Comet Observatory was one of the locations visited in the multi-stage, Galaxy Tours in Super Smash Flash 2. It used to be the main form of the stage, until being replaced by Starship Mario. It based off in the game Super Mario Galaxy's main hub. In competitive play, it was considered as a banned stage, this was mainly because the size of the stage allows excessive stalling. This was prior its replacement.
From the Comet Observatory, players are transported into different galaxies that changes the stage layout. The Comet Observatory itself, however, also has a distinctive layout. In the middle of the stage are the Beacon and map of the domes. The Toad Brigade's ship can be seen on the far left of the stage, as well as the Terrace dome. The Fountain dome can be seen in the background. The Garden dome is at the top of the observatory. The Library can also be seen at the far right of the stage, but it seems to be missing the Bedroom dome, as well as the Kitchen dome. The small floating planet in the background is the Gate, but it is merely part of the background. The Trial Galaxies are also featured in this stage's background, but like the Gate, are unreachable. Rosalina also makes a cameo in this stage. As of demo v0.7, Lumas also appear, but have no effect on gameplay. As of v0.8, from time to time, the Starship Mario from Super Mario Galaxy 2 will fly by in the background.
The Comet Observatory acts as the hub world for the game Super Mario Galaxy. Prior the events of said game, Bowser attacked Rosalina's Comet Observatory, stealing its 121 Power Stars, including its seven Grand Stars. Without them, the Observatory rendered immobile and non functional. When Mario recovered the first Grand Star, some of the energy of the Observatory returned to the Beacon, with this energy, the dome of the Terrace was opened. Rosalina asked Mario to recover the Power Stars in order to give back the Observatory its functionality as a spaceship, then, allowing Mario to chase and defeat Bowser and rescue Princess Peach. Every time Mario collects a new Grand Star, a new dome is opened. After 60 Power Stars, Mario is able to travel to the center of the universe to fight Bowser for a last time.