I run a Pathfinder campaign where the PCs will soon fight undead in outer space. Probably on the exposed surface of the moon, maybe outside a large creature flying between stars, perhaps somewhere else. Definitely out of planetary atmosphere.

What is the full set of outer space environmental threats that the PCs will face, assuming they are on the surface of the moon, and it's identical to our moon around our earth? What I'm seeking are rules that describe the environmental effects. I suspect that Undead would be able to avoid most of those effects and living players would be severely hindered.

According to the rules,

Mass Planar Adaptation has no effect when cast upon your native plane

but if it did have the appropriate effect (if I fudged things and treated "surface of an earth-like moon" as a 'different plane'), I am wondering if Mass Planar Adaptation would address all the environmental effects.


3 Answers 3


Low Gravity

The Distant Worlds Campaign Setting actually has a few rules about low/no gravity:

Low Gravity (×1/2): Characters can jump twice as high and as far and can lift twice as much. Movement speed is unaffected. Each range increment for ranged weapons is doubled. Creatures that fall within an area of low gravity take 1d6 points of damage for every 20 feet fallen.

No Gravity (×0): Without magical flight, moving in an area with no gravity is difficult. A character with a surface to push off from can move up to half speed in any direction. A double move or charge can be performed this way, but not a run. A character can move at his full climb speed or his full land speed by succeeding at a DC 20 Climb check as long as he remains adjacent to a surface with sufficient handholds. He adds his Dexterity modifier (minimum 0) on this Climb check in addition to his Strength modifier. Once a character starts moving, he continues moving at the same speed in the same direction each round without using an action until he latches onto an object to stop himself, pushes off in another direction, or creates thrust somehow (each of which requires a move action). Creatures with nonmagical means of flight lose the ability to fly for 2d6 rounds after entering a no gravity area. A character in a no gravity environment can lift and carry 10 times his normal limit. Ranged weapons have no maximum range, and their range increment distances are multiplied by 10.

Since the moon has about 1/6 the gravity of the Earth, the rules for 1/2 Gravity suits us better for the desired effect.

As a house rule, you could extend the jump bonus and ranged weapon increment by 3 times what is listed, so a character could jump 6 times as high and lift 6 times as much weight, while ranged weapons would reach 6 times as far. This rule could be extended to any gravity that is different from earth, on a planet that has 1/10 the gravity, you could jump 10 times as high, while on a planet that has twice the gravity, you could only jump 1/2 what you could jump on earth.

Do notice that, despite the name, the rules for no gravity can actually be used on both to Zero Gravity and enviroments such as outside of a space ship or on the surface of an asteroid (Microgravity), since they do allow your character to move if you have a surface to push off from.

No air

The Enviromental Rules cover the lack of breathable air, or water, for aquatic creatures, or whatever you breath to stay alive:


A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. If a character takes a standard or full-round action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The check must be repeated each round, with the DC increasing by +1 for each previous success.

When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.

The space is not cold

Since the atmosphere of the moon is practically nonexistant, creatures on the surface of the moon are subject to the enviroment of space, which, unlike popular belief, is not exactly cold, that will depend on the surrounding stars (in our case, the Sun).

The temperature on the surface of the moon will depend on whether those creatures are on the bright side of the moon (about 123°C, or on her shadow (which is about -150°C).

But creatures are subject to the lack of warm from a planetary normal atmosphere, and will eventually lose body heat.

Again, the Enviromental Rules have a set of rules regarding cold enviroments, though these rules were designed for cold weather rather than space travel.

If the creatures are on the moon's shadow, you could say they are dead within a few turns unless they have cold immunity somehow.

Extreme cold (below –20° F) deals 1d6 points of lethal damage per minute (no save). In addition, a character must make a Fortitude save (DC 15, +1 per previous check) or take 1d4 points of nonlethal damage.

If they are on the moon's bright side, they should actually be vulnerably to Extreme Heat instead of cold, and are probably vulnerable to solar radiation. But rules as written, they only really need Energy Resistance (cold) 6, as these rules don't make a distinction between -20°C and -150°C.

In severe heat (above 110° F), a character must make a Fortitude save once every 10 minutes (DC 15, +1 for each previous check) or take 1d4 points of nonlethal damage. Characters wearing heavy clothing or armor of any sort take a –4 penalty on their saves. A character with the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the Survival skill in Using Skills). Characters reduced to unconsciousness begin taking lethal damage (1d4 points per each 10-minute period).

For solar radiation, there are published rules on the Technology Guide aswell that you could use those on top of the rules for Extreme Heat. The rules for radiation, however, do not account for a creature that has fire resistance, so you might want to reconsider using them if the character is resistant/immune to fire damage.


The Starsoul Sorcerer Bloodline has an ability called Breaching the Gulf:

(...) once per day you can teleport a single creature within 30 feet into the void of space if it fails a Will save. The save DC is equal to 10 + 1/2 your sorcerer level + your Charisma modifier. The target can attempt a new saving throw as a full-round action each round to return. While trapped in the airless void, the target suffers 6d6 points of cold damage per round and must hold its breath or begin to suffocate.

However, this is one ability and does not cover the enviroment in general.

This topic on paizo's forum has interesting ideas on how to handle vacuum situations. One of which is from d20 Future and i will quote here:

On the third round of exposure to vacuum, a creature must succeed on a Constitution check (DC 20) each round or suffer from aeroembolism (“the bends”). A creature that fails the save experiences excruciating pain as small air bubbles form in its bloodstream; such a creature is considered stunned and remains so until returned to normal atmospheric pressure. A creature that fails the Constitution check by 5 or more falls unconscious.

The real danger of vacuum comes from suffocation, though holding one’s breath in vacuum damages the lungs. A character who attempts to hold his breath must make a Constitution check (DC 15) every round; the DC increases by 1 each round, and on a successful check the character takes 1 point of Constitution damage (from the pressure on the linings of his lungs). If the check fails, or when the character simply stops holding his breath, he begins to suffocate. In the next round, he falls unconscious with 0 hit points. The following round, he drops to –1 hit points. On the third round, he drops to –10 hit points and dies.

In my opinion, though this rule is unofficial and for a different d20-based system, it does accurately simulate the desired effect.

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – mxyzplk
    Commented May 8, 2017 at 21:17

I can think of four environmental effects:

  1. extreme cold
  2. no air to breathe, so suffocation
  3. vacuum (lack of pressure)
  4. radiation

The Core Rulebook has details in the Environment section on Extreme cold and Suffocation. Radiation is detailed on page 55 of the Pathfinder Technology Guide from Paizo as a poison.

For "lack of pressure" the closest official rules I found are the CR rules for excessive pressure under water:

Very deep water... deals water pressure damage of 1d6 points per minute for every 100 feet the character is below the surface. A successful Fortitude save (DC 15, +1 for each previous check) means the diver takes no damage in that minute.

  • \$\begingroup\$ There are rules for low gravity environments which are also a good choice to include in this answer. Anyway, the effect of vacuum on the human body has been grossly overstated by various media: if you can ensure adequate air despite the vacuum, the vacuum itself will be extremely uncomfortable or painful, but not actually hazardous in and of itself. On the other hand, the vacuum has a strong effect on suffocation (it would violently force the air from your lungs, and trying to hold your breath would only damage the various tissues you put in the way), and might also cause the bends. \$\endgroup\$
    – KRyan
    Commented May 8, 2017 at 20:23

Low Gravity

Pathfinder has rules for low gravity:

Low Gravity

Low-gravity areas, such as those in which the effects of multiple graviton-based artifacts interact in unpredictable ways, are PC playgrounds, in which characters’ relatively hyper-developed muscles are far more effective than normal.

In an area with only a third of standard gravity, for example, PCs can jump three times as high and as far and lift three times as much. (Movement speed, however, stays the same, as moving in great bounds can be awkward and difficult to control.) Projectiles have their range categories tripled.

The moon’s gravity is a sixth of Earth’s, so for the moon it would be jumping six times as high and as far, lifting six times as much, projectiles having range multiplied by six, and so on.


Contrary to popular conception (and the only existing Pathfinder rules on the subject, the starsoul sorcerer bloodline; thanks ShadowKras for finding that one), space is not cold. Frost would form as any moisture on the skin rapidly evaporates (which, of course, cools the surface it evaporates from), but the damage of that would be minimal.

After that, the concept of “temperature” is kind of difficult to apply to space, but the long and short of it would be that someone on the moon would effectively have exceptionally good thermal insulation. Without an atmosphere, there is no convection on the moon and conduction would only be through the feet to the lunar surface—which is a pretty poor conductor itself.

But their body would still produce heat. And if they were standing in direct sunlight, they would also be absorbing quite a lot of heat, since relatively slow or not, the sun is a ridiculously colossal heat source and its radiation is intense. So heat, rather than cold, is the primary concern.

The exception may be the dark side of the moon. On the dark side of the moon, people are standing on extremely cold lunar dust (−250°F), but it is a poor conductor of heat and that is only a small point of contact, so it may not offset the heat produced by their bodies—it is conceivable that they would get frostbite while also suffering from heat stroke!

(For reference, the Apollo missions avoided all this by scheduling all extra-vehicular activity at lunar dawn, and of course by having highly reflective and insulated spacesuits. I have not been able to find much in the way of convincing data on what would happen if standing on the moon bare foot.)

Pathfinder has rules for heat dangers, but they don’t apply very well to the moon. Even the rules for frostbite are pretty limited, so only “A character who takes any nonlethal damage from cold or exposure is beset by frostbite or hypothermia (treat her as fatigued).”

Work in Progress


The moon lacks a magnetic field, which is the primary protection a planet extends against radiation for its inhabitants. Pathfinder has rules for radiation, but they annoyingly lack any good way to determine if something is a low, medium, high, or severe danger. But I would go with low here—being out in space is not the same as sitting next to an unshielded nuclear reactor or something, and the damage caused by the radiation would, realistically, be a very long-term thing. Acute radiation poisoning does not seem to be a risk of exposure to outer space.

Moon dust sucks

This was actually one of the more surprising scientific discoveries by the Apollo missions: the stuff gets everywhere, and sticks to things surprisingly well.

It’s also really abrasive.

The dust was so abrasive that it actually wore through three layers of Kevlar-like material on Jack [Schmitt’s] boot.

– Professor Larry Taylor, Director of the Planetary Geosciences Institute, University of Tennessee (2008), quoted here

So if your unwitting lunar explorers are barefoot, or even wearing regular shoes, damage to the feet is quite likely. Caltrop rules are appropriate here.

On top of that, super-fine, sharp, abrasive material that gets everywhere? You do not want that in your eyes or lungs. Breathing in moon dust would be a serious hazard, getting it in your eyes would cause serious irritation at the very least. So if some magic effect allows them to breathe, but doesn’t cover their mouths, the dust itself could easily interfere with breathing. Without eye protection, blindness is potential hazard. And low gravity makes it surprisingly easy for it to get places, even in the absence of atmosphere.

Work in Progress


The moon has no atmosphere. Pathfinder has rules for suffocating due to lack of oxygen, but those rules still presume a pressurized environment of some sort, which the moon isn’t.

The effects of vacuums on the human body have been grossly overstated by media; you do not explode nor does your blood boil if subjected to a vacuum. However, the results are still quite unpleasant.

To begin with, you cannot hold your breath in a vacuum without some kind of pressurized breathing apparatus (e.g. oxygen mask). Moreover, the reduced ambient pressure can actually force oxygen out of a person’s blood, causing rapid progression to hypoxia. In game terms, that amounts to applying the suffocation rules immediately, rather than after many rounds and a failed Constitution check:

In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.

That results in 18 seconds before death. The reality is not that severe (estimates as high as 90 seconds exist), but this is more consistent with Pathfinder rules.

Additionally, attempting to hold one’s breath when subjected to vacuum could badly damage lung and throat tissues. If someone is exposed to vacuum very suddenly (decompression from 1 atmosphere to 0 in under half a second), it may be physically impossible to safely expel air from one’s lungs fast enough to avoid this. At slower time frames on the order of a few seconds, I would probably call for a Reflex save around DC 15-ish—not terribly hard by PC adventurer standards, but not automatic, either, and you only get to attempt the save if your character knows enough to exhale immediately (attempting to pin a Knowledge DC on this would be far too campaign-specific for me to venture a guess here, I think).

In terms of turning that damage into rules, I think attempting to describe this damage in HP terms is kind of missing the point: this would be a death sentence on a very short time limit. I allow (any) magical healing to fix the damage, but it would have to be applied before that person’s third turn after being subjected to the vacuum, even if pressure was restored.

Considering how quickly suffocation occurs, without an oxygen mask we can probably stop considering the effects of the moon’s environment there.

But if we have a pressurized oxygen mask (or magical replacement of some sort), but no pressurized space suit, then the other effects of vacuum also apply. Human skin and blood vessels are tough enough to maintain structural stability and prevent blood boiling, but there is still paralyzingly-painful swelling and expansion of blood vessels can cause a dangerous drop in blood pressure. Moreover, the formation of bubbles in the vessels can be extremely dangerous (decompression sickness or the bends).

To turn those into game terms, I say that anyone subjected to vacuum without any kind of pressure protection, must make a stiff Fortitude save—DC 25 seems appropriate—or else be staggered by the pain, swelling, and/or decompression sickness (which might be generous, paralysis might be more appropriate, but I prefer to still give someone in such situations a chance to do something about it). If such a person also has some way to breathe despite the vacuum (or doesn’t need to), they could operate in this way for some minutes.

As for what happens if someone with an oxygen mask but no other protection from the vacuum continues to operate for extended periods of time, so far as I can tell, the human race does not know. That situation has never happened to anyone, and no one’s volunteering to try. Personally, I just leave it at periodically rerolling Fortitude saves against staggering every minute.

  • \$\begingroup\$ Need to get going even though this is incomplete. I figure there’s enough here to be useful, even if I haven’t written up how to handle moon dust and temperature within Pathfinder yet. \$\endgroup\$
    – KRyan
    Commented May 8, 2017 at 21:43

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