List of external resources used and their locations/copyright info.

Asteroids models/textures:

Learning XNA 4.0 - Chapter 17 sample code

Skybox Shader:

http://rbwhitaker.wikidot.com/skyboxes-1

Heightmap terrain heavily influenced by:

Riemers XNA Tutorial
"Graphics with XNA Game Studio 4.0" by Sean James
Terrain code from class

The powerUp.wav is listed as "Power Up Ray" on this site: http://soundbible.com/tags-game.html
It is released under a Non-commercial 3.0 license.

The background.wav is listed as "Photo Op" in UniqueTracks WebLoops collection.
The licensing information for this track is provided in the accompanying html file.

The stateTransition.wav is listed as "match2.wav" on this site: http://inventwithpython.com/pygame/resources/
I can't find any licensing information, but I assume that it is safe to use it for non-commercial/educational purposes.

The alien and laser gun models are from TurboSquid.

flashbang.wav: http://soundbible.com/1793-Flashbang.html

meteor_impact.wav and laser.wav: http://soundjax.com/