I know you put a lot of time and effort into making your game, Programmer, and I do appreciate it, don't get me wrong. But in the future you might want to consider putting just as much effort into the name for your game. Space Battle isn't just a really dull name, it's, well, it's already taken. See?
It was a pretty popular game. I don't know how you could have missed it. A little research is never a bad thing. That brings me to my next point. A lot of progress has been made in cloning Space Invaders for the Atari 2600 by games like Space Instigators and INV+. It might behoove you spend some time with those games. Not to copy them of course. Just to see what can be done with this sort of game.
I'm not saying Space Battle is bad. Not by any means. The landing sequence is a nice little addition. You could maybe change the color palette a little though. The enemy ship is really hard to see against that black background. And maybe add some more obstacles, like in Lunar Rescue and This Planet Sucks.
You could maybe make the invaders themselves a little more of a threat too. As it is now they just sort of drift back and forth and fire at random. In games like Space Invaders and Megamania, the invaders not only move across the screen, they move down towards my ship at the bottom of the screen. This greatly enhances the sense of urgency and adds to the thrill of playing those games.
I'm sorry if it seems like I'm only pointing out the flaws in your game, but I'm trying to help you out here and there's no need to fix what already works, right?
This is all just the opinion of one person, of course. I'm sure you have your reasons for doing things the way you did.