Impossible mode

Impossible Mode is the hardest difficulty of Dead Space, and can be unlocked by completing the game on any difficulty level. Impossible mode is notably different from standard (Easy, Medium and Hard) difficulties, providing a more tense, gripping experience. As the title suggests, Impossible Mode is very, very, VERY hard, but can be beaten despite its name.

On Impossible Mode, you must be tactical, conservative, and careful. However, to ease this discomfort, you start off the bat with the last suit you used to beat the game that session, without exiting the game or shutting off the console. If you exit or shut off the console, and don't beat the game again before starting an Impossible game, you will have to start with the Level 1 RIG.

Differences from standard gameplay

Being the hardest level of difficulty means that enemies hit harder, take less damage, and fewer resources can be found in the game world -- every shot and decision counts.

• The Necromorphs encountered on Impossible are around Hard mode in terms of health and damage and are more likely to sidestep the player's shots, making precise aiming key.
• Item drops are even more scarce than before; pre-set item pickups yields much less quantities compared to lesser difficulties.
• Most of the time the amount of ammo collected from fallen enemies wouldn't be enough to put down the next. Killed enemies are also less likely to drop an item, so don't expect your ammo to be "refunded" by the creature you have just killed.
• Credits, though dropping at a somewhat higher rate than ammo and health, are in disappointingly low amounts, often leaving the player with barely enough money for anything.
• Hits from even standard Necromorphs are dangerous and can tear off huge chunks of health if Isaac's RIG is unupgraded. Be aware of your surroundings at all times.

Preparation

• Impossible Mode is unlocked after completing the game on any difficulty, including Easy! Therefore, it is recommended that you at least play through the game on Hard or Medium before attempting it.
• There is a possible glitch where you start this mode in the military suit. If you finished a New Game+ (Round 2) in a Level 6 Military Suit, then start a new Impossible mode game immediately after without powering off, you may (not 100% chance of success, many report consistent failure) see Isaac wearing the military suit in the opening cutscene.
  • It is recommended keeping a save file just before fighting the Hive Mind for this reason. Defeat the Hive Mind, skip the credits, do not save, and immediately start a new Impossible game. Some steps may not be necessary, but the ones outlined above help maximize potential success.
    • Note: Merely loading a save right before the boss will not suffice. You must begin and play through Chapter 12 (possibly even from Chapter 11) in the suit in order to trigger. See Talk.
  • If you plan to buy one of the DLC suits to beat Impossible Mode, the Tank Suit is your best choice since it can take the most damage.
• Plan ahead. Choose which weapons you wish to use and focus your meager resources only on them; you should know the locations of each weapon Schematic from your prior playthrough. The safety buffer of being able to play around with weapons and see which one you like best does not exist in this mode; misspent power nodes or an insufficient supply of ammo/health packs may hinder progress.
  • Keep a hitscan weapon like the Plasma Cutter, Contact Beam, or Pulse Rifle .
  • Although counter-intuitive, Impossible Mode is the best candidate for a one-gun run with the Plasma Cutter. It is the best weapon cost/benefit wise for ammo consumption, inventory space stacking, and quantity of ammo to kill per ammo pickup. Ammo is relatively cheap to buy from the store, and it fulfills most gameplay requirements admirably.
• Stasis. Be fluent in its use, and treat it as a weapon. Practice in lesser difficulty modes to hone effective tactics, like stasing multiple enemies at once. Remember to use Stasis frequently.

Ammo	Ammo per pickup	Stack size	Sell price	Kills per pickup
Contact Energy	1	4	1,000	1
Line Racks	2	6	1,000	2+
Ripper Blades	4	15	900	3
Pulse Rounds	25	100	625	1
Plasma Energy	6	25	600	2
Flame Fuel	25	150	500	1/2
Force Energy	3	15	450	1

• Know what it's worth. Some item pickups are worth more than others. Judge what is the best use of your inventory space to carry/vend, and act accordingly. The table to the right is sorted in descending order by sell price:
• Manage Med Packs. Financially, it is optimal to sell all Medium Med Packs, and use Large Med Packs when you need to restore 5+ health bars.

Strategy

• Always save. Every time you come across a save station, use it, because even the weakest of enemies are a threat.
• Impossible is not that hard if you know the right way around:
  • During the first stretches of the game when you (and your enemies) are still weak, try and utilize the shoot-stomp combo (dismember a leg then stomp until the enemy stops moving), with Stasis for insurance. Regular Necromorphs go down very quickly with this method and the only ammo expended during the process is to shoot off their legs. Be cautious of your surroundings, though.
  • Use the Plasma Cutter, and the Plasma Cutter ONLY. For some reasons ammo drops for this weapon is absurdly common if you happen to have only it in your inventory. A single pickup is worth 6 shots, the same amount as on Medium setting. 
  • Health is also not much of an issue if the player is cautious. Health packs DO drop at a relatively pleasant rate, though most of them are initially Small-sized which forces the player to avoid being hit to prevent rapid consumption of healing items.
• Though it is not entirely legit, memorizing and using cheats to give yourself some money and Nodes can be a lifesaver during the first parts of the game. The stasis and oxygen cheats sees extensive use and should be exploited to give yourself an easier time with additional Stasis packs/Air cans on hand to sell for credits.
• Rotate saves. Keep two or more saves for one playthrough, and alternate between them each time you save. This way, if you end up stuck in one chapter without enough ammunition or health to get past, you have old saves to fall back on, so that you don't lose too much progress.
• Conserve ammunition. If you have decent aim, improvise, and use Stasis effectively with melee attacks in order to save ammunition for when it is needed. You could also reload save files in certain situations where you used too much ammunition.
• Make use of DLC Suits and weapons to the max! Not only they provide a better head start, they are FREE. On a difficulty where spending is crucial such as this, the extra 7000 Credits saved from buying a Pulse Rifle can really save your life.
• Restrict buying to a maximum. Weigh out your needs and wants and prioritize survival. If your gun is nearly out of shots while your health is around medium, buy more ammo. 
• Power Nodes are expensive. While the prospect of buying Power Nodes to open storage rooms for extra supplies or upgrading your gear to deal more damage sounds good, most of the time the reward doesn't worth the 10000 Credits spent to buy the Node itself and may cause disastrous shortages of money when you needed it most.
• In order to maximize ammo efficiency, fire in controlled bursts to the limbs, and avoid the ammo consuming secondary fire unless absolutely necessary when using automatic weapons such as the Pulse Rifle or Flamethrower. With precision weapons like the Plasma Cutter, always aim for the limbs of enemies (particularly the legs to topple them and really slow them down) to save ammunition and cause more damage to the target.
• You may wish to consider using a smaller number of weapons in order to maximize the ammo drops for your guns. However, always make sure you have the gun for the occasion. For example, at least one precision weapon like the Plasma Cutter or Pulse Rifle should be used on Drag Tentacles, and at least one area-of-effect weapon such as the Force Gun or Flamethrower should be used against crowds of enemies or Swarmers. Also, try not to bring the Flamethrower into vacuum areas, or the Ripper into areas where you will be fighting at a distance, as these weapons will become dead weight.
• Learn from your mistakes. If you die, for example, due to the Slasher that pounced on you that was just around the corner and cut you to the floor, think of other ways of approaching the problem rather than running in and bludgeoning it with the butt of your gun.
• If you are low on ammunition, or just wish to conserve it for later use, it is possible to avoid fights by running. Just make sure not to let yourself get mowed down while you're on the run, and briefly defend yourself when you're forced to stop (at a slow door, for example).

Trivia

• There is no bonus cinematic or extra for completing the game on Impossible mode, bar the Achievement.