This mission begins in Sandy Shores, slightly northwest of Davis Quartz, and is only available to Franklin. Omega will tell you of an encounter he had with some aliens, when their spaceship crashed and broke into pieces. After he's done talking, he will ask you to find any spaceship parts (click the link to bring you to the page with all of the locations listed) you come across.
Although the mission can only be started by Franklin, the spaceship parts can be collected by any character. There are a total of 50 pieces scattered all around San Andreas and once you find them all, bring them back to Omega to start the next mission.