Outer space

Outer Space is the term for the massive amount of empty space surrounding the entire World of Avatar. It takes up the entire universe.

History
Outer space was first seen in "The Guru" when Aang had a vision of him walking across a bridge of energy to reach his celestial spirit, thus gaining him mastery over the Avatar State. Before he can reach his spirit, he sensed Katara was in trouble, ergo he left to save her, causing the bridge of energy to disolve away. It was seen again in "The Crossroads of Destiny" when Aang succeeded in merging with his spirit. He soon falls down again when Azula strikes his back with lightning.

Outer space is seen briefly in Sozin's Comet, Part 2: The Old Masters when Sozin's Comet was shown skimming Earth's atmosphere as it does every one hundred years.

Description
Outer space is currently a little known subject in Avatar World, though much of it seems comparable to the real world given the similarity of objects, such as stars, comets, meteors, as well as the Sun and Moon. The basic observations of astronomy have given an understanding of planetary movements as well as basic predictions; from basic wax and waning of the moon to the arrival of comets and eclipses.

Another similarity is that the planet of Avatar orbits the Sun. However, the moon is more complicated; it exists due to the presence of the Moon Spirit. The Moon's force is also dependent on the Moon spirit. It is not known if the spirit world's actual location (if there is a physical location) is within the realms of the Avatar World or somewhere in outer space. This emphasizes the fact that outer space is more of a mystery rather than scientific.

There appears to be no scientific or philosophical theories about what the outer space consists of. Experiments and research on outer space by any of the nations has not been evident as yet.

Trivia

 * Like acupuncture, full understanding is less important than knowing how to use it. It has been said of Chinese philosophy, "if it works - use it". It is possible that the philosophy of believing in spiritual power coupled with the natural abilities of bending, that the need for for further knowledge of outer space is less of a priority.
 * The word "space" is specifically mentioned in Sokka's Master as the place where meteorite that made Toph's Bracelet is from. The characters demonstrated knowledge that the meteorite did not originate from the Earth.

Known Visitors

 * Tui
 * Yue
 * Aang (in vision)