Too little things
I really think the idea for this game could be really great, except there are way too little things. Adress issues such as-
-Art; having the same f22 all the time is boring; try to draw something original. also, try to change the enemy speed, as well as waves. get them to shoot lasers, rockets, etc. I mean, in real life, i doubt if you're a UFO, you'd die because of a freak plane crash
-Upgrades: Upgrade your ship! Make it shoot faster, and have add ons that actually show up on the ship, so an upgraded ship looks different from the first. Add coins/points with a timer, so there isnt just a kill quota: you'd also have to quickly kill the enemies.
Honestly, this could really be a great game, but take some more time; NG doesnt have a 'sale quota/deadline', so take your time.