Airless world

Airless world
The full Moon, photographed from the Earth · Gregory H. Revera, CC BY-SA 3.0, Wikimedia Commons

Read off: The Moon, Mercury

Grey dust and craters on craters under a black sky, where the sun burns white and the stars do not twinkle. With no air there is no weather, no sound and no shade that is not pitch dark; the ground swings between an oven by day and a freezer by night. Fine, sharp dust clings to everything. Water ice may hide in the craters at the poles that the sun never reaches, worth more than gold to anyone living there.

In figures

Gravity
0.17 g (the Moon); 0.38 g (Mercury)
Day
29.5 Earth days from sunrise to sunrise (the Moon); 176 (Mercury)
Air pressure
A few thousandths of a billionth of a bar: vacuum
Mean temperature
about −23 °C at the Moon's equator
Coldest
about −238 °C in craters the sun never reaches
Hottest
about 427 °C at noon on Mercury's equator; about 117 °C on the Moon
Radiation
1.37 mSv a day at the Moon's surface (Chang'e 4), about 200 times Earth's background
Liquids
None; water ice in polar craters that never see the sun
Weather
No weather; about 5 tonnes of comet dust strike the Moon every day
Light
Lit
Going
Slow

Hear it

Hazards

  • Vacuum. No air at all: an unprotected person stays conscious for about ten to fifteen seconds, the moisture on the tongue boils, and death follows within minutes. A suit, a helmet that seals and an air supply are the price of every step. D&D 5e: With no breath to hold, a creature without air gains 1 Exhaustion level at the end of each of its turns until it can breathe; it then loses all the Exhaustion it gained this way. Starfinder 2e: Vacuum (GM Core): no breathable air, so the creature suffocates, and it takes minor bludgeoning damage (1d6 to 2d6) every minute; with no propulsion it is untethered. Armour environmental protections cover vacuum for a number of days equal to the armour's item level (minimum 1). Archives of Nethys
  • Low gravity. Everything weighs a fraction of what it does on Earth: a person bounds rather than walks, falls slowly, and finds that stopping, turning and carrying momentum is harder than lifting. D&D 5e (House rule): House rule: jump distances are tripled, falling damage is halved, and a creature can lift and carry twice its usual weight. A creature unused to it has Disadvantage on Dexterity (Acrobatics) checks for its first day. Starfinder 2e: Low gravity (GM Core): Bulk is halved, so a creature carries twice as much and jumps twice as high and far; physical ranged attacks reach the 12th range increment. A fall deals nothing for the first 3 m (10 ft), then bludgeoning damage equal to a quarter of the rest of the distance. Archives of Nethys
  • Surface radiation. No thick air and no magnetic field to stop the particles of the sun and the stars. A day outside is harmless; months add up to a real risk of cancer, and a solar storm can bring a dangerous dose in hours. Shelters are buried or walled in rock and water. D&D 5e (House rule): House rule: no effect at the table for a few days. After each month spent mostly outside without shelter, a creature makes a DC 10 Constitution saving throw; on a failure its Hit Point maximum drops by 1d4 until it spends a month under shelter. Starfinder 2e: Cosmic radiation (GM Core): on a world with no atmosphere, creatures are usually exposed all the time to mild to severe radiation, which brings radiation sickness (mild: disease 1, Fortitude DC 17). A radiation buffer armour upgrade gives resistance and a bonus to the save. Archives of Nethys
  • Micrometeoroids. With no air to burn them up, grains of rock and metal arrive at many kilometres a second. Most are dust; now and then one punches a hole in a suit or a dome. D&D 5e (House rule): House rule: once a day outside, roll a d20; on a 1 a random creature's suit is holed. Patching it takes an action and a DC 10 Dexterity check with tinker's tools; until then the creature is in vacuum.
  • Day and night extremes. With no air to hold heat, sunlit ground bakes and shadowed ground freezes, a few steps apart. Work is planned for the dawn and dusk lines. D&D 5e (House rule): House rule: in full sun, a suit's cooling must be switched on or the wearer plays as in Extreme Heat; in shadow, its heating, or the wearer plays as in Deep cold. Each switch-over that fails (a DC 10 Intelligence check when a suit is damaged) costs 1 Exhaustion level. Starfinder 2e: Temperature (GM Core): in full sun the ground is incredible heat, in shadow incredible cold; each brings fatigue after 2 hours of travel and moderate fire or cold damage (4d6 to 6d6) every minute without a thermal capacitor. Archives of Nethys

What may happen

  1. A blue world hangs in the black sky, the only colour in sight.
  2. Old footprints, sharp as the day they were made: with no wind, nothing here is ever erased.
  3. The party steps into a crater's shadow and the world goes black but for helmet lamps, and the suit heaters click on.
  4. A tiny flash on the dark ground far off: a meteoroid striking at many kilometres a second.
  5. After sunset, a faint glow hangs above the horizon where charged dust floats over the ground.
  6. A crater at the pole whose floor has not seen sunlight in billions of years; the instruments read water ice.
  7. At a sixth of a g, walking is clumsy; the party ends up hopping like children in a sack race, kicking up dust that falls in perfect arcs.
  8. Back inside, the dust on the suits smells of spent gunpowder, and the smell stays in the hab for hours.
  9. The dust is sharp as ground glass. A visor wiped with a glove comes away scored with fine scratches.
  10. With no haze, a hill that looks an hour’s walk away is a day’s drive. Every distance here lies.
  11. A hammer falls onto the rock without a sound. Only the jolt of it comes up through the boots.
  12. Earth hangs in the same spot of sky as yesterday and the day before, only its phase changing: half lit today.
  13. The two-week night begins. The base dims its lights, the heaters take most of the power, and everyone grows quiet.
  14. At the polar ice mine a crew in heated suits cuts frozen soil under floodlights, in a crater that has never seen the sun.
  15. On this crater rim near the pole the sun hardly ever sets: it skims the horizon all month, and the solar farm here seldom sleeps.
  16. A flag left by the first visitors still stands, bleached white by decades of unfiltered sun.
  17. The habitat is buried under bags of packed soil against the radiation. Someone has painted a door and windows on the outside, for fun.
  18. A round pit opens in the plain, dropping into darkness: a skylight into an old lava tube, the best shelter on this world.
  19. A tour from the polar base, eight paying visitors in rented suits, follows a guide named Amara who has told the same jokes for six years.
  20. A winding valley crosses the plain like a dry riverbed. It was cut by lava long ago, not by water.
  21. On the inner world the sun is more than twice as wide as it looks from home, and its noon would melt lead.
  22. On the inner world the dawn line creeps across the ground at little more than walking pace. The convoy keeps just ahead of it, in the shade.
  23. From this spot on the inner world the sun rises, halts, sinks back a little and rises again, a quirk of its orbit.
  24. A panel left outside for a year is peppered with tiny glassy pits, each one a grain of dust that struck faster than a bullet.
  25. A call home lags just over a second each way, enough to make every joke land late.

Adventure hooks

  1. The ice in a polar crater was claimed by two colonies on the same day, and both sent armed crews.
  2. Footprints lead away from a sealed habitat into the night side, and nobody is missing.
  3. A solar storm is due in nine hours, and the expedition is twelve hours from shelter.
  4. An old mass driver still fires a capsule into space every dawn, and nobody knows who loads it.

Who and what is met

Each is an SRD stat block under another name, as game masters reskin them.

Other worlds

Cold desert world, Ice moon with a hidden ocean, Methane world, Hothouse world, Volcanic world, Ocean world, Tidally locked world, Earthlike world, Acid world, Jungle world, Swamp world

Sources

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

This product is licensed under the ORC License located at the Library of Congress at TX 9-307-067 and available online at various locations including paizo.com/orclicense, azoralaw.com/orclicense, and others. All warranties are disclaimed as set forth therein. Attribution: this rules layer is based on the following Licensed Material: Starfinder Player Core © 2025 Paizo Inc., Authors: Jessica Catalan, Thurston Hillman, Jenny Jarzabski, Mike Kimmel, and Dustin Knight; Starfinder GM Core © 2025 Paizo Inc., Authors: Jessica Catalan, Thurston Hillman, Jenny Jarzabski, Mike Kimmel, and Dustin Knight; Starfinder Alien Core © 2025 Paizo Inc. Authors: Kate Baker, Lau Bannenberg, Rigby Bendele, Vishesh Bhartiya, Joseph Blomquist, Jeremy Blum, Tineke Bolleman, Brent Bowser, Michael Bramnik, Patrick Brennan, Charlie Brooks, Jessica Catalan, Brite Cheney, Jeremy Corff, Caryn DiMarco, Anthony Dollinger, Steve Fidler, Kim Frandsen, Andrew D. Geels, Basheer Ghouse, Sen H.H.S., Katrina Hennessy, Thurston Hillman, Joan Hong, Jenny Jarzabski, Sara Jeffers, Mikko Kallio, Lysle Kapp, Mike Kimmel, Dustin Knight, Cole Kronewitter, Mahpiya, Letterio Mammoliti, Randal Meyer, Jacob W. Michaels, Matt Morris, Dennis Muldoon, Quinn Murphy, Elizabeth V Nold, Chesley Oxendine, Emily Parks, Glen Parnell, Randy Price, Kyle T. Raes, Jessica Redekop, Erin Roberts, James Rodehaver, David N. Ross, Pidj Sorensen, Joel Southall, Kendra Leigh Speedling, Drew Taylor, Sam W Tennyson, Jalen Todd, Ruvaid Virk, Viditya Voleti, Christopher Wasko, Nicholas Wasko, Jackson Wery, Shan Wolf, and Basil Wright. If you use our Licensed Material in your own published work, please credit us as follows: Biomes, Starfinder Second Edition rules layer © 2026 Michael Ghelfi. Reserved Material: the names of Biomes and its catalogues, its descriptions, sounds, art and code. Starfinder is a registered trademark of Paizo Inc.; this site is not published, endorsed, or specifically approved by Paizo Inc.

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