Space Junk Galaxy

The Space Junk Galaxy is a galaxy in the game Super Mario Galaxy. Like its name implies, the Space Junk Galaxy contains lots of discarded space junk, including old rocket ships, floating debris, and various other objects and platforms.

The Pull Star is used much more here than in many of the other galaxies. Also, the Toad Brigade is found here on a large T-shaped platform covered in crystals; if Mario frees the Toads trapped in the crystals, the main Starshroom will appear to take Mario to the next location. There is also a hidden wooden planet shaped like Yoshi's head.

The last unique feature of this galaxy is that it has a path that builds itself when Mario approaches it, but crumbles after he leaves. This is later re-used in Bowser's Galaxy Reactor, except the path crumbles after a few seconds automatically. Mario fights two bosses in this galaxy: Kamella and Tarantox.

Starting Planets
The Star Dust Start Planet is a small planet with a glass core. A metallic outer shell shaped like a square is attached to the core by small metal triangles. Its core resembles a miniature Star Dust Rolling Step A. Mario starts out on this planet in only 1 mission. In the others he starts on the Red and Yellow Starshrooms.

Star Dust Rolling Step A
The Star Dust Rolling Step A is a small prism-shaped planet made out of glass. It has two metal plates on either base. There are three Crystals that contain Star Bits on its surface.

G Capture Tutorial Zone
These are two small fields of asteroids that Mario passes through in all of the missions but the second mission in the G Capture Tutorial Zone. There are two fields one at the beginning of the galaxy and one near the Assembly Block area. Mario uses small paths of pull stars to navigate himself through the fields. Strangely one of asteroids in the first field is shaped like an "L". If Mario positions himself correctly he can land on these asteroids by using nearby planet's gravity, as these asteroids do not have their own gravity.

Lava Transparent Planet
The Lava Transparent Planets are three spherical glass planets with Amps moving around them. These planets have a ring of metal around their surface and two metal plates on either side. There are five Star Chips moving about on these planets. After collecting the Star Chips, Mario can use a Launch Star to go to the Rocket Planet.

Planet Rocket
The Planet Rocket is a big, long, red-and-tan space ship floating around in space. It is encountered in the first and third missions of the galaxy. It has two Crystals containing coins growing out of its portholes. On the route to Tarantox in the third mission, the end of the planet features a Sling Pod, along with a few Spoings. This planet resembles Olimar and Louie's ship from Pikmin 2.

Blue Star Capsule Planets
The Blue Star Capsule Planets are three large, blue versions of the orb planets with Pull Stars inside of them. These planets act as large Pull Stars that Mario can walk on. The planets seem to amplify the power of the pull stars inside of them seen from the blue glow the planets emit. This means the player is able to grab onto any part of the planets with their cursor and be affected by the pull star.

T Shape Planet
The T Shape Planet resembles a tower. It contains some Spoings. It is also where Mario finds the Toad Brigade, all of them trapped in a Crystal. After Mario frees them all, the Starshroom will appear. Various items and a Goomba are also trapped in crystals. On the top of the Starshroom is a Sling Star, which will take Mario to the Assembly Block Area.

Assembly Block Planet
The only static planet in the Assembly Block Planet is the platform on which Mario starts out on. This planet somewhat resembles the starting planet of the galaxy. The "Space Junk" in this area is made up of Assembly Blocks. Here, Mario must move around to find that Assembly Blocks will help him by giving him a floor to walk on. There are five Silver Stars scattered about that, once collected, will combine to create a Power Star. The player however, must be very careful as to not fall off the Assembly Blocks and into a black hole. Later, a more stylized Assembly Block Area is used for a Purple Coin mission.

Airships
In the second mission, several airships appear. Members of the Toad Brigade are trapped on the airships. Mario must hit a lever on the first airship to open the bridge, leading to a Sling Star and the second airship. The second airship has a Green Shell. Mario must throw the Green Shell at the Treasure Chest to unlock a Launch Star. The third airship has green shells, which Mario should throw at a few enemies on the fourth airship to open the bridge to it. The fourth and final airship is purple and red, different than the other airships. It is also where Mario must fight Kamella. Mario will get a Power Star for defeating Kamella.

Small Maze Planet
The Small Maze Planet is a planet that is encountered shortly before the battle with Tarantox. At first, Mario walks on top of it, which is practically a transparent crystal orb. Once he finds the passageway in, he can go under the crystal layer and explore underneath, where there are 1-Up Mushrooms, coins, Star Bits, and Goombas. There is a Launch Star in this planet, which will blast Mario out of its glassy surface, and take him to the Rock Planet. There is also a Hungry Luma on the surface of the planet, which will take Mario to the Yoshi Head Planet.

Grave Stone A
The Grave Stone A is a planet mostly made of gray stone. It has a sort of arch, as seen in the picture. This planet is an entrance to Tarantox's lair. The pre-boss music begins when Mario lands on this planet, and the Grave Stone B planet is nearby. Spoings also dwell here. To proceed, Mario must use the Sling Pod to launch the Toad to Grave Stone B, before launching himself also.

Grave Stone B
Grave Stone B is a small block with a Sling Pod that launches Mario to the nearby space ship.

Grave Stone C
Grave Stone C is an L-shaped platform that contains several Spoings. The player must use the Sling Pod at the end to launch Mario to the Phantom Boss Spider Zone.

Phantom Boss Spider Zone
The Phantom Boss Spider Zone is exceptionally large and looks like a giant version of the top of a Sling Pod. In fact, Mario uses a Sling Pod to get to the planet, breaking its shell open and revealing a giant sticky spider web, where he fights Tarantox, who is frozen inside the Sling Pod Planet. Several Coins are also stuck in the web, and can be freed using the Star Cursor to restore some of Mario's health during the battle. After defeating Tarantox, Mario will get a Power Star. There's also a green Toad inside the planet, stuck in a Sling Pod.

Yoshi Head Planet
The Yoshi Head Planet is a small wooden planet in the shape of Yoshi's head that shoots steam out of its nostrils. This appears to be the third reference to Yoshi in Super Mario Galaxy, the first being the Yoshi game file icon, and the second being the Yoshi Egg planet from the Good Egg Galaxy. This planet is home to a number of Goombas. These Goombas do not release any Star Bits or coins after they are defeated. When all of them have been destroyed by Mario, he will earn a hidden Power Star. In addition, when Mario defeats two Goombas in a row by jumping on top of them without touching the ground in between, a counting total will appear on the screen beginning with the number 2. If Mario can continue doing this until he defeats a total of eight Goombas, he will receive an extra life. Any further Goombas that are defeated without touching the ground will also earn additional extra lives for Mario. This is the only planet in the entire game on which this is possible.

Trivia

 * There appears to be unused data in the source code of the game for a Cosmic Mario race which was never added in the official release of Super Mario Galaxy, being replaced with the Speedy Comet mission instead, likely due to the Pull Stars. However, if the data for the Speed Comet mission is replaced by Cosmic Comet data, the mission can be played like a regular Cosmic Mario race.