NASA just posted a ton of free sound files on their Soundcloud page for free download and holy cow, these space sounds are nuts!
Another thing: you’re free to use all of these sounds as you wish, because NASA’s own audio isn’t copyrighted. It’s meant to be a public service to the American people of their taxpayer-funded government program, but that extends to everyone. There are some restrictions – not everything NASA publishes is covered by the same license, though it appears to be on SoundCloud. And you aren’t free to use NASA’s name or logo or imply commercial endorsement. (The Eagle didn’t land on a bag of Doritos.) But that means just about any imaginable musical application is fair game. They do ask you to list NASA as source, but that’s only reasonable.
Solar System & Beyond Sounds
Rocket Engine Sounds
Space Shuttle Mission Sounds