Instructions and the link to the game are on the project page.
FYI, here’s how I made the animated GIF: I used Quicktime Player to record a portion of the screen, then used ImageMagick to convert the .mov file to a series of PNGs:
convert asteroids.mov png/asteroids_%03d.png
I deleted the extra frames at beginning and end that I didn’t care about (unpausing the game, figuring out how to turn the recording off), then used ImageMagick again to make the GIF:
convert -delay 1x30 png/*.png -layers optimize +dither -colors 32 asteroids.gif