Hello, I am David Andersson, and welcome to the guide to completing the prologue to 100%. In order to get 100% you must pick up the artifact, the S.C.A.R.F. artifact, and complete the prologue on the impossible difficulty.
Watch on YouTube: http://youtu.be/HUbWwoEGRl4
Load into the prologue, or Codex Crash Site, on the impossible difficulty. Instead of following the blue line turn around and go backwards. A short while later you will see a blue shimmer in the distance:
Approach it and pick up the .C.A.R.F. artifact.
Now complete the mission on the impossible difficulty. I have already written a guide which can find here: http://wp.me/p1SfP0-1kS
Once you have done all that you will have completed the Prologue to 100%: