The universe is an endless expanse of adventuring potential. On its billions of worlds, physics create every possible permutation of geology, while life’s endless creativity gives rise to organisms both eerily familiar and defying imagination. Regardless of their design, all of these creatures struggle to survive and thrive in their native habitats, from icy seas and lush fungus jungles to the savage pyroclastic flows of tidally heated moons or the rusting hulks of ancient alien megastructures.

The following section contains rules to help you as GM adjudicate the game universe, including rules for the vastness of space, for various types of planets and the different terrains that may be found on them, and for environmental effects and hazards that may come into play in a variety of settings.


The immeasurable gulf of space is home to everything on the Material Plane, housing more stars and planets than could ever be recorded. During their careers, the player characters will undoubtedly need to venture into space. Traveling from one planet to another, exiting the atmosphere of a planetoid, or visiting an orbiting space station are all examples of common travel that require at least a brief time in space. Many hazards of space can be mitigated by wearing armor or a standard space suit, but sometimes unlucky spacefaring adventurers get caught without them!

Cosmic Rays

“Cosmic rays” is a catchall term for various interstellar radiation effects. They use the same rules as radiation. Most habitable planets maintain atmospheres capable of repelling these emissions. Such protected planets allow, at most, a low amount of radiation in infrequent bursts. Planets devoid of a protective atmosphere are constantly assailed by radiation of medium to severe intensity.


The void of space is effectively empty of matter, and this vacuum is perhaps the greatest danger of outer space. A creature introduced to a vacuum immediately begins to suffocate (see Suffocation and Drowning) and takes 1d6 bludgeoning damage per round (no saving throw). Because a vacuum has no effective temperature, the void of outer space presents no dangers from cold temperatures. A creature retains its body heat for several hours in a vacuum. Sound doesn’t travel in a vacuum.

Decompression occurs when a creature suddenly transitions from a pressurized environment to a vacuum, such as by being flung out of an airlock or being inside a sealed structure that becomes heavily damaged. Such a creature takes 3d6 bludgeoning damage (no saving throw) in addition to any suffocation damage.

Most creatures travel the vacuum of space in a starship.

Astronomical Objects

Most living beings begin their lives on floating astronomical objects. These planets, planetoids, and stars are the hub of much adventure and vary in complexity of design and makeup. A brief summary of the different types of astronomical objects is presented below, along with various rules associated with each.

Classification of Astronomical Objects

There exist several different types of astronomical objects. Summarized below are the most prominent types encountered during interstellar exploration.


An asteroid is a fractured chunk of matter, notable for being too small to be considered a proper planetoid. Asteroids commonly lack any sort of ecosystem and are often bereft of an atmosphere and breathable air. Many see asteroids as exploitable resources, given that they are often rich in minerals of varying rarity.

Gas Giant

As their name suggests, gas giants are worlds composed entirely of gas—frequently elements such as hydrogen and helium. They lack any natural solid surfaces to walk on and so have no proper ground. Creatures unable to fly or without flight-capable equipment or magic tumble toward the dense core of the world at the falling speed of a standard-gravity planet. Such a fall often takes days, given the immense size of these worlds. Near the center of a gas giant, a creature is subject to extreme gravity. The heart of a gas giant acts in many ways like a star (see Star below), including destroying creatures that don’t have full immunity to fire.

Irregular World

Some planets exist outside of the typical description of a (mostly) spherical mass of gases or silicate rocks and metals. These irregular worlds come in a variety of shapes, many of which are still considered theoretical. Some worlds might be artificially designed in the shape of a torus. Other worlds, like a planet in the form of a cube or a world that is entirely flat, exist as the result of cosmic abnormalities or the direct intervention of the divine.


Satellites are objects, such as moons, orbiting any other form of planetoid. “Satellite” is a classification that can be applied to other astronomical objects as well, as many asteroids and terrestrial worlds are also satellites. Unlike other types of astronomical objects, a satellite isn’t necessarily a natural object. Alien markers and space stations are but a few types of artificial constructs that hang in the gravitational field of planets. Some planets have only a single moon, while others (such as gas giants) boast dozens of objects caught in their gravitational fields.


A star—sometimes multiple stars—typically rests at the heart of a planetary system. Stars are massive balls of incandescent plasma that blast their orbiting planetoids with heat. While there are various categorizations of stars, from blue dwarf stars to yellow hypergiants, all stars produce enough heat to pose similar hazards to most adventurers. The surface of a star is so hot that only full immunity to fire allows a creature to survive there. Any creatures or items not immune to fire are instantly and utterly consumed down to the molecular level—only spells such as miracle or wish can bring back such victims.

Terrestrial World

Most people use the word “planet” to refer to a terrestrial world. The ones closest to the star of a solar system are the worlds most likely to be naturally habitable. They’re home to varying ecosystems, from barren, rocky landscapes to vibrant jungles of lush plant life and rushing waterways. Such worlds are sometimes categorized by their predominant features, leading to titles such as desert world, ice world, jungle world, and lava world.


An atmosphere is a layer of gases held in place by the pull of a planetoid’s gravity. The gravity and temperature of a planetoid impact its ability to retain an atmosphere. Most planets and planetoids support some manner of atmosphere. In addition to hospitable atmospheres, there are various other types of atmosphere that serve as hazards to most life.


As the name suggests, a corrosive atmosphere eats away at matter. The type and speed of the erosion varies, but the most common use of this term describes atmospheres capable of dissolving most matter. A typical corrosive atmosphere deals anywhere from 1 acid damage per minute up to 10d6 acid damage per round to creatures and objects within. Certain metals and treated materials may be immune to the specific atmosphere of a planet, and often the corrosion can be mitigated with dutiful preparation.

No Atmosphere

A creature on a planet without an atmosphere (or with an atmosphere so thin that it is effectively airless) is exposed to a vacuum.


A normal atmosphere is one that can support the majority of breathing life-forms. Most such atmospheres are composed of some combination of oxygen, nitrogen, and other nontoxic gases.


A nonacclimated creature operating in a thick atmosphere treats it as somewhat harmful, due to the extra chemical compounds in the air and the increased atmospheric pressure. Every hour, such a creature must succeed at a Fortitude save (DC = 15 + 1 per previous check) or become sickened. This condition ends when the creature returns to a normal atmosphere. Conversely, the increased weight of the air grants a +4 circumstance bonus to Acrobatics checks to fly or Piloting checks to keep an aircraft in flight.

Severely thick atmospheres are far more dangerous. Every minute, a creature in such an atmosphere must succeed at a Fortitude save (DC = 15 + 1 per previous check) or begin to suffocate (see Suffocation and Drowning) as its lungs cease coping with the density of the oxygen inhaled and lose the strength to keep pumping air into its bloodstream.


Thinner atmospheres tend to cause a nonacclimated creature to have difficulty breathing and become extremely tired. A typical thin atmosphere requires such a creature to succeed at a Fortitude save each hour (DC = 15 +1 per previous check) or become fatigued. The fatigue ends when the creature returns to a normal atmosphere.

Severely thin atmospheres can cause long-term oxygen deprivation to those affected in addition to the effects of a standard thin atmosphere. The first time a creature in a severely thin atmosphere fails its Fortitude save, it must succeed at a DC 25 Fortitude save or take 1 damage to all ability scores. A creature acclimated to high altitude (see Hill and Mountain Terrain) gains a +4 insight bonus to its saving throw to resist this effect.


Toxic atmospheres are composed of poisonous compounds and vary radically in their consistencies. Some toxic atmospheres are capable of sustaining oxygen-breathing life-forms, while others immediately suffocate those within them. Regardless of whether or not they allow creatures to breathe, toxic atmospheres are threats to most living creatures, as they act as an inhaled poison. Though the specific type of poison varies, many toxic atmospheres act as existing poisons but with radically different onset times and save DCs. Low-level toxic atmospheres can have onset times measured in hours or days, while heavily toxic atmospheres have onset times measured in rounds.


The following section includes information on a variety of biomes found on planets. Some planets could be entirely made up of a single biome, such as desert or forest worlds, while other planets contain a mix of the following terrain types.

Aerial Terrain

On worlds where the atmosphere expands high above the physical boundaries of the surface, there exists a region of open air. Similarly, gas giants are made up of nothing more than a vast atmosphere, held in place by a starlike core. The most common rules sections to reference when using aerial terrain are Falling, Gravity, Suffocation and Drowning, and Weather. The rules for flying with the Acrobatics skill are also critical for many creatures operating in an aerial environment.


Most clouds are little more than condensed gas that obfuscates vision. Treat a cloud in an aerial environment using the same rules as fog cloud, except it’s a nonmagical effect. Other types of cloud exist, such as corrosive or toxic clouds, which operate in the same manner as those types of atmospheres (see above).

Stealth and Detection in Aerial Terrain

How far a character can see in the air depends on the presence or absence of clouds. Creatures can usually see 5d8×100 feet if the sky is completely clear, with minimal clouds (or other aerial objects) blocking their views. Clouds generally provide enough concealment to hide within (though the hiding creature might have difficulty seeing out from its hiding place).

Aquatic Terrain

Aquatic terrain can be one of the least hospitable to PCs because most can’t breathe underwater. The ocean floor holds many marvels, including undersea analogues of any of the other terrain elements described in this section, but if characters find themselves in the water because they were bull-rushed off the back of a transport ship, the kelp beds or volcanic vents hundreds of feet below them don’t matter. The most common rules sections to reference when using aquatic terrain are Suffocation and Drowning and Underwater Combat. The rules for swimming with the Athletics skill are also critical for many creatures operating in an aquatic environment.

Deep Water

Lakes and oceans simply require a swim speed or successful Athletics checks to move through (typically, DC 10 in calm water, DC 15 in rough water, DC 20 in stormy water, and DC 30 in maelstrom water). Characters need a way to breathe if they’re underwater; lacking that, they risk drowning. When underwater, characters can move in any direction, including up and down.

Extreme Depths

At certain depths, the pressure of the surrounding water becomes so great that characters might be affected as if they were in a thick or severely thick atmosphere, even if they can breathe underwater.

Stealth and Detection Underwater

How far a character can see underwater depends on the water’s clarity. As a guideline, creatures can see 4d8×100 feet if the water is clear and 1d8×10 feet in murky water. Running water is always murky, unless it’s in a particularly large, slow-moving river. It is hard to find cover or concealment to hide underwater (except along the sea floor).

Desert Terrain

Desert terrain exists in cold, temperate, and warm climates, but all deserts share one common trait: very little precipitation. The three categories of desert terrain are tundra (cold desert), rocky deserts (often temperate), and sandy deserts (often warm). The most common rules sections to reference for adventures in these areas are Cold Dangers, Heat Dangers, Starvation and Thirst, and Weather.

Stealth and Detection in the Desert

In general, the maximum distance in desert terrain at which a creature can succeed at a Perception check to detect the presence of others is 6d6×20 feet; beyond this distance, elevation changes and heat distortion in warm deserts makes sight-based Perception checks impossible. The presence of dunes in sandy deserts limits spotting distance to 6d6×10 feet. The scarcity of undergrowth or other elements that offer concealment or cover makes using Stealth more difficult.

Forest Terrain

A forest can be composed of more than trees. On some worlds, vast fungal growths tower into the sky, while on others metallic veins rise from the ground and connect in spidery canopies. Common rules sections to reference for forests are Catching on Fire, Falling Objects, Smoke Effects, and Vision and Light.


Most forests are filled with trees, or something akin to trees, which provide partial cover to those standing in the same square as a tree. An average tree has an AC of 4, a hardness of 5, and 150 HP. A successful DC 15 Athletics check is enough to climb most trees.


Fungal blooms, vines, roots, and short bushes cover much of the ground in a forest. Undergrowth counts as difficult terrain, provides concealment (20% miss chance), and increases the DCs of Acrobatics and Stealth checks by 2. Squares with undergrowth are often clustered together. Undergrowth and trees aren’t mutually exclusive; it’s common for a 5-foot square to have both a tree and undergrowth.

Stealth and Detection in a Forest

In a sparse forest, the maximum distance at which a creature can succeed at a Perception check to detect the presence of others is 3d6×10 feet. In a medium forest, this distance is 2d8×10 feet, and in a dense forest it is 2d6×10 feet.

Because any square with undergrowth provides concealment, it’s usually easy for a creature to use the Stealth skill to hide. Logs and massive trees provide cover, which also makes hiding possible.

The background noise of a forest makes Perception checks that rely on sound more difficult, increasing the DC of the check by 2 (not 1) per 10 feet.

Hill and Mountain Terrain

Hill terrain describes rises in the immediate area, often multiple hills spread over miles. This type of terrain can occur in any other biome. Mountains are steeply rising rock, metal, or even the organic crust of the planet. The most common rules sections to reference when using hill and mountain terrain are Cold Dangers, Falling, and Weather.


Usually formed by natural geological processes, chasms are common dangers in mountainous areas. Chasms aren’t hidden, so characters won’t (usually) fall into them by accident. A typical chasm is 2d4×10 feet deep, at least 20 feet long, and anywhere from 5 to 20 feet wide. It usually requires a successful DC 15 Athletics check to climb the wall of a chasm. In mountain terrain, chasms are typically 2d8×10 feet deep.

Rock Wall

A vertical plane of stone, a rock wall requires one or more successful DC 25 Athletics checks to ascend. A typical rock wall is from 2d4×10 feet tall to 2d8×10 feet tall.

High Altitude

At particularly high altitudes, the thinning atmosphere poses a challenge for many creatures, with the same effects as a thin atmosphere. A creature residing at a high altitude for 1 month becomes acclimated and no longer takes these penalties, but it loses this benefit if it spends more than 2 months away from high-altitude terrain and must reacclimatize upon returning.

Stealth and Detection in Hills and Mountains

As a guideline, the maximum distance in mountain terrain at which a creature can succeed at a Perception check to detect the presence of others is 4d10×10 feet. In hill terrain, the maximum distance is 2d10×10 feet. It’s easier to hear distant sounds in the mountains. The DCs of Perception checks that rely on sound increase by 1 per 20 feet between listener and source, not 1 per 10 feet.

Marsh Terrain

Two categories of marsh exist: relatively dry moors and watery swamps. Both are often bordered by lakes, which are effectively a third category of terrain found in marshes. The most common rules sections to reference for marshes and swamps are Suffocation and Drowning, Underwater Combat, and Weather.


If a square is part of a shallow bog, it has deep mud or standing water of about 1 foot in depth. It counts as difficult terrain, and the DCs of Acrobatics checks attempted in such a square increase by 2.

A square that is part of a deep bog has roughly 4 feet of standing water. It counts as difficult terrain, and Medium or larger creatures must spend 4 squares of movement to move into a square with a deep bog, or characters can swim if they wish. Small or smaller creatures must swim to move through a deep bog. Tumbling is impossible in a deep bog.

The water in a deep bog provides cover for Medium or larger creatures. Smaller creatures gain improved cover. Medium or larger creatures can crouch as a move action to gain this improved cover. A creature with this improved cover takes a –10 penalty to attacks against creatures that aren’t underwater.

Deep bog squares are usually clustered together and surrounded by an irregular ring of shallow bog squares.

Stealth and Detection in a Marsh

In a moor, the maximum distance at which a creature can succeed at a Perception check to detect the presence of others is 6d6×10 feet. In a swamp, this distance is 2d8×10 feet. Vegetation and deep bogs provide plentiful concealment (20% miss chance), so it is possible to use Stealth to hide in a marsh.

Urban Terrain

Urban terrain can be found in most settlements where the people have greatly exerted their influence over the surrounding environment, constructing buildings where they can live and work in comfort and laying well-defined roads, usually paved. This type of terrain can occur in just about any biome, and it often supersedes the environmental effects of that biome. Urban terrain can include space stations, and it is often replete with technology. The most common rules sections to reference when using urban terrain are Settlements, Structures, and Vehicles, as well as Breaking Objects and sometimes Radiation.

Stealth and Detection in Urban Terrain

In a settlement with wide, open streets, the maximum distance at which a creature can succeed at a Perception check to detect the presence of others is 2d6×10 feet. In a settlement where the buildings are more crowded, standing close together, this distance is 1d6×10 feet. The presence of crowds might reduce this distance.

Thanks to twisting side streets and vehicles that can provide cover, it’s usually easy for a creature to use Stealth to hide in a settlement. In addition, settlements are often noisy, making Perception checks that rely on sound more difficult; this increases the DC of any such checks by 2 per 10 feet.


Weather can play an important role in an adventure. The following section describes weather common on most habitable worlds. Additional rules for cold and heat dangers can be found in Environmental Rules.

Rain and Snow

Bad weather frequently slows or halts travel and makes it virtually impossible to navigate from one spot to another. Torrential downpours and blizzards obscure vision as effectively as dense fog. Most precipitation is rain, but in cold conditions it can manifest as snow, sleet, or hail. If the temperature drops from above freezing to 32° F or below, it might produce ice.


Rain reduces visibility ranges by half, resulting in a –4 penalty to Perception checks. It has the same effect on flames and Perception checks as severe wind (see below).


Falling snow has the same effects on visibility and skill checks as rain. Snow-covered squares count as difficult terrain. A day of snowfall leaves 1d6 inches of snow on the ground.

Heavy Snow

Heavy snow has the same effects as normal snowfall but also restricts visibility as fog does (see Fog below). A day of heavy snow leaves 1d4 feet of snow on the ground. Snow at this depth counts as difficult terrain, and it costs 4 squares of movement to enter a square covered with heavy snow. Heavy snow accompanied by strong or severe winds might result in snowdrifts 1d4×5 feet deep, especially in and around objects big enough to deflect the wind—a reinforced wall or a large force field, for instance. There’s a 10% chance that a heavy snowfall is accompanied by lightning (see Thunderstorm).

Other Precipitation

There are other forms of precipitation, such as freezing rain, hail, and sleet. These generally function as rain when falling, but at the GM’s discretion, they may also have effects on movement similar to snow once they accumulate on the ground.


The combined effects of precipitation (or dust) and wind that accompany storms reduce visibility ranges by three-quarters, imposing a –8 penalty to Perception checks. Storms make aiming with ranged weapons difficult, imposing a –2 penalty to attack rolls, and archaic ranged weapons can’t be fired at all. Storms automatically extinguish unprotected flames. Storms commonly appear in three types: dust, snow, or thunder.

Dust Storm

These desert storms differ from other storms in that they have no precipitation. Instead, a dust storm blows fine grains of sand that obscure vision, smother unprotected flames, and can even choke protected flames (50% chance). Most dust storms are accompanied by severe winds and leave behind a deposit of 1d6 inches of sand. There is a 10% chance for a dust storm to be accompanied by windstorm-magnitude winds (see Table: Wind Effects); this greater dust storm deals 1d3 nonlethal damage each round to anyone caught out in the open without shelter and also poses a choking hazard (see Suffocation and Drowning). A greater dust storm leaves 2d3–1 feet of fine sand in its wake.


In addition to the wind and precipitation common to other types of storms, a snowstorm leaves 1d6 inches of snow on the ground afterward.


In addition to wind and precipitation, a thunderstorm is accompanied by lightning that can pose a hazard to characters who don’t have proper shelter (especially those in metal armor). As a rule of thumb, assume one bolt per minute for a 1-hour period at the center of the storm (GM rolls to hit). Each bolt deals between 4d8 and 10d8 electricity damage. One in 10 thunderstorms is accompanied by a tornado.

Powerful Storms

Very high winds and torrential precipitation reduce visibility to zero, making Perception checks and all ranged weapon attacks impossible. Powerful storms are divided into the following types.


Whether in the form of a low-lying cloud or a mist rising from the ground, fog obscures all sight beyond 5 feet, including darkvision. Creatures 5 feet away have concealment (20% miss chance).


Wind can create a stinging spray of dust, sand, or water, fan a large fire, rock an atmospheric transport midflight, and blow gases or vapors away. If powerful enough, it can even interfere with some ranged attacks and knock characters down. Below are the most common wind forces seen on habitable worlds.

Light Wind

A gentle breeze, having little or no game effect.

Moderate Wind

A steady wind often extinguishing small, unprotected flames.

Strong Wind

Gusts that automatically put out any unprotected flames. Such gusts impose a –2 penalty to nonenergy ranged weapon attack rolls.

Severe Wind

Nonenergy ranged weapon attack rolls take a –4 penalty.


Powerful enough to bring down branches, if not whole trees. Nonenergy ranged weapon attack rolls take a –4 penalty, while attacks with archaic ranged weapons are impossible. Perception checks that rely on sound take a –8 penalty due to the howling of the wind. Small characters might be knocked down.

Hurricane-Force Wind

Nonenergy ranged weapon attack rolls take a –8 penalty, and archaic ranged weapon attacks are impossible. Perception checks based on sound are impossible: all characters can hear is the roaring of the wind. Hurricane-force winds often fell trees. Most characters are knocked down due to the force of these winds.


All flames are extinguished. All nonenergy ranged weapon attacks are impossible, as are sound-based Perception checks. A creature in close proximity to a tornado that fails a DC 15 Strength check is sucked toward the tornado. All creatures that come into contact with the actual funnel cloud are picked up and whirled around for 1d10 rounds, taking 6d6 bludgeoning damage per round, before being violently expelled in a random direction (falling damage, described below, might apply). While a tornado’s rotational speed can be as great as 300 mph, the funnel itself moves forward at an average of 30 mph (roughly 250 feet per round). A tornado uproots trees, destroys buildings, and causes similar forms of major destruction.

Table: Wind Effects
Wind ForceWind SpeedRanged Attack Penalty
Light0-10 mph
Moderate11-20 mph
Strong21-30 mph-2
Severe31-50 mph-4
Windstorm51-74 mph-4
Hurricane75-174 mph-8
Tornado175-300 mphImpossible
* This applies only to nonenergy ranged weapons. Larger weapons, such as starship weapons, ignore this penalty.

Section 15: Copyright Notice

Starfinder Core Rulebook © 2017, Paizo Inc.; Authors: Logan Bonner, Jason Bulmahn, Amanda Hamon Kunz, Jason Keeley, Robert G. McCreary, Stephen Radney-MacFarland, Mark Seifter, Owen K.C. Stephens, and James L. Sutter, with Alexander Augunas, Judy Bauer, John Compton, Adam Daigle, Crystal Frasier, Lissa Guillet, Thurston Hillman, Erik Mona, Mark Moreland, Jessica Price, F. Wesley Schneider, Amber E. Scott, and Josh Vogt.