I was gonna say, power-ups would make this perfect! A little harsh when your ship randomly moves into a ship that was on the whole other side but I guess it makes destroying them more important than just outlasting.
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You never know which direction player's ship will go next time, so yea, you have to choose would you try to destroy alien ship before it reaches player's ship or take a risk and try to dodge it. There's a tip: ships that flew past the player appear again at the bottom of the screen, so sooner or later you'll have to destroy them one way or another, otherwise you'll... run out of space!