Bowser Jr.'s Robot Reactor is the first boss galaxy in Super Mario Galaxy. It is located in the Terrace. To unlock this galaxy, Mario must get eight Power Stars. In this galaxy, Mario battles Megaleg, who is summoned to attack him by Bowser Jr. When Megaleg is defeated, Mario earns his second Grand Star in the game (the first appears in the first mission of the Gateway Galaxy).
Starting PlanetThis planet is a very small planet. Here, Mario must let a Bullet Bill notice him and follow him so it can break open a cage, revealing the Launch Star to the Robot Reactor. This metal planet has several holes and a Black Hole in its center, which must be avoided. There are also several other cages on this planet that can be broken for Coins or a 1-Up Mushroom by luring a Bullet Bill into one. The planet is also infested by Astro Goombas and regular Goombas, which can make running to a cage with a Bullet Bill following the player somewhat difficult. There is a trick and odd shortcut here. If Mario Triple Jumps off the cage with the Launch Star and Spins, he can reach Megaleg faster.
Robot ReactorThis is the planet where Megaleg is found and battled. It is a small brown planet with many technologic symbols around it. There are small rectangular areas on the planet that are transparent and seem to have computer components inside them. Even though Megaleg is a huge boss, these transparent areas do not break if Megaleg stomps on them, for all that happens is a cloud of dust that rises up. On the underside of this planet, there are cages that can be broken with the help of Bullet Bills for items. This planet orbits the Starting Planet.
Megaleg's MoonMario begins on the Starting Planet. There are three cages of on this planet, one which is mandatory to be broken. There are also several Bill Blasters as well as Goombas and Astro Goombas on this planet. Mario must get a Bullet Bill to spot him by coming close to one, then have it follow him to the third cage. At this cage, Mario can run behind the cage to trick the Bullet Bill into crashing into the cage. Once this is done, Mario can use the freed Launch Star to blast to the Robot Reactor. Here, Bowser Jr. spawns and taunts Mario before activating Megaleg to attack him. Here, Megaleg will blast Bullet Bills at Mario as well as trying to stomp on him. There are also cages on the other side of this planet, which Mario can break by getting a Bullet Bill to follow him and collide into the cage. Anyway, avoiding Megaleg's attacks, Mario must run up one of Megaleg's three legs when its on the ground. Here, he will soon reach an area where three Bullet Bills are blasted at once. After dodging the Bullet Bills, Mario will come to a spinning gear area where he must hop on the gears. Finally, he will reach Megaleg's head, where there is a cage and a Bill Blaster. After luring a Bullet Bill into this cage, a regenerative cage will show up, blocking the cage with the pink/purple light on it. After breaking this cage with the use of a Bullet Bill, Mario must quickly get another Bullet Bill to hit the cage in the middle of this cage with the purple light to defeat Megaleg for good. However, he must be quick, for if he is too slow, the regenerative cage will grow back, forcing Mario to have to destroy part of the regenerative cage and then hit the cage in the middle with the pink/purple light. Once Megaleg is defeated, Megaleg explodes and Mario falls to the ground. He can then take the Grand Star.
- Starting Planet
- Robot Reactor
- If Mario goes to the top of Megaleg's head and uses first-person view to view the Starting Planet, it will look differently than when Mario was on it.
- Around the square panels on the base of the Robot Reactor where the glass cages are, the technological symbols are S-Block Tetriminoes in different orientations. This is a reference to Tetris, which is another game made by Nintendo.
- If the player goes to the back leg of Megaleg, they will find a 1-Up Mushroom.