Luna knew what would happen to the world if the sun was badly damaged or destroyed.
However, she didn't know what would happen if the same thing happened to the moon, so when they found out about the comet, once all other options had been gone over, she approached the scientists in secret and asked them.
If the moon was destroyed by the comet, a large number of fragments would rain down on the earth. Cities would be destroyed, and countries wiped off the map. The remaining fragments would form a ring around the world, and periodically would break off and slam to the surface for ages to come. Not only that, but the moon blocked the world from being pounded by meteorites. They were what caused the craters on the moon. Also, losing the moon would effect the ocean tides. They would become much calmer. Why was that a problem? Because the movement of the water distributed nutrients to where they were needed to all the sea life. Without it, many ocean species would go extinct. Then the creatues that ate them would be in jeopardy, all down the food chain.
In other words, the world needed both the sun and the moon to survive.
However, even with all the problems losing the moon would cause, solutions could probably be come up with to buy time until those problems could be solved and the moon either pieced back together or somehow replaced. Magic could do great things these days.
But if she was careful, that wouldn't be necessary.
She asked the scientists all kinds of questions. How big was the moon compared to the world? How strong would it need to be to withstand a hit from the comet?
When she was sure she had all the information she needed, she started to strengthen the moon. She had to do it carefully to make sure it didn't effect things too much. She also couldn't do it all the time. It was tiring and she had to make sure that nobody noticed her horn glowing.
It wasn't easy, pretending that she wasn't up to anything. Pretending to go along with everything everyone else was doing. She hoped that another solution would come up, but there was nothing.
She understood now what Starswirl's prophecy truly meant.
For the moon so loves the world she will make a decision that will change everything. For the nightmare that wanted to bring the world darkness will leave it in light.
Leave it in light. Leave it in light.
She knew what that meant. Luna didn't lie to herself. She was scared. She didn't want to die. She didn't want to have to leave again, to leave her sister again. A part of her felt guilty, knowing what she was going to put Celestia through, but it was her duty to protect her subjects.
When Nightmare Night came, she was glad that she was able to convince Celestia to go with her. She wanted to be able to spend one last night with her. She had been careful to leave her note on her bed where she could find it.
Even knowing what she had to do, what she must do, seeing Celestia smiling and laughing almost made her change her mind. She didn't want to cause her sister pain. Maybe... maybe she shouldn't do it...
Then she had spoken to Pipsqueak.
"I just wish I'd gotten my cutie mark first. It would've been nice."
How many foals would never get their cutie mark if the comet came here? She knew then that she would go through with it.
She made the most of her time, knowing it was running out. She had been pleased when she won the costume contest. After celebrating with everyone it was nice to spend the next day quietly with her sister. Catching up with each other, laughing about happy memories, and simply being together.
That evening as Celestia set the sun and she raised the moon, and everyone could see the comet in the darkness, she quietly took a deep breath. This was it.
"Celestia, I love you, very much."
"I know Luna, I love you too."
I know you know, but I had to tell you, one last time. It's funny, but I'm not scared anymore. I feel... peaceful. I love you Celestia. I love you all.
"So I hope that you can forgive me for what I'm about to do." Luna gave everyone, gave her sister, one last look, seeing Celestia's confusion before she turned back to look at the moon.
"What do you mean what you're about to do?"
Luna knew she had to wait till the last possible second. Otherwise Celestia would fight her for control. She had to wait, then do it fast, before she had the chance. Almost... not yet... now!
She moved the moon, and heard Celestia scream as she realized what she was doing. "Luna no!" She felt her sister try to grab the moon and yank it out of the way, but she had timed it just right, and there was no stopping it now.
Luna never felt the blow. There was too much force, and it happened so fast. One moment she was looking at the moon, then she saw the comet about to hit...
Then there was nothing. It was like she had lost all her senses, all but one. She couldn't feel, couldn't see, but she could hear.
"No! Luna! Luna!" She was holding her. She had to be, for her voice to be so close. "Luuuuunnnnaaaaa!" She heard the agony in Celestia's voice as she screamed and cried, as she begged her to come back, as she pleaded for this to not be true.
Luna could feel herself start to drift as pony's voices joined their princess in shock, pain, and confusion. But at least they were alive to feel that pain, to go through that grief. She hoped that eventually they could move on, but she knew Celestia wouldn't. The sounds that she was making was as if she was already dying with her.
Oh my dearest sister, I really am so very sorry...
"She's gone. She's gone Princess." She could hear Twilight say in a choked voice. That and Celestia's sobs was the last thing she heard as everything faded out and she could feel herself being gently pulled away, though she knew not to where.
But this is goodbye.