Hey, congrats on your first game! It's a big accomplishment to get that first one out there.
I just downloaded and played it for a little bit. The idea is solid and I like the basic gameplay. I love the style of the Earth and moon, but I wish the asteroids used the same style (they look like 2d chicken nugget sprites).
Your presentation is too minimal for my taste. It makes the game feel unfinished before I've even started playing (there is no splash screen, title screen, intro... just straight into the gameplay screen) The little mini explanation of the controls is perfect, but I had to read your description of the game before I knew how to "win." You might want to add a brief explanation of the rules into the game.
Like I said, the idea is solid and immediately gives me a lot of ideas for expanding it. The most obvious would be to make the asteroids not always aim directly at the Earth - alot of them could be near-misses unless you use the moon's gravity to direct them into the Earth. Asteroids could also do a few elliptical orbits before they hit.
You could also introduce other planets with different configurations (bigger/smaller planet, bigger/smaller moon... moons at different distances.... multiple moons? You could also use the moon to absorb the impact of a planet-killer asteroid, breaking it into smaller asteroids that won't destroy the planet in one hit.
Good start though. Keep it up!