Here is the full list of all 16 Galactic Explorers achievements.
Defeat the Soviet Spacecraft
Defeat Ares II
Finally defeat Orion King of Titan
Free the sun from it's soviet prison
Travel the rift and destroy the white hole
Defeat the Noxious Beast
Defeat the space worm
Defeat the Blazing Leviathan
Defeat the Hellish Nightmare
Defeat the Soul of Ares II
Defeat the Hideous Glob
A glitch in the simulation?
Defeat Typhon
Defeat Gabriel
Experience your first death. I can assure you it won't be your last.