A Prankster Comet is a comet that sometimes circles galaxies in Super Mario Galaxy. If Mario goes to a galaxy with a Prankster Comet circling it, he will have to do a special challenge. They can be moved around by feeding a special Luma 20 Star Bits. In Super Mario Galaxy 2, however, Prankster Comets need to be summoned by a Comet Medal.
In a Speedy Comet challenge, Mario must get the Power Star before the time runs out. In Super Mario Galaxy 2, Speedy Comets reappear but there are clocks that add ten more seconds and less time to begin with.
In a Cosmic Comet challenge, Mario or Luigi must race their cosmic counterparts to the Power Star. Cosmic Luigi takes more of the shortcuts than Cosmic Mario does. Cosmic Comets do not appear in Super Mario Galaxy 2.
In a Daredevil Comet challenge, Mario has a maximum of one health point, and will usually fight a boss. Daredevil Comets reappear in Super Mario Galaxy 2.
Fast Foe Comet
In a Fast Foe Comet challenge, all of the enemies move faster than normal. In Super Mario Galaxy 2, there are similar comets called Double Time Prankster Comets, where certain elements of the world switch faster.
In a Purple Comet challenge, Mario must collect 100 Purple Coins. There is sometimes a time limit. They only appear after beating Bowser's Galaxy Reactor in Super Mario Galaxy. Purple Comets reappear in Super Mario Galaxy 2.
Double Time Comet