From the Super Mario Wiki
A Meteorite is an object found in space. It makes a notable appearance in Super Mario Galaxy 2 as an item that Mario can use to his advantage. These round meteorites are gold and have the Ground Pound symbol on them. To use a meteorite, Mario simply has to jump into the air near a meteorite, which will automatically attract him via its gravitational pull. Once on the meteorite, Mario must position himself in exactly the right place, and then perform a Ground Pound to launch it toward a target. This simple action enables Mario to forcefully hit objects that are much larger than he is. He must first use them to break apart large stone locks blocking the way through giant doors in several Bowser levels. Later, Mario must actually use them during Bowser battles to attack Giant Bowser. He must hit him with meteorites four times to defeat him (in the final battle, Mario must hit Bowser eight times instead). However, if Mario is not able to Ground Pound a meteorite quick enough during a Bowser battle, it will lose its golden color and turn black, after which it will crumble and disappear. They first appeared in Bowser's Lava Lair and later in Bowser's Gravity Gauntlet and Bowser's Galaxy Generator respectively.