Collisions, Score, and Game Over

With bullets and asteroids both on screen, it's time to make them actually interact: destroy asteroids you shoot, and end the game if one hits your ship.

Distance-based collision detection

import math

def distance(a, b):
    return math.hypot(a.x - b.x, a.y - b.y)

For circular objects (ships, asteroids, bullets all drawn as circles/triangles with a radius), you don't need pixel-perfect collision, just check whether the distance between two centers is less than the sum of their radii:

def circles_collide(a, b):
    return distance(a, b) < (a.radius + b.radius)

Bullets destroying asteroids

score = 0

for bullet in bullets:
    for asteroid in asteroids:
        if distance(bullet, asteroid) < asteroid.size:
            bullet.life = 0        # mark the bullet dead so it gets filtered out
            asteroid.hit = True    # mark this asteroid for removal
            score += 100

asteroids = [a for a in asteroids if not getattr(a, 'hit', False)]
bullets = [b for b in bullets if not b.is_dead()]

Rather than removing items from asteroids/bullets in the middle of the nested loop (which corrupts the loop you're iterating over), the pattern is the same as the bullet-cleanup from the last lesson: mark things as "should be removed" first, then filter both lists once, after the collision checks are done.

Ending the game

game_over = False

for asteroid in asteroids:
    if circles_collide(player, asteroid):
        game_over = True

if game_over:
    font = pygame.font.SysFont(None, 64)
    text = font.render("GAME OVER", True, (255, 60, 60))
    screen.blit(text, (250, 260))

pygame.font.SysFont(None, 64) loads the operating system's default font at size 64, .render(text, antialias, color) turns a string into a drawable Surface, and screen.blit(surface, position) is how you draw any image or rendered text onto the screen, it's the same function you'd use to draw a sprite image.