Location: While playing as Franklin, you need to reach the trailer in the South-East of Sandy Shores, i.e. the one where Trevor has his trailer. After you get there, (the above screenshot) talk to the man called Omega to learn about his ideas about liens. You may now take to searching for the spacecraft parts. There is 50 of them, in total.
I recommend that you use the "GTA V - maps and collectibles" to locate the parts. An important piece of information is that also Michael and Trevor can engage themselves in searching for the collectibles of this group. All of the items you find will be included into one pool. Personally, I recommend that you use Trevor because many of the parts are located in inaccessible areas and the best way to get there is on the chopper.
After you have collected all 50 parts, while playing as Franklin, wait for a text message from Omega and get to a small trailer park located in the Eastern part of the map, to the North of the hills with wind farms. Approach Omega to talk to him again, and to give hi what you have found. The game does not award you anything for that, per se, but you will be allowed to take a buggy car from the garage (Space Docker), which has been modified by Omega.
Rewards for the completion of the mission: none