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
Like most, the Arrowhead Comet is largely made of ice and dust. It was suspected that it might have a previously undiscovered type of that doesn't fit into the six known types, and thus might hold the key to secrets concerning the solar system's birth.

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

Parasatellite 01
Parasatellite 01 (パラサテライト01) is an unnamed probe which was built by Academy City to land on the comet and investigate it. In order to land, the probe has to match the comet's one hundred thousand kph relative speed. Parasatellite 01 was intended to be launched from an underground silo-type mass driver in School District 23 in a single-stage launch, with the probe leaving the atmosphere and entering the desired orbit less than ten minutes after launch. A campaign was underway for sponsors, with the chance to win the right to name the seventh if it was discovered by the probe.

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.