. The first three or so levels are perfect. I like that it basically teaches you to make nice circular orbits as they are the least likely to collide.
But it gets really frustrating quite quickly, once you need to place 3 or more planets. Having them not collide with each other is much, much too hard and punishing. Ironically this game makes space feel claustrophobic: there's hardly any room. Even on the level with no asteroids, just a sun and 3 planets, there isn't really enough space (due to the large size of the planets and limited screen space) to have 3 planets in non-overlapping circular orbits. The biggest issue is that when two of your planets collide, you didn't just fail the new planet but you also broke your original planet. It's like reaching a checkpoint in a platform game only to have the checkpoint deleted because you made a mistake later. I wonder if it would be better to say when two of your planets collide, only the most recently placed one gets destroyed, so you don't ruin your existing planets. Or just add more space. Let planets go off screen by a few hundred pixels before they're destroyed. Or shrink the planets' collision radius so they can graze each other without being destroyed. Let the difficulty of this game be about figuring out orbits, not having to redo work because of a collision.
Another idea would be to show the trajectory before you take a shot, to take the guesswork out of it and make it more about intuitively understanding the mechanics.
I liked the level with many asteriods placed to one side of the sun, so you were forced to create elliptical orbits. I love the harmonies. I love the feeling of having completed three concentric orbits and watching them go around.