You obtain this mission upon cementing the alliance between the turians and krogan. Admiral Hackett has requested assistance with this matter upon Noveria. Specialist Traynor mentions this mission in passing, which then adds it to the journal.
This mission occurs on the multiplayer map Firebase White (as it looked prior to the Retaliation Pack DLC). You'll find the usual Cerberus opposition - no Phantoms. Lots of cramped corridors make shotguns really shine on this mission.
Travel to the Horsehead Nebula and then land on the planet Noveria. Hackett will brief you on the way down, and Cortez will drop you on a small landing platform above the main pad. As soon as you land you will find Assault Troopers and Centurions already present on the location. Take out the enemies in the upper area: note that they are dropping down from above to attack you and can throw grenades from there. Then move forward and clear the enemies in the lower area. There are no Nemeses, Combat Engineers, or Guardians in this first wave, so you need not be overcautious.
Control Panel EditPistol Magazine Upgrade on the chairs. Head around and examine the computer console on the other side of the room. Go down the stairs, turn the corner to the right and salvage the Spare Parts (2,000 credits). Head forward to the console in front of you to salvage a datapad (2,000 credits). In the corner where the datapad was found high up on the wall are Heating Unit Schematics. Access the medical station on the wall before going outside. Head to the right and at the back of the catwalk you'll find more Spare Parts to salvage (2,000 credits). Turn around and in the middle of the two catwalks is another med-kit. Head to the right and in the far corner by one of the red rectangle marked pillars are some more Spare Parts to salvage (2,000 credits).
Once you have collected all the items, return back into the building with the heating unit stabilizers. On the opposite wall to the right of the display, you can find the security console to deactivate. Pick a team member to deactivate the console. Once the security panel is activated by one of your team members, you will have to fight off a wave of Troopers plus a Centurion and a Guardian with only your remaining squadmate. It's not a large wave but it deploys from all directions, so watch your flanks and be ready to move quickly when a surprise grenade arrives.
Landing Pad and Atlas EditCombat Engineers, and a Nemesis. You can choose the indoor path or the outdoor path, though note that the outdoor path gives your enemies the vertical advantage as well as allowing the Nemesis more chances to snipe you. An Atlas will be sent as the last unit you have to deal with, but only once all the other forces are cleared, so you'll be able to concentrate on it exclusively. As soon as you kill the Atlas you will be returned to the Normandy to de-brief with Admiral Hackett.
Note: Because the mission will end immediately upon destruction of the Atlas, be sure to grab all the items before that happens.