Collectibles:
2 Artifacts
2 Logs
4 Weapon Parts
2 Upgrade Circuits

And here we are in the orbit around the C.M.S. Greely. The first time you can access this optional Mission is at the end of Chapter 4/the beginning of Chapter 5, the last time in Chapter 7, if you proceeded past Chapter 7 you can't go back to this optional mission. Before you fly towards the entrance of the C.M.S. Greely to start the optional mission, you might want to collect an Artifact, a weapon part and some more goodies. To do so look down from your exit point of the skip to see an L-formed piece of space trash with a white eagle on red ground and the letters S C on it.
Fly to the lower right end of the L and get into the door there. Inside the room you will find a box with a Compact Standard Frame Weapon Part (1/4) and some more goodies and in the corner of the room is the S.C.A.F. Artifact (1/2). There are also 3 lockers in here for even more goodies, so it is worth it to pay this room a visit.

And now we can get into the C.M.S. Greely. Set your questmarker to the optional mission and you will easily find the entrance where you get into the C.M.S. Greely and all other quests.
You enter a few of the kinesis lock doors until you reach the top of the main chamber in the Greely with 2 lockers, an ammo clip and the Compact Standard Frame Weapon Part (2/4) on a small shelf next to the ladder. Collect it all and climb down the ladder to enter the real main chamber of the Greely.
Get to the Bench you see and turn around as there will be one single Lurker appearing. While the music is way too dramatic for this single annoyance, kill it and chat with Santos to get a new objective

Mission Objective: Restore power and override the lock-down

Before we do this we want to grab the 2 Lockers and the 2 Cabinets. The Torque room can only be opened once there is power to the ship. And then we can also use the Bench to create a Torque Bar. So go down the stairs between the lockers and the cabinet to turn on the power.
Open 2 Lockers and stomp a box and enter the next door. Exit the narrow short room through the door and open the 2 Boxes in the next room. Climb down the ladder ahead and open the 2 Lockers.
And voila, there is the generator. Kickstart it with kinesis, you might have to hammer your kinesis button to get it running but sooner or later it will work. Please note that another Upgrade Circuit is located here on a New game +.

Mission Objective: Explore the Greely

Climb up the ladder and run straight and stand in front of the door and turn around. To your right a Puker will break out of the vents, so take him out before the 2 Slashers from the other end of the room appear here. You should use stasis on them to kill them pretty easy. I haven't tried running away with the elevator from all 3 necromorphs, but I guess it takes all the time until all the necromorphs are dead until the elevator arrives. Just a guess. And now we take the elevator up as the game blocks the way we just came from.
Up here we open the 3 Lockers and then enter the radio room with 2 Cabinets. You should exit through the next door, but it is not powered, so open the glowing blue fuse box and just push the two sliders with 60 on it to the right and you are done and can exit while an Audio Log (1/2) plays. Please don't miss the upgrade Circuit to the left of the door on a new game +.

Through yet another door we are back in the main chamber of the Greely. But there are 1 Exploder and 2 Slashers coming in from towards the Bench and the set of stairs to the right of the Bench, so you want to get a good aim at both of them. After they are dead a Puker comes in and then this ambush is over.
And now we can use the Bench to get rid of all the Upgrade Circuits and Weapon Parts and also create a Torque bar to get more of these Upgrade Circuits and weapon parts...



Also the door next to the stairs down to the generator is open now and you can get yet another Upgrade Circuit (2/2) and more goodies from the 2 Lockers in there. At this door is also a good Scavenging Point (in case you have your Scavenger Bot already).
Through the door next to the Bench we exit, collect the one Locker and enter the next door. In this frozen room are a 6 Boxes to stomp on, and also 1 Puker and 2-3 Lurkers from the vents on the very left when entering the room. There is also an Alien Artifact (2/2) to be found in the left pink alcove.
Once you got it all take the cargo lift down one floor. You'll encounter a Slasher behind some boxes, kill it before it can get too close to you. Once you are where you killed the Slasher another one will appear out of the vent next to the cargo lift. Now pass through the door and while you try to open the next door a Slasher will attack you from behind, which is not nice and not very avoidable. So have some health on your RIG in case he gets a hold on you. Just kill it, take the next door to find 2 Boxes and an ammo clip. And another box and 3 more ammo clips around the Bench in this room. But watch out, there are some Head Infectors coming from the next door past the Bench and of course they also get it on with you when you use the Bench, so get rid of them before doing that.

Use the Bench if needed, then kinesis the door open by using kinesis on the gears to the right of the door, and proceed around the corner to kinesis open the doorhatch on the generator. Take out the semiconductor to turn the electricity off. But this activates some anti gravity fields, so watch your step, these are deadly.
You also want to stomp all the corpses here as there are Head Infectors around. There is a box on the left and 4 ammo clips on the ground, at the end of the room you can find the Electric Charge Weapon Part (4/4) (it is the MK-II Version on New Game +), use kinesis on the blue circle to listen to the Audiolog (2/2) and the most interesting thing here is Engstrom's Office Key behind the audio log.
There is a good Scavenging Point in the back of the room here, if you have your Scavenger Bot.

Mission Objective: Download the encryption codes

When you leave the room a swarm of Head Infectors rush in, shake them off if they get you and continue back. A Slasher walks in but steps on one of the anti gravity plates and is history real quick.
You now head back to the main chamber, but meet some more Necromorphs on your way there. In the ice room are 6-7 Slashers and a puker and lots of anti gravity fields, so some of the Slashers take care of themselves via the anti gravity fields, others are lucky enough to die through you. Don't step into the anti gravity fields!

In the main chamber are 5 Exploders coming at you, but with a good aimed hit on their explosive part they are easy to handle.
And now we can unlock and enter the office next to the Bench. Get the ammo clip, the tungsten, the 2 Boxes and also finally download the encryption code via the computer.

That's it for the C.M.S. Greely, quite some good stuff was collected along the way and now you can get out of the Ship. On your way out you will pass by a Suit Kiosk. I recommend upgrading your health to 2 or 3, depending on if you are on chapter 5 or 7. Also upgrade your stasis a lot with all the goodies you collected along the way on this optional mission. Once you are outside in space return to the end of chapter 4 or skip this and jump to the beginning of chapter 5 and board the Terra Nova or get back to chapter 7 and get the engine for the shuttle to finally get down to Tau Volantis.