Clover's Comet, first observed and recorded by Clover the Clever approximately twelve years before The Exodus, is a periodical comet, on an orbit that takes 102 years to complete. Since its initial sighting, astronomers have been charting the comet's course, and have predicted that one day, thanks to the comet's highly eccentric orbit, that it may impact directly onto Equus' surface.
Astropony Monthly, Issue 5, Year 654 AB.
*********
"I've always loved Clover's Comet," said Twilight, as she fiddled with the eyepiece on her telescope with the moon high overhead, "it's one of my favourite astronomical objects since I read about it when I was a child, and I've been looking forwards towards the day when I could see it for myself for actual years."
"Indeed," said Princess Luna, "It is good to see that ponies are still interested in what happens in the heavens. Though I too must admit a fascination with the comet, as it is one of few objects in the night sky that is not ours to control."
"Really?" said Twilight," I mean, I should have guessed, especially given your absence, but to know that you don't control everything in the sky..." Twilight trailed off as something new in the sky began to make itself known. A new star sputtered and spurted into existence, a faint trail blowing behind it, and Twilight trained her telescope on it. She flicked back and forth between her telescope and several reference guides and her own amateur astronomy journal, making notes. Princess Luna, on the other hand, had appeared to slip into a meditative trance, her senses slipping beyond her body and out to the space that was her domain, and only minute twitches of her face proved that she was not simply sleeping.
"Well, that's definitely Clover's Comet," said Twilight, at length, turning around to face Princess Luna, who had stood, and had a grim expression fixed on her face.
"That comet will be our doom," said Luna, "We have until the next passage of the comet to prepare."
"Princess Luna?"
"Clover's Comet will not pass by Equestria again, Twilight. On its next cycle, it will collide with the planet."
********
Princess Luna strode down the halls of Canterlot Castle, smiling broadly. Ever since that night so long ago, the Lunar Princess had been more absent and distant, even with the other alicorns, and that distance had only grown more and more pronounced as the years rolled on. Only Twilight had an inkling why: the return on Clover's Comet was approaching. This was the year it was due to arrive, and by her calculations, in just a few months the comet would be visible.
"Luna?" asked Twilight, forcing her stride to match Luna's, "What's happened?"
"Clover's comet is returning, Twilight," said Luna.
"But the comet will impact Equus, Luna. Nopony, nothing will survive. I did the calculations, remember?"
"Yes, Twilight, I remember. I am smiling because we are ready."
"Princess," said Twilight, "We are not ready. We have made no preparations for this. As far as I know, almost nopony, no one outside of a few astronomers know about this."
Luna paused by a window, and guestured out with a hoof. "Tell me, Twilight, what do you see in the sky?"
"The moon," sighed Twilight. Luna's smile grew wider, and her horn lit up. Twilight watched as the moon began to slowly revolve, and her eyes caught sight of something on the previously-hidden face of the moon. Artificial structures, and what looked like thrusters.
"That is not a moon, Twilight Sparkle," said Luna, "that is Equus' escape vessel."