Dead Space 3
58 Achievements
The Explorer
Complete all optional missions.
How to unlock the The Explorer achievement in Dead Space 3 - Definitive Guide
There are 10 optional missions in the game. Seven can be played solo and three require a co-op partner.
Completing the three co-op missions will unlock the Ghosts of the Past achievement.
You can start at any optional mission from the chapter select menu. Be sure select "co-op campaign" instead of "solo campaign" to set up a co-op game.
In Standard, Pure Survival, or Hardcore modes, you must complete all 10 optional missions to unlock this achievement. You can unlock this achievement by completing just the seven optional missions in Classic mode as there is no option for co-op campaign.
Mission Breakdown
Chapter 4
Optional mission: C.M.S Greely
Co-op mission: C.M.S. Brusilov
Chapter 5
Optional mission: Conning Tower
Chapter 9
Optional mission: Supply Depot
Chapter 11
Optional mission: Armory
Co-op mission: Archaeology
Chapter 14
Optional mission: Disposal Services
Optional mission: Reaper Barracks
Co-op mission: Marker Containment
Chapter 17
Optional mission: Artifact Storage
Edit: I can confirm that this cannot be earned in standard mode, as I did all available missions solo. Also it looks like collectibles are different by a little bit for classic mode, but since I haven't collected them all, I can't confirm if the collectible achievements are attainable in classic. And the collectibles do not carry over save files. For example, you can't get half in your first playthrough and get the rest on classic mode. All collectibles must be found on one save file.
Edit 2: The only optional mission that's not really pointed out to you is the one in Chapter 17. I forget the name of it, but you find it in a corridor near one of the first alien word doors after fighting a few alien necromorphs in a cave full of red markers.
Online Trophy. There are a total of 7 Optional Missions that can be completed in Single Player or Co-op mode, and 3 Co-op Missions that must be played with 2 players. The Online Pass is required to play the game in Co-op mode. You can either work on this as you play, or return and complete them using chapter select at any time.
In Co-op mode, it does not matter if you’re playing as Isaac (host) or Carver (guest) as you will be credited for completing the mission as either character.
Chapter 4 - Optional Mission: CMS Greely, Co-op Mission: CMS Brusilov
Chapter 5 - Optional Mission: Conning Tower
Chapter 9 - Optional Mission: Supply Depot
Chapter 11 – Optional Mission: Armory, Co-op Mission: Archeology
Chapter 14 – Optional Mission: Disposal Services, Optional Mission: Reaper Barracks, Co-op Mission: Marker Containment
Chapter 17 - Optional Mission: Artifact Storage