Complete the following tasks to unlock each bonus mission.

Air Docking: Accumulate a very high score.
Shuttle Defense: Quickly complete all the levels before the "Desert" level.
Goliath Defense: Successfully complete the first two bonus levels.
Space: Successfully complete all regular missions and "Air Docking", "Shuttle Defense", and "Goliath Defense" before reaching the "Ice Cave" level. Successfully complete the "Ice Cave" level to unlock the "Space" level.