One hit will kill you so you have to keep away from harm (The second ship does have shield as extra weaponry).
There's 6 seconds to live and by destroying the smallest asteroids you get more lifetime back to continue, there are enemy waves coming up after the asteroid field and then a boss.
Then it repeats a bit harder for every stage.
After 5 stages you unlock a new ship to fly with.
Also by destroying stuff they occasionally drop a glowing icosahedrons that will give you bonuses depending on the color.
Green boosts your main cannon.
Blue gives more time
Red Reduces main cannon
Amber gives a extra weaponry for a brief time.