The mission Far Out becomes available after completing the story mission Fame or Shame. Have Franklin go to the green question mark in east Sandy Shores. It is just a little East of the Senora Desert Airport.

Once there, you will speak with a crazed hippie who has the codename Omega. Eventually he will tell you about a crashed alien ship and ask you to help recover the parts.

After speaking with Omega, 50 Spaceship Parts will spawn around the map. After collecting all 50, you will trigger a new event called The Final Frontier.