Eggsteroid 18 is the 18th and last Eggsteroid found in the game, it's OSIRis-REx.
The OSIRIS-REx is located in Beak impact 8-25 above the stone planet, simply launch Lazer Chuck up and activate his ability.
Hitting the pigs is hard as they wander, but Lazer Chuck's ability is useful.
Launch Lazer Chuck at the bottom Medium Pig and activate his ability to the switch, then defeat Foreman Pig by using Lazer Chuck's ability below the UFO, the path must be clear. Then launch Chuck near the Asteroid next to the Medium Pig and activate his ability to the other Medium Pig