When you’ve finished the “Far Out” mission you’ll be able to collect Spaceship parts in the game. The Spaceship parts glow in the dark and generate a sound which is helpful for finding them. There are a total of 50 spaceship parts spread out through the map. Here’s the location map, thanks to yourfriendlygerman from reddit
GTA 5 Spaceship Location Map
(Click to enlarge)
GTA 5 Alien Spaceship Parts Location Video
Once you’ve collected all the spaceship parts, bring them to Omega to get the Space Docker.