Thank you for your long feedback!
Yes, each destroyed asteroid makes appear some particles and also 5 small meteorites.
The improvements of the planet are totally essential, once developed the asteroids and meteorites are not a problem at all because totally intercepted by the defenses. If I remember correctly the first alien ships arrive at wave 5. 25 waves in all.
The beginning is too difficult, I didn't realize it while doing the tests. But once I bought a few cannons or planetary missiles, it gets much better.
I continued the development of the game since I posted it. The bugs when you die are now solved. I redesigned and rethought the user interface completely. The waves of enemies are also redesigned so that you don't get "surprised" anymore. I also added a purchase confirmation system.
Upgrades are more numerous, less expensive and it is now possible to accumulate the bonuses offered.
I am currently developing two more ships for the player offering a choice of gameplay and different weapons for each of them.
Then I will work on two game modes, a "campaign" mode with waves and a story and an infinite mode.
Don't hesitate to follow the development to give me your feedback after the updates, it's extremely beneficial for the development.
Thanks again!