Original asteroids code:
License: MIT (https://opensource.org/licenses/MIT)
By paramat (available from https://github.com/paramat/asteroid)

Original asteroid_ textures:
License: CC BY-SA 3.0 (https://creativecommons.org/licenses/by/3.0/)
Attribution: paramat

Crystal models:
License: MIT (https://opensource.org/licenses/MIT)
By Electra Gizen

Skybox texture:
License: CC BY-SA 3.0 (https://creativecommons.org/licenses/by/3.0/)
Attribution: Ulukai (available from http://opengameart.org/content/ulukais-space-skyboxes)

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Recoloured textures from various mods:
Grass textures based on default mod in minetest_game (WTFPL). 
Redgrass texture based on junglegrass texture in default mod in minetest_game (assumed to be CC BY-SA 3.0).
Red versions of asteroid textures (CC BY-SA 3.0).

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Textures created for this mod:
CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
Attribution: Shara RedCat

All original code and edits to code for this mod:
License: MIT (https://opensource.org/licenses/MIT)
By Shara RedCat, tenplus1 and Mewmon

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Thanks also to DonBatman for help with the initial idea, and to the players of Red Cat Creative who gave ideas and helped in testing.