The Celest Ion Accelerator, or Celest-IA, was an ancient Omni-spell mega weapon built by Equestria and left orbiting in space. It was mere weeks from completion prior to the apocalypse, and orbited the world impotently for centuries before finally being completed by Golden Hoop. The device focused sunlight into an intense blast of rainbow energy powerful enough to level entire cities. It was mere weeks from completion before the apocalypse.
It figured heavily in Golden Hoop's plans to supplant Prince Solstice and become the new messiah. He would use it to simulate an attack from a malicious "divine" being which only he could stop. This would unite all of the Church and the Tundra denizens beneath him, giving him the authority to depose Solstice and become the next Alicorn.
Chamberlain destroys it toward the end of the original novel, along with Golden Hoop, stating that it is a "a mechanical blasphemy; another mockery raising wires and sparks above divinity".
The destruction of the Celest-IA occurs during the first chapter of Borrowed Grace, inciting the story's primary conflict. It is the catalyst event for the colonists to attempt their return to Equestria.