Arrowhead Comet

The Arrowhead Comet (アローヘッド彗星) is a comet said to approach Earth once every 1700 years.

Principles
The Arrowhead Comet is largely made of ice and dust. It is suspected that it might have a new type of that doesn't fit into the six known types.

It only approaches Earth once every 1700 years, with its latest closest approach being around December of the current year of the story timeline.

Magic God Invasion Arc
On December 3rd, there is a large amount of media coverage in Academy City about the comet's imminent arrival and the possibility of a seventh achondrite being discovered. After being launched into space by the mass driver intended to send Parasatellite 01 to investigate the comet, the High Priest seizes control of the comet and attempts to use it to return to Earth. However Kihara Noukan kills the High Priest with the Anti-Art Attachment and destroys the comet before it can impact. The explosion from the collision at over Mach 20 shatters all the glass in Academy City.