Bowser Jr's Robot Reactor is Bowser Jr.'s first galaxy in the game Super Mario Galaxy. In it, Mario fights Megaleg to earn a Grand Star.


NOTE: Unless otherwise marked, all the names are unofficial.

Starting Planet

This is a very small planet. Mario is required to entice a Bullet Bill to follow him and break open a cage to reveal a Launch Star, and then use that Launch Star to reach the Robot Reactor. This metal planet has a black hole that Mario needs to avoid. There are also other cages that can be broken to reveal Coins or a 1-Up Mushroom. It is also infested with several Goombeetles, which make it slightly harder to traverse the narrow pathways while being pursued by a Bullet Bill. There is an odd shortcut on this planet. If the player triple-jumps off the cage with the Launch Star and spin they can reach Megaleg faster.

Robot Reactor

This is where Megaleg is found and battled. It is fairly small and orange-brown, with many technologic symbols all over it. It has small rectangular areas on it that are transparent and seem to have computer components inside them. Despite Megaleg's probable immense weight, the transparent panels never break if Megaleg stomps on them; all that happens is a cloud of dust rises up. In the underside, there are several glass containers around the back of the planet that can be destroyed with the help of Bullet Bills for items. If players go to Megaleg's back leg, they find a 1-Up Mushroom. The Robot Reactor orbits the starting planet.


This galaxy consists of only the following mission.

Level Summary
Megaleg's Moon Mario must battle and defeat Megaleg to obtain the Grand Star of the Terrace.


  • A boss in Super Mario Galaxy 2, called Digga-Leg, resembles Megaleg.
  • A glitch was discovered in this galaxy which is if the player triple jumps on the glass cage containing the Launch Star the gravity will pull the player to Megaleg's planet, expect that Megaleg is unactivated. If the player walks on the brown square on Megaleg's planet, the boss battle will start.
  • If the player looks at the back of one of Megaleg's legs, they'll find three cages holding various prizes.
  • If the player goes to the top of Megaleg and use the first person view to look at the starting planet, the player will see that it looks like its grassy early appearance, instead of being metallic. This can also be seen after Megaleg is defeated.
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