Please forgive me if this has been asked/posted elsewhere. I've searched the internet and this forum for it but all I seem to get is that entering the warp near a planetary body is "bad, and bad things happen"…nothing really cannonical or mechanical is ever listed. It is merely stated that it's "not good" and nothing else. So…what "bad things" happen when a ship translates from realspace into the warp in orbit around a planet? Earthquakes? Warp-stuff enters the atmosphere? Planetwide psychic phenomenon? Warp encounters? Ship explodes? Planet explodes?

I ask because in a recent session I had to make a call and didn't have much to work with so I pretty much improvised. A player, to escape certain destruction at the hands of a powerful grand-cruiser decided to literally slip into the warp pretty much within orbit above a planet. I gave some pretty big penalties to the Navigation rolls and such, but as luck would have it they managed all right. Despite their ship not being shredded, the translation wrecked havoc on the planet's surface. Minor quakes, some planetwide phenom and a few warp encounters. Just wondering how any of you would've handled it. Or if there has been any actual discussion on the mechanical ramifications of attempting this kind of maneuver?

I don't think there have actually been any discussions on the mechanics of it, but fluff indicates that quite aside from the effects of a multi-megatonne mass disappearing (sudden tidal effects,potentially including earthquakes; a massive localised pressure drop as air rushes into the now-vacant spot, assuming someone was insane enough to do so in atmosphere), every time a ship enters or leaves the Immaterium, a certain amount of random warp energy leaks into real space (known, according to William King, as a Probability Wake).
If a ship is far ennough away from a planet (i.e. all the way out in the system's Jump Zone), the most damage that's likely to do to the planet is muck around with probability on a low level (a few coins may land the other way when flipped, maybe every animal simultaneously decides to face west- little things), and bad/prophetic dreams for people with some degree of psychic sensitivity.

Closer the ship enters/leaves the warp to a planetary body, the worse the effects will get, until a translation in, say geostationary orbit (or closer) basically floods the ionosphere and upper atmosphere with bits of quasi-daemonic effluvia, and one side of the planet is directly exposed to the strange energies and not-quite radiation of the unbridled Empyrean. Every psyker on the planet not in a psycurium-lined bunker or otherwise heavily shielded is going to go insane, probably spectacularly so; every one on that side of the planet is at immediate risk of psychneuein (enslaver) infestation.
Mutation rates will increase massively worldwide, and there is a (potentially permanent) global weakening of the veil (meaning that any sorcery, summonings and psychicanery are going to be more effective, and more likely to attract …attention from the denizens of the warp). Planetwide psychic phenomena are not out of the question, particularly when we consider that everyone on the planet is going to suffer "warp dreams". Expect widespread rioting, with peoples' emotions feeding back into the under- and overlying Phenomena. If sufficient deaths occur in the early days, it's possible the ship's entry point will open up into a fully-fledged warp rift, with all the unpleasantness and potential daemon invasions that implies.

On the ship's part, I'm not sure if it's ever been fully explored in canon, but the assumption has generally been that safe transition to (and from) the warp requires a locally flat Lorentzian metric (that is, no significant local gravity wells). Attempting to transition in a gravity well would is implied/speculated to put a far greater strain on the ship's warp drive, with effects ranging from "failed to transit, drives may or may not suffer damage as a result" through "successful transition, but the drive needs a serious overhaul" and "successful transition- but ship is massively off course/out of control" to "ship has 'bounced' of the veil between warp and realspace, ship is in considerably more parts than before it attempted it and drifting helplessly through whichever realm it was in."

Incidentally, if the theory about requiring merely negligible gravity acting on the ship when it translates is correct, then there are a number of points/areas a ship could translate close to a planet (assuming it had at least one moon) without risking itself (although the dangers of its probability wake to the planet are… non-trivial to say the least)- it could transition at one of the Lagrange points, although plotting them in a 3(+) body system is a real pain, and piloting to one in a combat situation (as implied in your post) would be …interesting, depending on how the engagement's geometry started and played out.

Ship runs a roughly 30% chance of exploding if you enter within-system according to Frozen Reaches.

I think I remember some fluff that indicates leaving/entering the warp too close to a planetary body also gives the population terrible nightmares due to the warp miasma leaking through.

In one of the black library books (Deliverance, I believe), a ship translates into the warp near another ship, the (intentional) effect being that the ship was caught in its wake and pulled into the warp -- without its geller fields up, causing the entire crew to be hideously warped and mutated and ultimately destroyed by demons/pure warp energy.

So that said, I would think something similar might happen depending on how close you are to a planetary body. For one thing, any other ships nearby in orbit on your side of the planet would potentially be sucked into the warp and destroyed, and I could also imagine there being warp incursions on the surface as the portal to the warp would probably extend all the way to the surface. It's possible you could even end up sucking buildings/cities/land masses into the warp…not sure though, but it would be a pretty big consequence for the dynasty to deal with!

Capt_Dymock said:

I'm thinking Abbadon wouldn't have bothered with the Planetkiller if it was that easy to knock over a world. I think I'd cap it at severe weather imbalance and nightmares across a good chunk of the hemi-sphere for a few days. Very Bad Things happen to the ship that caused it of course, and if they enter the Warp from extreme proximity, I'd be tempted to say "Everybody burn one permanent Fate Point. Yes, really."

I seem to recall that during the Battle of Maccarage, the Imperial Navy flagship translated into the warp while orbiting an uninhabited gas giant in a desparate suicide tactic, and destroyed the core of an entire Tyranid Hive Fleet.

As has already been pointed out by Plasmafest, the efficacy of this as a military tactic makes one wonder why it's not used more often. If one can sacrifice one ship to destroy an entire enemy fleet, why isn't it routinely done?

I suppose the reasons would include:-

• The crew of the ship concerned effectively all go straight to hell immediately
• A very expensive ship is destroyed
• A warp rift capable of destroying an enemy fleet presumably is quite a small phenomena, and as such only works against tightly packed opponents, or those who are actually grappling with your vessel (as Tyranids are wont to do)
• Creating a warp rift like this requires a planetary body's gravity well to work

As to the impact on a planetary body, I wonder if this depends on the type of planetary body? As a warp translation would create a warp rift, maybe the way this rift will act depends on whether the world is inhabited or not? After all, the souls of the living (psykers especially) draw Daemons like moths to a flame. So perhaps creating a warp rift over an inhabited world is far more dangerous than creating one over an uninhabited world?

So in the Battle of Maccarage, a temporary warp rift above an uninhabited world doesn't really have much impact long term - there are no psykers to sustain the rift, and any daemons emerging will be trapped in the void of space. Whereas if such a rift appears over an inhabited world, the psychic energy of the populance could keep the rift open, especially if Daemons appear and start sacrificing the locals to keep it open.

The 30% chance of complete ship destruction if you translate near a planetary system sounds like a good rule to me - it's simple and brutal. I'd go further:

D100

001-030 - The ship is sucked into the warp immediately. All aboard are hurled into the sea of souls and torn to shreds by Daemons. The ship is either destroyed immediately or becomes a warplocked hulk, according to the whims of the GM.

031-066 - As above, but the warp translation also creates a warp rift above the planetary system. If this world is uninhabited, the rift fades after 1D10 hours. If the world is inhabited, Daemons will pour from it and attempt to keep the warp open by sacrificing the souls of the world's inhabitants-particyularly psykers- to use their psychic energies to sustain it.

067-080- As above, but the warp rift remains open for longer (5D10 hours) even without the Daemonic entities attempting to sustain it. The warp rift is so large that it acts in a similar way to a small black hole, and pulls entire cities from the world below into its maw before closing.

081-099- The translation works, but the psychic shock of it instantly kills all psykers (including Navigators) aboard the vessel and causes any psykers on the world below to have nightmares for months.

000- Nothing happens. The translation takes place perfectly, as if the ship was in deep space.

Inquisitor_VonDible said:

Did you consider having a look on the house rule page? Specifically here

Granted the discussion is about entry not exit, but still

Plasmafest said:

I would agree, which is why I suggested perhaps a chunk of land being affected. I don't think it would be able to destroy an entire planet, or else it would only take a single warp capable ship to take out some stubborn fortress world.

Pages 75 to 77 of Rogue Star by Andy Hoare describe what happened when a bulk carrier make a “tiny miscalculation” during a jump into the Warp. The spreading Warp influence from the disintegrating starship that covered inhabited areas caused humans to go insane. The big “I” sent a strike force to purify a moon and a continent.