Cepheus Atom

Posted: 2021-06-13
Last Modified: 2023-04-06
Word Count: 1622
Tags: cepheus character-generation rpg traveller

While some of the Year Zero Engine games I considered last time make character generation and task resolution simple, they could be simpler.

Enter Cepheus Atom, a post-apocalyptic RPG using a radically simplified version of Cepheus, which itself is a debugged and somewhat streamlined derivative of Mongoose’s Traveller SRD. The author of Cepheus Atom, Omer Golan-Joel, describes his work as a “beer and pretzels” game in a “gonzo apocalypse” of mutants and futuristic weapons. Mutant Year Zero, which we touched on last time, is a somewhat more grim and gritty exploration of the genre.

Cepheus Atom – like Traveller and its many descendants – uses only six-sided dice, which the rules abbreviate as D. (1D is one die, 2D is two dice, etc.) I shall do the same below.

Making a Character

Creating a character is extremely easy:

  1. Roll 2D for Endurance.
  2. Roll 4D for Lifeblood.
  3. Distribute 5 points among the six skills, with none higher than 3. The skills are:
    • Combat - armed and unarmed conflict.
    • Knowledge - basic education, science, medicine, etc.
    • Physical - running, jumping, climbing, etc.
    • Social - communication with other sapient beings.
    • Survival - finding food and shelter and avoiding danger in the wasteland.
    • Technical - repairing and operating relics from the Before Time.
  4. Write down starting equipment, noted below. (I won’t walk through spending “trade units” in this post.)
  5. Give each ranged weapon 20 bullets/arrows/bolts worth of equipment.
  6. Decide on a name and a physical description.

CHANGED 2021-07-07: Reformatted using standard Markdown.

Name: Matthias the Graybeard

Description: A grizzled, taciturn man who hunts for technology from before The End.

Skills: Combat 1, Knowledge 0, Physical 0, Social 0, Survival 3, Technical 2

Endurance: 9

Lifeblood: 12

Equipment: food and water (3 days), backpack, waterskin, utility knife, … (100 Trade Units worth of items TBD)

The character sheet also has spaces for recording Contamination Levels (0 to 5) and Mutations acquired while poking around the wreckage.

Playing the Game

Basic Rules

As in all Traveller-based games, when the character attempts an action that may not succeed, the player rolls 2D, and adds modifiers for the relevant skill and other factors. If the total is greater than or equal to a Target Number (TN), usually 8 – written 8+ for historical reasons – the action succeeds; otherwise, it fails. Cepheus-based rules tend to call this a Throw.

So, for example, if Matthias came across an old car that didn’t work, he would roll 2d6 + 2 (for his Technology skill.) If the roll were 8 or more, he’d know why it didn’t work, and maybe start looking for parts to fix it. Otherwise he’d just strip out anything that looked valuable and move on.


Like in Traveller and many other games, combat proceeds in “rounds”. At the start of combat all participants roll 2D + Combat; the highest total goes first in each round, and so on until the lowest.

Each combatant makes a Combat Throw to hit their target, i.e. throw 2D, add Combat Skill, and try to get the target or more. If the target has a shield, the attacker subtracts 1 from the roll.1 The base TN is 8+ to hit within a weapon’s “effective range”, or 10+ for ranged weapons at longer range.

If an attack hits, the attacker rolls 1-5D depending on the weapon – 1D for a dagger, 4D for a battleaxe or shotgun – and the target takes that many points off their Endurance, or their Lifeblood when Endurance reaches 0. Damage first comes off Endurance, then Lifeblood. When Lifeblood reaches 0, the character or creature is dead. Assuming a character survives, Endurance points return after ten minutes of rest, while Lifeblood takes 5D days to heal.

The Rest

The rulebook handles most of the player-facing rules in six pages. The next six pages provide basic rules for wandering the wasteland, including movement rates in various terrain, weather, camping, foraging for food, scavenging for tech, and avoiding or surviving contaminated areas. Following those are six pages of rules for the effects of contamination and mutation, twelve pages of encounter tables and monsters, and six pages of high-tech relics to find in the wasteland.

Assessing the Game

What It Isn’t

In 50 pages Cepheus Atom defines a fully playable game. But it also, necesarily leaves things out:

Yes it’s unfair to compare a 50-page “beer and pretzels” RPG to longer and older works like Mutant Year Zero. But it does highlight how Cepheus Atom is primarily intended for one-shots or mini-campaigns.

Alternate Rules

On the other hand, as a streamlined, “beer and pretzels” game, the rules could conceivably be simpler still.

  1. In “Cepheus Quantum” by the same author all combat is simultaneous; all players and the GM roll in any order, and the results ony take effect at the end of the round. (The aforementioned AFF2 takes the same tack.) Perhaps that would make combat deadlier, in that without an initiative order a PC (for example) with a higher Combat skill and lucky dice couldn’t kill an opponent before said opponent got a chance to attack.

  2. The distinction between Endurance and Lifeblood reads as a holdover from the full Cepheus/Traveller rules where damage came first from Endurance and then from Strength or Dexterity. Barbarians of Lemuria, which I’ve also mentioned once or twice, has a single Lifeblood score; after combat, if characters get a chance to bind their wounds, they regain half the damage they lost in the immediately preceding fight. Everywhen, which extends the most recent BoL rules to other genres, even presents a convention for marking off temporary (/) vs persistent (X) damage.

  3. “Old School Rules” and their recent imitators use a “depletion die” mechanic to avoid tracking arrows, food, water, etc. Resources are tracked as a polyhedral die from D12 to D6 (or less); every time their used, the player rolls that die, and if it comes up a 1, the die is downgraded. (E.g. a D10 becomes a D8.) When the lowest lega die becumes a 1, the resource is out. In a system that only uses D6s, this might be hard, but not impossible; maybe D6 becomes D3, D3 becomes D2, and D2 becomes 0? (One could even simulate a D12 by rolling a D6; on a 1, reroll and downgrade if the result is odd.)

Arguably these simplifications might be going too far, although the second one (Lifeblood only) has no major downsides I can see.

What We Can Learn

All that said, Cepheus Atom’s core system has some good bones.

As written, Cepheus Atom doesn’t hold a candle to larger systems built around specific genres or engineered to be multi-genre. However, bigger systems have difficulty moving to new genres and settings because of assumptions baked into the system. By starting small one can build out the basic concepts to something like the full Cepheus system … or something more suited to a different genre entirely.


ADDED 2023-04-06

The author of Cepheus Atom has also written stripped-down versions of the main Cepheus system, including:

“Barbaric!”, “Lightning League”, and “Quantum Starfarer”, adapt the Quantum Engine to swords-and-sorcery, pulp superhero adventure, and star-faring science fiction, respectively.

  1. Subtracting 1 from the dice is equivalent to adding 1 to the TN. In many cases, e.g. a target with a shield, increasing the TN might be easier to calculate at the table than subtracting from every attack against that target … no matter what the Rules As Written may say. ↩︎