From the Super Mario Wiki
“This is the AMMO DEPOT, not a GARBAGE DUMP! What, do people think they can just toss their trash here and it'll magically disappear? Heyyyy... we have plenty of bombs. Maybe you could use them to blow up all this trash?”
Bob-omb Blasting is one of the four minigames found in Super Mario Galaxy along with Bubble Blowing, Manta Ray Surfing, and Star Ball Rolling. 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 garbage in the ammo depot, which is supposed to 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. 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.