Many years ago, I developed a phobia of outer space. It just sort of happened...I looked up into the night sky, and felt terror. It provoked a visceral response in me, similar to the feeling I get in my core from heights and roller coasters.
It has something to do with the sheer distance between myself and the stars, moon, and planets. I look at these celestial bodies and feel as though I will "fall" all the way out to them, like if I'm not careful, gravity may lose its hold on me. The longer I look, the more I focus on the feeling, the more panicked I become.
Sometimes I freak myself out by thinking of what it would be like if the Earth fell off its orbit. The world is plunged into darkness as it falls farther and farther from the sun. Would we feel anything?
A couple years back, I thought I had gotten over my phobia, but it came back. The feeling is not so strong as it once was, but it's still there.