The game looks great with a premise that's easy to understand. Only thing I'd improve upon would be to put some form of lose mechanic/challenge in the game that could cause a game over. Possibly a case where if the sputniks collide with the planet they would cause it damage, or if they collided with each other they would explode (wouldn't really be a game over case, but would be visually interesting to see).
One thing I'm confused on is the purpose of the arrows. When I first saw them I saw them as a way to indicate where the thrusters on the sputnik were located. I realize now after playing for a bit that this was actually the direction the sputnik was going to go once I pressed the spacebar again, but the way it was presented definitely threw me for a loop. Perhaps maybe some indication on the little tutorial given beforehand would be helpful for that, or just make it look more like an arrow.
Apart from that, the physics done for the sputniks in relation to the planet was very well done. I saw something earlier about someone mentioning the sputniks still being counted after they leave the game screen area, but I didn't see that as too much of an issue. Vastness of space and all. Great work.