How are all the Eggsteroid levels opened?

Solution 1:

Look at this guide for an answer:

http://www.angrybirdsnest.com/angry-birds-space-golden-eggsteroids-guide/

Eggsteroids #1 Pig Bang Level 1-9

Zoom out to see the Golden Eggsteroid hidden in the brush below the two bubbled-pigs. Fire any of the Blue birds to reach it. This Eggsteroid level pays homage to Space Invaders (1978).

Eggsteroids #2 Pig Bang Level 1-20

The Golden Eggsteroid is hidden in the tall brush atop the planet. Fire a Lazer bird above it, then redirect him into the Eggsteroid. This Eggsteroid level pays homage to Super Mario (1985).

Eggsteroids #3 Cold Cuts Level 2-13

Zoom out to see the Golden Eggsteroid in the brush on the planet below the slingshot platform. Launch the Lazer bird backwards, then redirect him into the Eggsteroid. This Eggsteroid level pays homage to Breakout (1976).

Eggsteroids #4 Cold Cuts Level 2-25

Zoom out to see the Golden Eggsteroid on the underside of the slingshot platform. Launch the Lazer bird downward to get a good angle, then redirect him into the Eggsteroid. This Eggsteroid level pays homage to Bubble Bobble (1986).

Eggsteroids #5 Cold Cuts Level 2-28

The Golden Eggsteroid is concealed under the snow pile, though the concealing is sort of undone by the large flashing arrow pointing at it. Launch any bird head-on into the snow pile to obtain the Eggsteriod. This Eggsteroid level pays homage to Star Castle (1980).