Spaceship Earth doesn't have a theme song anymore; it went out in 1995. The current descent music, I think you mean?, is actually Sinfonia 02 in C Minor by Bach.
As for theme songs, I gotta rally behind all of the above, plus New Horizons, Now is the Time, it's a small world, Let's All Sing Like the Birdies Sing, New World Bound, GOLDEN DREAM, Canada!, Universe of Energy...
... oy! :lol: