Unfortunately, to be able to access the mission gamers must use special tools that change some variables in it.
The latest news says, some gamers have found a way to access alien missions in GTA 5 without having to use special tools. Similarly, quoted from the page Gamerant.
Apparently, gamers can access the alien mission or Chiliad Mystery (in the game) by completing the Gunrunning mission 600 times on GTA Online between 21:00 and 23:00, and the mission will begin.
However, gamers only get one chance to complete the alien mission. If you can not finish successfully, gamers will not get a chance to try again.
Since the way to unlock the Chiliad Mystery mission is already known, it is only a matter of time before any player can complete the mission and what the rewards are.
For your information, Grand Theft Auto 5 is now available for PC, PlayStation 4, and Xbox One.