You may have read some of Bungie's weekly updates about the number of things going on behind the scenes in Halo 3. Specifically, the sky you see in Halo 3 is much more complex than you might believe. The game engine simulates the pollution and other particles in the atmosphere that cause the sky to appear blue, when in actuality the sky is not blue at all. Granted, you won't ever notice this effect as you're playing a game of Slayer, but it is there. As proof of this, we present to you this video of a Banshee flying far into the sky of Valhalla. The Banshee flies so high that it actually pierces the cloud cover and reaches an altitude at which the sky is no longer blue and stars are visible. Honestly, it's pretty serene up there. Just a little food for thought the next time you waste some fool with the Spartan Laser.
[Thanks, Muffin Man1]