Bowser Jr's Robot Reactor is the first boss galaxy found by Mario in Super Mario Galaxy and this is also the first boss battle with Bowser Jr. in the game. In this level Bowser Jr.'s Robot, Megaleg, uses the Grand Star for power on the top of its head.
It is a yellow planet with holes and a bullet bill shooter, the bullet bill shooted out of the bullet bill shooter needs to hit a launch star tank to move on to the Moon Planet
As the mission's name, Megaleg's Moon, this is the moon planet where Megaleg is battled. You need to climb up either one of Megaleg's legs, then you will reach the Bullet Bill Shooters, then after a bunch of gears, you will need to use a bullet bill to break the shielding gate of the Grand Star. Then you will need to break the tank containing the Grand Star to unlock the next dome, the Fountain.
- The second Grand Star is collected here.