From the Super Mario Wiki
Bob-omb Blasting is one of the two minigames found in Super Mario Galaxy, the other being ray surfing. It appears in the Battlerock Galaxy and the Dreadnought Galaxy, and involves the use of multiple Bob-omb Dispensers. When Mario (or Luigi) arrives on the small planet that the minigame takes place on, he is told by a furious Gearmo that people have been dumping trash in the ammo depot, which is supposed to be clean. Mario (or Luigi) therefore has to use Bob-ombs to blow up all of the trash in under thirty seconds. The Bob-ombs take a full ten seconds to explode, and the small yellow lights on the ground mark a good place to throw them. In the Dreadnought Galaxy, you will usually need six Bob-ombs to blow up all of the trash, but if you throw a Bob-omb into a specific spot, you can complete the level using only five. When the player successfully blows up all of the trash on the planet, the Gearmo rewards them with a Power Star. Unlike most minigames, failing Bob-omb Blasting does not result in the loss of a life. Instead, the player will only be told that they need to do it faster.