Arcane Game Lore

Gaming is not in the blood, it's in the dice!

Designing Out Loud – Skill System – Skill List

Okay, now that we’ve talked about the skill resolution mechanic and skill improvement costs, it’s time to start looking at the skills themselves.  As I mentioned in the previous post, this system will have broad skills divided into even broader skill categories.

The Skills

This week’s post will just be the skill categories and the skills they contain.  Each skill will be listed in the category that it most likely applies to.  However, with a good justification, some skills may be available through different skill categories.  In the places where I’ve thought of them, I’ll mention it.  In play if a player can justify why a skill should fall under their skill category, the referee should give it a serious consideration and allow it.  So let’s dive in.

Agent with trenchcoat, briefcase, and gunAgent Skill Category

These skills are about activities that are often on the wrong side of established laws or customs.  They cover activities such as espionage, theft, coercion, and other deceptions.

  • Detective – Covers activities related to gathering and connecting information  like listening for and spotting clues, surveillance, searching, gathering information, reading facial expressions and body language, etc.
  • Persuasion – Covers any activity trying to convince others to do something that you want whether it is against their will or not such as bribery, charm, intimidation, cons, etc.
  • Stealth – Covers activities related to moving silently and unobserved such as hiding, concealment, shadowing a target, etc.  This skill may also be gained through the Scout skill category.
  • Thievery – Covers activities related to acquiring others property such as lock-picking, forgery, slight of hand, pick-pocketing, etc.  It also applies to knowing where to dispose of such ill gotten gains safely.

Female artist working at desk.Artisan Skill Category

This skill category covers all types of artistic endeavors, including creation, interpretation, and evaluation of the various expression forms.  Those who specialize in this area have an easier time learning a musical instrument, putting on a show, giving a speech, or creating things whether it be written, a sculpture, a painting, a song, or some other form of creative, artistic expression.  This category includes the following skills, although as a Referee, if you want the skills to be more specialized, feel free to adapt them for your campaign by adding more.

  • Composition – Covers the various forms of writing whether it be poetry, prose, screenplays, music, or something else.
  • Performance – Covers all forms of acting as well as singing, dancing, and public speaking
  • Sculpture – Covers sculpting in various media as well as areas such as pottery, woodworking, leatherwork, and metalwork.
  • Visual Art – Covers the visual media such as painting, drawing, photography, and holography

Language Skill Category

Someone with the Language skill category as their primary or a secondary category has the gift of tongues and learns and masters languages easily.  Each language (or maybe language group) in your game is a separate skill.  All characters are assumed to have a level 5 skill in their native language as well as a level 4 skill in the common tongue if one exists in your campaign.  The various skill levels represent varying levels of language mastery as give below:

  • Level 1 – Memorized phrases, can reads a few words.  Allows for limited/halted conversations
  • Level 2 – Slightly better conversation with greater vocabulary and limited reading/writing capability
  • Level 3 – Basic fluency but possibly with a strong accent.  Able to read and write the language
  • Level 4 – Full fluency and literacy in the language
  • Level 5 – Ability to detect, pick up, and imitate local dialects.  This is the level of a native speaker raised speaking the language and anyone who achieves this level in a language can be mistaken for a native speaker.
  • Level 6 – Academic scholarship – This level is reserved for those who have dedicated much effort to understanding the history and entomology of the language and its subtle and obscure corners.

The exact skill list for this category depends on the campaign but should include at least one skill for each alien race (including humans) and a common language.  If playing a single species campaign (e.g. just humans) the Referee may assign skills for specific language groups (Slavic, Germanic, Romantic, Asian, etc) or even individual languages if that suits the style of the campaign.

soldier with body armor and laser rifleMilitary Skill Category

This category covers all of the martial arts from unarmed combat through pistols and rifles and on up to starship weaponry.  Characters with these skills are effective at acquiring and eliminating their targets.  These skills include the use and maintenance of the weapons they cover.  This category contains the following skills:

  • Archaic weapons- Covers older weapons like bows, crossbows, atlatls, ballistas, and even trebuchets.
  • Demolitions – Covers the use of explosives as well as setting, detecting, and deactivating charges
  • Energy weapons – Covers weapons that emit a beam of energy such as lasers, electrostunners, and sonic weapons
  • Melee weapons – Covers any type of melee weapon from brass knuckles to swords to clubs to morning stars.
  • Projectile weapons – Covers any weapon that fires a physical projectile such as regular guns, gyrojet weapons, rocket launchers, and needler weapons.
  • Starship weapons – covers the operation and firing of starship based weapons whether energy or projectile based.  (Note: if the referee desires they may split this into two skills for the two weapon types).
  • Thrown weapons – Covers all weapons thrown by hand such as grenades, knives, javelins, and spears
  • Unarmed combat – Covers any combat using just a creature’s natural limbs without using additional weapons

Pilot with helmetPilot Skill Category

This category covers the operation of vehicles, both civilian and military, in stressful for dangerous situations.  Characters with good pilot skills can maneuver their vehicles through confined areas, across dangerous or treacherous terrain, and quickly recover if control is lost.  Each skill in this category covers a different type of craft:

  • Air – Covers any fixed or rotary winged aircraft such as helicopters, air planes, air cars, jets and blimps
  • Ground – Covers any vehicle that moves via direct contact with the ground such as cars, trucks, motorcycles, and tanks.
  • Hover – Covers any vehicles that move on a cushion of air such as hovercycles, hovercars, etc.  These vehicles may or may not be able to move across water.
  • Water – Covers any water going vessels such as boats, ships, or submarines
  • Systemship – This covers any short range or in-system spacecraft that do not have FTL capability such as shuttles, inter planetary vessels, or fighters.
  • Starships – Covers any spacecraft large or small that is capable of FTL travel

A midevial scholar surrounded by notes, books, and globesScholar Skill Category

This category covers skills about knowledge, whether “scholarly” or not.  Scholars are good at making connections between different areas and ideas and using that information to assist them in whatever situation they may find themselves in.  The skills is this category cover both basic knowledge of a subject (i.e. roll to see if you know the information) and the ability to do research in the topic to discover the information (i.e. roll to see if you can find/discover/research the data).  The scholarly skills in this category are:

  • Architecture – Covers the design, construction, and history of buildings and other structures
  • Economics – Covers anything to do with financial matters, markets, industry, etc.
  • History – Covers knowledge of the past, whether people, places, or events
  • Law – Covers the legal systems in use and how to work with existing laws and law enforcement agencies and the judicial systems
  • Literature – Covers knowledge of authors and their works both past and present
  • Philosophy/Theology – covers knowledge of the various religions and philosophies in the game
  • Politics – Covers the understanding of politics and bureaucracies both government and corporate, large and small
  • Pop Culture – Covers a knowledge of current events, people, and places
  • Xenotechnology (Species) – Covers knowledge of the technology of a single alien species. See full description under the Technology skill category

Doctor holding a vialScience Skill Category

This category covers all the sciences, both physical and biological.  Someone with this skill category has an analytical mind and is good at reasoning out problems and puzzles.  Scientists develop hypotheses, design tests, and prove theories in an attempt to understand and control the world around them.  Skills in this category include:

  • Earth Sciences – This covers items related to land, water and air sciences such as geology, chemistry, and meteorology.
  • Life Sciences – This covers things like biology, zoology, and other living things including eco-systems and the things that impact them.
  • Medical – Covers all aspects of medicine from basic first aid, to treating diseases and poisons, to major surgery.  As an optional variation, the Referee could require this skill to be species specific, i.e a Medical (Human) skill being need to work on humans and a Medical (Klingon) skill being needed to treat a klingon.
  • Psyco-Social – Covers the study of the mind, the psyche and and things like hypnosis and the unconscious mind
  • Space Sciences – This covers the sciences related to outer space and includes astrogation, space physics, and astronomy.
  • Xenotechnology (Species) – Covers knowledge of the technology of a single alien species. See full description under the Technology skill category

Note:  The skills here are very broad.  This is for ease of play.  For a more realistic handling of these types of skills, the referee could have individual skills for each science or application.  For example the Space Sciences skill could be divided into Physics, Astronomy, and Astrogation as skills that have to be learned independently.  This is more realistic but makes skill advancement harder and creates more skills that have to be adjudicated during the game.

Woman with backpack overlooking a mountain valleyScout Skill Category

This category covers the skills of an outdoorsman or someone skilled at working in and with nature.  A good scout can live off the land and survive adversity given the necessary resources (or time to find them).  The skills in this category include:

  • Animal Handling – Covers the care, training, and use of animals
  • Athletics – Covers such things as climbing, running, jumping, etc
  • Mariner – Covers such things as swimming, diving, operating terrestrial water craft, and navigating on water ways, including rivers, lakes, seas, and oceans.
  • Navigation – Covers all aspects of terrestrial navigation, cartography, etc.
  • Stealth – Covers activities related to moving silently and unobserved such as hiding, concealment, shadowing a target, etc.  This skill may also be gained through the Agent skill category.
  • Survival – Covers the ability to live of the land, find/create shelter, hunting, tracking, etc.

Engineers in clean suits standing around a piece of space technologyTechnology Skill Category

This final category covers all technical skills from basic machinery up through starship drives and everything in between.  Characters with this skill category are good with their hands and like to tinker and build stuff.  They can repair existing items and keep the running or make new things as needed.  The skills in this category include:

  • Architecture – Covers the design, construction, and history of buildings and other structures (e.g. bridges, railways, radio towers, etc) and includes civil engineering.
  • Computers – Covers all aspects of computer use from building them to programming them to repairing them and also includes how to create and circumvent computer security measures
  • Robotics – Covers all aspects of working with robots including repair, programming, and modification
  • Starship Engineer – Covers all aspects of spacecraft engineering from designing ships to building them to keeping them running and an understanding of all systems on the ship.
  • Technician – Covers all basic mechanical and electrical skills both for machines and vehicles.
  • Xenotechnology (Species) -  his skill represents studying the alien culture and understanding how it works in relation to how it creates and operates it machinery and is designed to help overcome the -20% alien technology penalty. This skill only goes up to level 2, and is harder to earn than would be normal, costing one extra XP to earn level 1 and two extra XP to earn level 2. The skill must be learned for each race individually but each level overcomes 10% of the penalty applied to using alien technology. Thus if a human had a Level 1 Xenotechnology (Klingon), when using technology specifically designed for klingons, that would normally carry a -20% modifier, he only suffers a -10%. If he raised his skill to level 2, the penalty would be removed. This skill would not apply to any technology designed for any of the other races.

Final Thoughts

Whew, that was a long post with a lot of information.  This system is very heavily influenced by a skill system that Bill Logan developed as an optional skill system for the Star Frontiers game which was published in the A Skilled Frontier article in issue 9 of Star Frontiersman magazine.  Many of the names you see here are the same since there just isn’t anything better to call them and the categories are very similar.  I’ve made some changes to some of the skills and where they fall based on what I think works better but the parallels are very strong.

These skills are not necessarily exhaustive.  Referees and players are encouraged to develop additional skills in the specified categories that apply to their games as needed.  There will probably be some tweaking to this as we move along but I thing this sets a good skill foundation for the game.

What do you thing of this skill system?  Too broad, to specific? Overly complicated? or just right?  Let me know in the comments below.

3D Modeling – A Big Print

So at work we got a new 3D printer (It’s an Ultimaker 2 Extended or if you want the Amazon page).  I happily volunteered to test it out and put it through its paces.  And I have to say, this is one very nice 3D printer.  However, like all printers, it is not without its share of problems.

As part of testing we wanted to do some “torture tests” and run some large, long prints.  The first of these turned out to be something for one of the library patrons.  He was printing a large nose cone for a model rocket that was just over 3 inches in diameter and 8 inches tall.  Now our current printer can print that but it has to be on its side (build area is 10″ wide by 6″ deep by 6″ tall) which for this particular shape is less than desirable as you get supports running up the side that mar the surface.  That wasn’t working so we decided to try it on the new printer, which can print it straight up since its build area is 8″w x 8″d x 12″ tall.  The print took almost 17 hours at 0.2mm resolution and worked perfectly.  Although it looked like an 80mm artillery shell when it was done and we’re not supposed to print weapons :) .

A Big Ship

So after that it was time to try a 0.1 mm resolution test.  The goal was to print something both wide and tall and with some surface features and details that would require supports and a long print time.  I had just the thing.  I grabbed Jay’s Assault Scout model that I described printing and painting in an earlier post.  Instead of printing it 4″ tall, I scaled the model up to 12″, the full height of the printer.  This made the wing span 10.2″ which is just under the 11.3″ diagonal size of the print area.  This print would fill the bed both horizontally and vertically, a perfect test.

Setting the resolution to 0.1 mm per layer gave me an estimated 42.5 hours to complete the print.  One of the reasons for the long test is that our older printer has a tendency to clog up on longer prints (even at 0.2 mm resolution) and we don’t even print at 0.1 mm resolution on that printer anymore because it clogs even on small prints.  This would be a great test.  I started the print on Friday morning so I could watch it get started and make sure everything seemed to be running okay but then let it run over the weekend.  After a couple of hours it looked like this:

The first 15mm of the printed Assault Scout

Printing at about layer 150

At this point it is just starting to print the back side of the fuselage in the middle.  The back side of the engines are done and the wings are connected to the engines are just starting to connect to the fuselage.

I even grabbed a quick video of the printing.  The quality isn’t that great as I was just holding my phone in my hand. This is real time and it is printing just one of the 3000 layers in the model.  You also get to listen to the sound of the printer, which has been my constant companion for the last few weeks as I’ve been putting it through it’s paces.  This is one of the layers that takes longer to print as there are a lot of features on that level but it gives you an idea of the process.

I snapped this picture just before I left to go home for the weekend:

A bit further on in the print.

Printing layer 195

Here we’ve moved up a bit further in the print and it is just finishing up with the printing of all the details on the back of the fuselage.  At this point all there really is to print is the infill (10%) and the outer surface of the model and each layer will go much faster.   It took about 6 hours to get to this point and we’ve only printed 195 of 3000 layers but they were the most complicated layers in the print.

I almost came in on Saturday and took a look to see how it was going (it would finish sometime late Saturday night/early Sunday morning) but even though I was on campus, I didn’t have time to run over to my office in the library and peek. (I was helping my wife get her night’s observing set up on the campus observatory.)

When I got to work Monday morning, this was the sight I was greeted with:

The fully completed Assault Scout Model still sitting on the printer.

The completed print

What a beautiful sight!  As you can see it pretty much fills the print volume.  When I picked it up off the print bed, I was surprised at how light it was.  (It weighed in at 170 grams or 6 oz).  For some reason, I was expecting a little more weight to it.

The print was almost perfect.  If you look closely at the image, there is a lighter color line running across the print right at the level of the top of the wing where it connects to the engines.  For some reason, about 15 layers here suffered an under extrusion problem (not enough plastic flowing through the nozzle).  That would have happened some time around Saturday morning during the print process.  I’m not sure if it is a problem with the slicing (the code that drives the printer) or a filament issue.  The only way to be sure would be to print another one and see if the exact same thing happens.  The good news is it recovered and to some extent the print quality above that point is actually better than below it.

Other than that one little section of under extrusion, which I’ll need to do something to fill in the small holes before I paint this, the print is amazing.  The surface is really really smooth and the surface details came out nice and clean.  Additionally, because the print was so large, some of the details, especially on the back of the ship, showed up much better than they did on the smaller 4″ model.  Speaking of which, here are the two models side by side:

The smaller painted Assault Scout model next to the newly printed large one. The larger print is 3 times taller and wider than the smaller one.

The smaller painted Assault Scout model next to the newly printed large one.  I haven’t yet removed the printing supports.

I can’t wait to get this big one painted.  I’ll be sure to post pictures when I do but that may not be for a while.  As a Tom Verreault pointed out, this one is almost big enough to be to scale with the character miniatures.  Which of course got me thinking about how to actually print one that is to the right scale.  But that’s a later project.

One last picture.  Here is what the back of the ship looks like with all the supports removed.

A view of the back of the ship showing the full detail of the model

The back of the ship

There is some clean-up still needed as all of the support material didn’t come off cleanly.  So far this has just been cleaned up with the pliers on my Leatherman tool and my fingers.  The little bits of features that you see running upper left to bottom right are residual support material.  Plus the first few layers right about the supports don’t print fully and I’ll want to sand them down for maximum quality.

Thoughts on 3D Printing

If there is anything I’ve learned about 3D printing over the past few weeks while testing out this printer is that it is definitely not yet to the “fire and forget” stage.  There is still a long way to go.  You need to know your printer, how 3D printing works, and be willing to tweak and play with settings and even your models if you want the best possible outcomes.

Every printer is different.  There are things that printed just fine on the older printer that I’ve had to work a bit to get to print on the new one and vice versa.  Another difference is the slicer used to prepare the models for printing.  I’ve noticed that subtle surface features are not as pronounced with this new printer as they were with the old one.  The two use different software to make the printer files and it seems that the older printer’s software would do something that would make surface features more pronounced.  This is something beyond your control and you just have to design for it.

In the end, there is really no substitute for practice and hands-on experience.  Make a model, print it, see what works and what doesn’t, make changes, and do it again.  If you’ve got a single printer you’re working with, you can optimize your prints for that printer.  In a way, that’s what I’ve done as all of my prints have been made for the older printer we have here at the library.  Seeing how this new printer handles those same files has given me some more ideas on what to do for models I make in the future.

The other thing to remember is that the settings that work for one model may not be good at all on another or even on the same model at a different print resolution.  You really need to look at the model, what your goals are for the print and what settings will allow you to print that successfully based on the characteristics of your printer.

3D printing has a bright future ahead of it.  The technology continues to improve and the costs continue to come down.  But right now it is definitely a hobbyist’s area that takes a certain level of dedication and commitment to if you really want to do it right and create and print models of the highest quality.

Have you done any 3D printing?  Any success stories or failures you want to share?  Let us know in the comments below.


On-line resources

Hopefully this will be the last of the off schedule posts.  I just finished the final project for my class this week and a have a couple weeks before the next one starts.  I had originally planned to continue with the skill mechanics discussion in my “Designing Out Loud” series but that post is taking longer to write up than I expected.  So today, I thought I’d give a shout out to some of the blogs and podcasts I follow for gaming inspiration and ideas.  This list is by no means exhaustive but these resources are great and you should check them out if you’re not already following them.


  • Campaign Mastery – This blog is full of great times for GMs looking to build and run campaigns.  The archives go back to 2008 and every article is gold.
  • Role-playing Tips – A weekly newsletter of tips and ideas for players and GMs.
  • Delta’s D&D Hotspot – A technical look at items related primarily to early editions of D&D but also occasional love for my favorite game Star Frontiers
  • Dyson’s Dodecahedron (formerly A Character for Every Game) – Amazing maps by Dyson Logos.  All are free for personal use and many are now free for commercial use thanks to his Patreon campaign
  • Explore: Beneath & Beyond – A blog exploring game design and mechanics details that is at least partially spawned by Star Frontiers.


  • Ken & Robin Talk About Stuff – Game designers and writers, Ken and Robin talk about just about anything.  Each podcast typically has four segments (called huts) were they delve into details about specific topics related to gaming, movies, food, and other items and how to use them as background or hooks in your games.
  • Fear the Boot – A general RPG podcast covering a variety of topics.
  • Saving the Game – Another general RPG podcast that starts each episode with a scripture.  They have an amazing Virtues and Vices series in their archive covering the seven deadly sins and seven virtues and how those ideas relate to gaming.
  • The Gameable Podcast (formerly known as the Gameable Disney Podcast) – A podcast reviewing all the full length Disney (and now Pixar) movies looking at how the can be used for gaming inspiration.  I learned about just recently and am still months behind on going through their archive.


Hopefully some of those blogs and podcasts will be of interest to you and help you improve your game whether it is generating ideas, being a better player, or becoming a better GM.  Do you have a favorite on-line RPG resources.  Let us know about it in the comments below.

Designing Out Loud – Skill System – Basic Mechanics

Okay, I seem to be running a bit late this week as well due to fallout from the camping trip (I developed a kidney stone the night we got back.  I don’t recommend this to anyone).  But better late than never.

My involvement with the experiment to create the SPACER RPG as a community via polls that I posted about two weeks ago has gotten me back to thinking about my game design and what I want to do.  So we’re back to our Designing Out Loud series this week to talk about the basic mechanics of the skill system.

I’ve gone back and forth on this several times trying to decide exactly how I want to do the skills but in the end I think that for now, I’ve decided to go with the broader skill definitions based somewhat directly on the character’s ability scores.  So let’s dive in and see what we have.

Skill Resolution Mechanic

Every skill will have the same basic mechanic.  You roll d100 and try to roll under the target number for the skill.  That target number will be determined by:

TN = 1/2 × relevant Ability Score + 10% × skill level + modifiers

The relevant ability score will depend on the skill being used.  For example, most weapons skills, especially weapons skills, will depend on your Dexterity ability although some melee weapons might call for using your Strength score.  A computer skill check might be based on your Intelligence while a scouting skill my rely on your Wisdom score.  And since the skills are fairly broad covering a range of actions, maybe the same skill in a slightly different situation will base off a different ability score.  It will be up to the referee and the player to decide which ability score is directly applicable.

Each level of skill will grant the character an increase of 10% in their skill chance.  Thus at lower levels the majority of success will come from their raw natural talent but at higher levels their skills and training will contribute more and more to their chance of success.

Finally modifiers can be applied to make the chance of success harder or easier based on situational modifiers.  Some of these may be defined in the skills (e.g. a negative modifier based on weapon ranges or computer levels) and some may simply be situational and assigned by the referee (e.g. blowing sand is making tracking your quarry across the desert slightly more difficult).


I’m still up in the air about this one.  On one hand I like the specificity of sub-skills.  It calls out specific actions and you can apply predetermined modifiers based on the action being attempted.

On the other hand, that may be to restricting.  It’s much easier if you just have a computer skill as opposed to a computer skill that allows you to do x, y, and z things.  What if you want to do w?  What skill (or sub-skill) covers that and what is the chance of success?  I can’t think of everything and there are sure to be things that players will want to do that I didn’t think of a skill/sub-skill for.

So for now, no sub-skills.  Instead of a computer skill with sub-skills like ‘defeat security’, ‘repair computer’, etc, we’ll just have a computer skill and when a player wants to do something related to computers, the referee will give an appropriate modifier to the basic resolution mechanic described above.

Skill Categories

Every skill will belong to a broader category.  For example the military skill area will include all the weapon skills, the technical skill area will cover things like computers, robotics, spaceship engineer, technician, and other related skills.  There will be a variety of skill categories.  The exact number and content is the subject of a later post but I’m looking at something like 8-10 categories.

At character creation each character will select one category to be their Primary Skill Category (PSC).  This represents the area where they have a high natural talent or disposition toward and want to focus their training.  In addition they will select two Secondary Skill Categories (SSC).  These also represent areas that they have an interest or ability in but to a lesser extent than their PSC.  All the other skill categories will be considered Tertiary Skill Categories (TSC) for that character.  A character can acquire and improve any skill be skills in the character’s PSC will be the easiest to increase, skills in their SSCs will be a bit harder, and skills that fall into their TSCs will be the hardest to improve.

These categories are used to define what is important and relevant to the character concept and what the player desires the character to excel as.  Maybe you envision your character as a hacker who likes to paint and is a gun enthusiast.  You pick the Tech PSC, with Military and Artisan as your SSCs.  On the other hand, maybe you want to be a roboticist that works with cybernetics and tries to keep abreast of all the relevant laws.  In that case you might have Tech as your PSC but have Bio-science and Scholar as your SSCs.

Improving Skills

Skills will be improved by spending experience points (XP).  The number of XP required to improve a skill will depend on the level of skill desired and which of the skill categories (PSC, SSC, or TSC) the skill falls into the for the character.  Basically there will be some multiplier times the desired skill level with the multiplier depending on the skill category.

As a working value, I’m looking at making the multipliers 3, 4 and 6 for PSC, SSC, and TSC skills respectively.  This means that if the skill in in your Primary Skill Category, getting level 1 will cost you 3 XP, level 2 will cost you and additional 6 XP, level 3 and additional 9 XP and so forth.  On the other hand, levels 1, 2, and 3 of a skill in one of your SSCs would cost 4, 8, and 12 XP respectively while a skill in one of your TSCs would cost 6, 12, and 18 XP for the same levels.

I haven’t decided on a maximum level yet but it will be probably be 6 unless I change the skill progression cost.  With the system described above,  Once you’ve hit level 3 in a TSC skill, you might be better off improving your ability scores (something I have talked about yet).  That assumes that you can trade 1 XP for 1 ability point.  For TSC, level 4 costs you 24 XP and gives you a +10% but you could get that same +10% by spending 20 XP on the relevant ability score.  For SSC skill, that point comes at level 5, and for PSC skills it comes at level 6.  This breaks down somewhat because different ability scores can apply in different situations but for the most part each skill will be based on a single ability score.

Now, if the cost of increasing ability scores is different than 1-to-1, this will change the balance a little as well.  That is something I’ll need to look at going forward.

Coming up

In the next Designing Out Loud article, I’ll be looking at the actual skills and skill categories.  If there is a skill you think is relevant to a sci-fi game, let me know and I’ll be sure to include it in the final list.

What do you think of this basic system?  Is it too simplistic, to complicated, not enough information?  Any other thought or things I should be considering when building the skills system?  Let me know in the comments below.



Frontier Explorer issue 13 Available

By the time you’re reading this, I’m off camping in Yosemite National Park with my family.  (Edit: Actually, I’m back.  For some reason, I forgot to hit the publish button on the post after I scheduled it so it didn’t post on Tuesday.  Here it is now.)  The preparation for that, combined with the rest of my regular work and school responsibilities, has squeezed out any extra time I’ve had for writing blog posts.  However, getting ready for the camping trip has got me thinking of some new topics that I’ll probably cover once I get back.  I just don’t have time for them now and the post will be better with personal experiences (and pictures).

In the mean time (and another item that sucked up my free time from blogging), I finished this and got it up on-line:

That’s right, lucky issue 13 of the Frontier Explorer is now available either at the magazine’s website or from DriveThruRPG. This issue’s mini theme is racing and character mounts. We have a number of articles describing new creatures that can be used for races or as mounts for your characters when away from civilization as well as rule for running racing events in game or in your campaign.

Additionally we have another new race, some new ships and a compilation of all the Star Frontiers planetary and system data in to one comprehensive guide which was done by one of our Patreon supporters, Scott Holliday. We also have the usual comics and a couple of guides to existing races that flesh them out a bit more and give ideas for incorporating them into your game.

This issue’s full article list is:

  • Zebulon’s Guide to the Humma
  • Zebulon’s Guide to the Boon’Sheh
  • Off to the Races
  • Running With the Whitefooted Storch
  • New Pale Centisteed
  • Racing Mounts in the Frontier
  • Titan Rising: 2299
  • Trafalgar Trade Lines
  • Meet the Delphusians
  • Survey of the Frontier
  • Is There Anybody There?
  • Grimz Guide Comic

So grab a copy and explore a bit, maybe you’ll be inspired to fold something from the issue into your game.  I’ll be back next week with some thoughts about camping and travel.  See you then.

Community Game Design

Have you ever wanted to be part of a RPG game design project?  Do you have an interest in building a science fiction RPG?  Have you ever wanted to watch that process as it unfolds even if you’re not participating?  If so, you’re in luck.  My friend Bill Logan, just started up a new community on Google+ with exactly that purpose.

Spacer game lgooThe game will be called SPACER and it’s being developed completely openly and by the community on Google+ with Bill sort of guiding the direction and compiling the results.  When completed the game will be free and open sourced for anyone to use and add to and is being released under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

The basic format of development is for someone to throw out a topic and some options on how the game mechanic or idea should world.  Others suggest alternatives or variations and the community discusses the pros and cons of each.  Then Bill puts together a poll and everyone votes.  The option with the most votes is selected, although any decision is up for change later as more gets developed.  Especially if we come up with something new and better that makes one of the earlier decisions not work.

The community just started on July 16th so we’re barely getting going.  There have been votes on a few things already such as how to do stats, the resolution system, and whether to be class-based, skill-based, or something else.  Jump over to the community and check out the completed polls to find out the results.  I have to say I’ve been out voted on every one so far.  I guess I just like my games a little different that the majority of people in the community.  The decisions so far have been interesting and are definitely pushing towards a very rules-light system.

This will be an interesting exercise, both in seeing what will come out of it as well as watching the process unfold.  I’ll probably have more thoughts on this as it unfolds and we see the dynamics develop (and as we get closer to a working game).  If you’re interested in participating, come join the community.  We’d love to have you help out.  We’re 40 strong as I write this and the more the merrier.  Now I need to go read the combat mechanic posts as that’s the next item up for discussion.

Have you had any experience developing a game in a community setting or as part of a group?  Are there any pitfalls or dangerous rapids ahead that we should watch out for?  And advice on how to do it well?  Give us your voice of warning or cheers of encouragement in the comments below.

New Horizions Reaches Pluto

Pluto as seen by New Horizions at a distance of 766,000 kilometers

Best image of Pluto that had been downloaded from New Horizons as I was writing. Taken at a distance of 766,000 kilometers (about 2x the distance from the Earth to the Moon)

If you’ve been following my twitter feed (@dagorym), you’ll have noticed that I’ve been tweeting a lot using the #NewHorizons and #PlutoFlyby hashtags about the New Horizons mission to Pluto.  By the time you’re reading this the closest approach encounter will be over, although we may not have the first images back yet.

Needless to say, I’ve been quite excited about this mission.  I can honestly say I’ve been waiting for these pictures for nearly my entire life.  I’ve been interested in astronomy for as long as I can remember and Clyde Tombaugh, the discoverer of Pluto, was always one of my heroes as a kid.  I remember going to the local university to watch pictures come down live from Voyager 2′s encounter with Neptune back in 1989 and being disappointed that there was no way for that amazing spacecraft (which is still sending us data nearly 40 years after launch) to make it to Pluto.  And then all the proposed missions to Pluto not being funded or canceled.

If I had been in a closer field of astronomy, I may very well have ended up working on this mission in some way (I ended up working in high energy astrophysics instead of planetary astronomy).  I’ve met the Primary Investigator of New Horizons, Alan Stern, and my wife knows him as well.  I was lucky enough to see the New Horizons spacecraft in the clean room at Goddard Spaceflight Center just before it was sent down to Kennedy Space Center to be mounted on it’s launch vehicle.  And like the science and mission operations teams of New Horizons, I’ve been patiently waiting for the past nine years for the spacecraft to complete this first leg of its journey.

So what does all of this have to do with gaming?  I mean, this is supposed to be a gaming blog.  Well for today there are a couple of things I want to talk about.

Space is Vast

We all know that, but sometimes, maybe always, we forget what that really means.

Communication Lag

I mentioned at the beginning that the closest approach was over but we may not have images yet.  There is two reasons for that.  First, the spacecraft can’t transmit while taking data as it’s radio antenna isn’t pointed at Earth and it has a busy observing schedule for the next few hours to collect as much data as possible as it zips through the Pluto system.  But just as importantly, even if it could transmit at the same time, the light travel time from Pluto to Earth is just around 4.5 hours at the moment.

Closest approach was at 7:49 EDT.  That means that the soonest we’d possibly start getting data from that closest approach image is sometime after noon. (This if it started transmitting immediately.  It truth it will be about 6.5 hours later) Think about the scales involved here.  When bouncing a radio signal (or laser) off the Moon, the round trip travel time is about 1.5 seconds.  Here we’re talking about 9 hours.

What does this mean for your game.  Well, if you’re out in the outer parts of your system, away from any bits of civilization, it’s going to be awfully quiet.  You’re not going to just get on the radio and have a conversation.  You might send a message but it will be hours before you get a reply.  It means no one will be looking over your shoulder but it also means you’re not calling in the cavalry either.  You are on your own.

This is even more true if you’re off exploring other star systems.  In that case, normal radio communications will take years to make the one-way trip and then may not even be detectable unless you’ve got a really big antenna.

Of course this all assumes you’re using “normal” communication methods that are limited by the speed of light (i.e. radio, modulated laser beams, etc.)  If you have some faster than light communication method this may not apply.  But if it’s not instantaneous, then there may still be a lag of some sort.

Have you ever role-played that delay with your players?

Travel Times

I mentioned that it took nine years for New Horizons to make it to Pluto after launch.  That may seem like a long time, and I guess it is.  But New Horizons is the fastest spacecraft to ever leave Earth orbit.  (It doesn’t hold the record for the spacecraft which has achieved the highest velocity any longer but it still is the one that started out the fastest.)

New Horizons made it to Jupiter in 13 months.  Compare that to the nearly 2 years it took Voyager 2 to make the same trip.  That’s still a long time to be coasting along.  Most sci-fi games have some sort of “magic” propulsion system that cuts these travel times down significantly but even so, the travel times can be measured in days, weeks, or months.  Even if you could accelerate at 1 g to the half way point, flip over and decelerate at 1 g to your destination, the trip to Jupiter takes almost 6 days and the trip to Pluto would take about 38 days.  What are your players doing during this time?  And if you don’t have something like that to speed up the travel, go rewatch 2001: A Space Odyssey to remind yourself what real space travel is like on these long trips.

There are some other insights to be takes from this mission but I’ll save those for a future post.

Using it in Your Game

Do you convey the vastness of space in your game?  Do you make your players aware of the time passing and the distances involved in their travels?  It’s a great way to convey a little bit of the wonder and grandeur of the cosmos and maybe make them feel just a little bit small an insignificant.   Share your space scale stories in the comments below.

How to Determine the Mass of a Planet

Planets on a scale

By NASA/JPL-Caltech [Public domain], via Wikimedia Commons

In my post on calculating planetary orbital periods, I mentioned that I might do a post on determining the mass of the planet.  Well, here it is.  And like that process, it sounds a lot harder than it really is.  So let’s dive in.

What’s Needed

In order to calculate the mass of your planet you again only need two things: the surface gravity and the size of the planet.  The relationship between these quantities is give by

surface gravity is equal to the gravitational constant times the mass divided by the radius squared

where g is the surface gravity, G is our friend the gravitational constant (6.67384×10-11 m3 kg-1 s-2), M is the mass of the planet, and r is the radius of the planet.  The units here are SI (meters, kg, seconds).  If you want to work in multiples of Earth masses, radii, and surface gravities you can drop the constant G.

Calculating the Mass

So the calculation is straight forward.  To get the mass, you just take the surface gravity, multiply it by the radius squared and divide by the gravitational constant. i.e.

Mass equals surface gravity times radius squared divided by the gravitational constant

So for example, the surface gravity of Earth is 9.807 ms-2 and it has a radius of 6,371,000 m.  Plugging this in gives us a mass of 5.9645×1024 kg which is really close to the actual value of 5.9726×1024 kg.

For fun, let’s do Venus as well.  Venus’s surface gravity is 8.87 m s-2 and it has a radius of  6,052,000 m.  Plugging that in gives us a mass of 4.8679×1024 kg which again is really close to the actual value of 4.8676×1024 kg.  Or doing the calculation in terms of Earth ratios we have that Venus’s surface gravity is 0.905 times that of Earth and it’s radius is 0.950 times that of Earth.  Using the equation without the G you get M = 0.905 x 0.950 x 0.950 = 0.817 Earth masses which (unsurprisingly) is the same 4.8679×1024 kg value we computed before.

And that’s all it takes.

Comparison of planetary sizes

Kepler-62 System and the Solar System

Time for a Reality Check

That’s all well and good for planets we know about, but what about a random planet you’ve made up for your game (or found in a supplement).

Let’s say I want a big planet, say 50% bigger than the Earth but with a surface gravity of only 0.8g.  So we do our calculation and we get that M = 0.8 x 1.5 x 1.5 = 1.8 Earth masses or about 1.075×1024 kg which sounds reasonable.  It’s a larger planet and has more mass.

But let’s look at the density.  The density of an object is just its mass divided by its volume.  So we have the mass above and the volume of a sphere is just 4/3 × π r3 which gives us a volume of 3.66×1021 m3. Dividing the two gives us a density of 2940 kg/m3.

You may not realize it yet but that’s awfully low.  At least for a terrestrial planet.  In fact, it’s a really weird density for object we know about.  Let’s look at some comparisons (remember that the density of water is 1000 kg/m3).

Object Density (kg/m3)
Sun  1408
Mercury  5427
Venus  5243
Earth  5514
  Moon  3346
Mars  3934
Ceres  2170
Jupiter  1326
  Io  3528
  Europa  3013
  Ganymede  1428
  Callisto  1236
Saturn  687
  Titan  1880
Uranus  1270
Neptune  1638
  Triton  2061
Pluto  2030
  Charon  1650
Halley’s Comet  600

From this we see that the terrestrial planets have densities mostly around 5500 kg/m3, although the Moon and Mars are lighter,while the gas giant planets have densities around 1500 kg/m3, and the ice moons and planets are in the 1500 – 2000 kg/m3 range.  So our hypothetical planet doesn’t really fit any of those models.  The closest match is Europa, one of Jupiter’s moons that is mostly made of silicate rock and water ice.

So what does this mean?  Well it means that the planet we just created is probably very deficient in metals.  It is a large abundance of nickel and iron in the cores of the planets that make Mercury, Venus, and Earth so dense.  Mars and the Moon have a lower iron content and so are a lower density and the outer planets and moons are even more iron poor (relatively.  There is probably more iron in Jupiter than the entire mass of the Earth).

Could such a planet exist?  Sure.  But you might want to think about the implications.  Similarly if you found that the density of your planet is really high, you might have a problem in the other direction.  It is just something to think about when designing your world and your system.


On Space Combat

I listen to several podcasts during the week on my way to and from work and when working around the house.  Most of them are RPG podcasts and one of those is Fear the Boot.  Last week in their bonus episode #60, Dan had a wonderful solo episode where he talked about Capital Ship Combat.  It was more an episode on general space combat and it was great.  I’ve been thinking about writing posts on this topic but Dan hit most of the points and covered it thoroughly. Everything that follows was sparked by comments Dan made.  If you haven’t listened to the episode, you should do so now.  (It’s a little long at ~90 minutes but it is well worth the time.)  Go ahead, I’ll wait…


Back?  I told you it was worth it.  Now on to my thoughts.  I was actually listening to this while out grocery shopping over the weekend so my notes are just short jotted down thoughts on my week’s grocery list.  I will probably be revisiting this again in the future but these are the thoughts that stood out.  And I’ll admit up front I’m one of those guys Dan mentioned that is very much into and comfortable with the physics and math involved (B.S. in Physics, M.S. & Ph.D. in Astronomy).

Beam Diffusion

Dan talks about ranges in space combat.  One of the things about lasers is that they spread out.  They don’t stay a tight beam of light forever.  I touched on this a bit in the laser rifle post a couple of weeks ago when I talked about focusing the laser beam at the proper range for the target.  This effect will tend to limit the range of your laser weapons.  So while in theory you could shoot a ship that is orbiting Mars from Earth as Dan talks about, the laser may not have much power when it arrives.  At least not concentrated enough to do any damage.

We use lasers to regularly measure the distance between the Earth and the Moon.  The lasers are fired from Earth to the Moon where they bounce off retroreflectors left by the Apollo missions and the reflected photons are measured to determine the roundtrip time (you can read more about it in this Wikipedia article and this one about a specific experiment at Apache Point Observatory in New Mexico, where I went to school).  The relevant point is, that even over just the distance from the Earth to the Moon, about 385,000 km (or 38 hexes on the Knight Hawks map for you Star Frontiers fans out there), the beam has spread out to be about 6.5 kilometers (4 miles) in diameter.  So while you can hit targets at great distances, there may not be much power left in your beam when you do.

Long Laser Cavity

Which brings us to another topic.  In order to get around the beam diffusion issue you need really long lasing cavities.  This means that the laser weapons inside your ship are going to be long and skinny if you want longer ranges.  Dan talked about the fact that you can have your weapons pointing out any side of your ship and why you might want to do that in his discussions about “broadsides” and he’s completely right.

However, this may be a reason to have long skinny ships with a forward firing laser cannon.  You need that entire length of the ship to hold the long lasing cavity to generate a tight enough beam to get the long range you want.  And depending on how your ship is designed, you might have that equipment running through all your decks.

Using All Your Weapons

That relates to another point talked about, namely bringing all your weapons to bear on a target when the weapons are spaced out all around your ship.  Battery type weapons (that have a 180 degree field of fire) are much better for this.  Half of your weapons can swivel in their mounts to come to bear on the target and to get the others into position, all it takes is a roll of the ship around the thrust axis which can typically be done fairly easily.  So weapons are fired in a 1-2 punch but this can be fairly close together.

A fixed weapon is not so easy as it requires you to orient your ship so that the weapons is aimed at the target which may not be along the direction of thrust.  However, all is not lost.  As Dan points out your ship can point any direction and doesn’t have to be pointed along the direction of motion.  As long as you’re not thrusting to attempt to change your velocity (speed and/or direction) you can have your ship pointing in any direction you want to fire your weapons.

It’s only when you’re trying to maneuver and line up a fixed weapon that you might run into problems. Basically what happens in this situation is that you have to rotate your ship, fire, and rotate back to the proper maneuvering position.  This can be done with or without turning off the main engines.  Turning them off provides more exact maneuvering but reduces the effective thrust you have for that time period.  Leaving them on introduces some randomness into your velocity depending on how you perform the maneuver (which may be a good thing, see below) and still reduces the effective thrust available as you’re now using it in a direction that isn’t where you want to go.

Here what is possible comes down to the details of the game mechanics.  If you’re playing with short time scales and large ships, maybe there will be a significant  loss of capability.  For example, you can still fire your fixed weapon anywhere you want but can only use half of your available thrust.  If your target is in a direction close to where your ship is pointed based on your desired maneuver, then there is no loss.  Of course this requires understanding the physics and orientation of the ship so maybe just a rule that reduces effective thrust when firing the fixed weapon regardless of target location would be easier to play.

On the other hand, if you’re playing a game with long time scales and/or smaller ships, you can just allow the weapon to be fired with no loss of capability.  Once upon a time I was developing a Star Frontiers combat plugin for the Orbiter Space Flight Simulator program.  The plugin was very nearly fully functional but we never released it as we were waiting on a usable multi-player plugin.  All it was lacking was automating the turn, fire, return to orientation maneuver.  In the process of developing that, I looked at what was required to pull that off.  And what I found that even for the largest ship in the Star Frontiers game, the UPF battleship (length 600 meters) you could turn the ship to any direction, fire, and return to your original orientation in less than 90 seconds and never experience more than 0.5g sideways acceleration at the extremities of the ship (I’m pretty sure it was 0.5g and not 0.5m/s/s or 0.05g.  It’s been many years and my memory is fuzzy).  The smaller ships could rotate even faster.  Since the Star Frontiers combat turns are 10 minutes long, this represents a relatively small part of the turn (<15%).  It would have real impact in the computer simulator game but for the table top version, using real vector physics (Star Frontiersman issue 11) we could ignore it and allow the fixed weapons to fire any direction without affecting the capabilities of the ship.

By the way, if you really want to get a feel for what maneuvering in space with real physics is like, grab Orbiter and play with it.  I highly recommend it.

Leading the Target

Dan also talked about leading your target and having to shoot where it will be and not where it currently is or where your sensors say it was when they got a detection.  This is very true and something that modern militaries deal with already.  And while it can be compounded as Dan described by the longer range distances, there is actually a mitigating factor with space combat that makes it easier.  And that goes back to the fundamental difference between the difference between movement in space and movement in an atmosphere or on the ground: no friction.

Unless your universe has ships capable of unbelievably high accelerations without smashing the crews to jelly, they can’t change direction or velocity very quickly and the way they do so is somewhat predictable.  To the first point, let me give an example.  A ship that fires it engines non-stop for one minute at 1g (where I’m using 1g = 10m/s/s instead of 9.8 to keep the math simple) can change it’s position by 18 kilometers (11.25 miles). That may seem like a lot but remember, space is big, and that change is the same regardless of what it’s initial velocity was.  Basically, if you knew it’s original velocity, you know know that it will be somewhere within a sphere 18 km in radius centered on it’s projected position.

But it’s not even as bad as that because of the second point.  If you have some idea of the ship’s design (i.e. you know where the engines are mounted) and you can determine roughly the ship’s orientation, you know which direction it’s thrusting and the number of possibilities for its ending position are greatly reduced because the ship has to obey the laws of physics, specifically Newton’s laws of motion.  The better you can estimate the acceleration being used and the direction the ship is pointing, the easier it is to lead your target accurately.  There will still be some error but it’s not really as hard it you might think.

Sensor Lag

Which brings me to the final point I wanted to mention.  And this isn’t so much commentary (as Dan covered it quite well talking about the time delays involved) but a reading suggestion.  What’s needed is some way to represent the uncertainty involved.  There is an example of this that I have always loved and it is the way I envision it when I think about it.  And that is the sensor displays from C. J. Cherryh’s Downbelow Station.  If you’ve never read this book, I highly recommend it.  The ships are moving at near light speed in the book but the same type of error display would work at the velocities and ranges we’re talking about here as well.

Last Words

Actually I don’t really have much else to add at the moment.  I just wanted a section heading to separate this bit from the section above.  Dan did a great podcast.  Hopefully you enjoyed my ramblings as well.  Feel free to share your thoughts in the comment section below.

Calculating Planetary Orbital Periods

In my previous article on determining the current season, I showed that you needed to know the length of the world’s year in order to track the season.  But what if you don’t know the length of the year.  Maybe you’ve got a setting book that gives the orbital distance but not the year length, or maybe you don’t have anything at all.  In this post I’ll walk you through what you need to know to determine the orbital period of the planet (i.e. its year length) and how exactly to calculate that period.

What’s Needed

In order to calculate the orbital period you actually only need two things.  You need to know the mass of the central star and the distance from the star of the planet’s orbit.  Now, at this point you’re probably thinking “How the heck do I get the mass of the star?”  Don’t worry, it’s actually not that hard.  But I’m going to walk you through the calculating length of year part first and then we’ll go back and talk about finding the information needed.

How Long is a Year Anyway?

plot of planetary period squared against orbital distance cubed


Okay, so we’ve got the mass of our star and the orbital distance, how do we find the length of the year?  Before we answer that, a brief history lesson.  “It’s a warm summer evening, circa 600 B.C. …”  No.  Wait.  We don’t have to go back that far.  In the early 1600′s Johannes Kepler, using the observational data of Tycho Brahe, derived his three laws of planetary motion for planets in our solar system.  The third of which is the one were interested in.  It says that the square of the orbital period (in years) is equal to the cube of the orbital distance (in astronomical units – the distance from the Earth to the Sun).  These laws are true for planets around other stars except the units don’t work for the 3rd law.  Those only work for our solar system and you’re probably not doing that calculation.

Kepler’s work was empirical, he simply fit the data that he had.  It would not be until about 70 years later, 1687, that it would be possible to generalize this to other systems.  1687 was when Issac Newton published Philosophiæ Naturalis Principia Mathematica which described his theory of gravity.  With this law of gravity, it was possible to derive a version of Kepler’s third law that would work for any planetary system.  That’s the one were interested in. (If you’re interested in the actual derivation you can read about it on this site.)  This form of the law looks like this:

Kepler's third law

where P is the orbital period in years, a is the orbital distance in meters, G is the gravitational constant (6.67384 × 10-11 m3 kg-1 s-2), and M1 and M2 are the masses of the two objects.

“Hold on!” I hear you shouting. “You said we only needed the distance between the planet and the star and the mass of the star.  Now we need the mass of the planet too?”  Well, not really.  If you’re calculating the orbital period of an Earth-like planet around a star, the truth is that the mass of the planet doesn’t really matter.  The mass of the sun is about 2×1030 kg (that’s 2,000,000,000,000,000,000,000,000,000,000) while the mass of the earth is about 6×1024 kg (that’s a six with 24 zeros after it, you can write it out yourself if you really want to). Adding those together is 2.000006×1030 which isn’t much different from just the mass of the sun. If the star is much smaller than the sun or the planet is much larger, there might be a small effect but those combinations don’t typically result in habitable planets so we’re not going to worry about it. If it ever is important you have the full equation (and I may just do a post on determining the mass of your planet someday.  Edit:  Look I did!).

Okay, so we treat M2 as zero.  Now it’s just a matter of plugging in the values and crunching the numbers.  Here is an example using the Earth:

The orbital radius of the earth is about 150 million kilometers or 150 billion (US) meters.  We have the mass of the sun and G from above so we get:

P = sqrt( 4 * pi2  * (1.5×1011 m)3 / ( 6.67384×10-11 m3 kg-1 s-2 * 2.0×1030kg))

P= sqrt( 1.33×1035 / 1.33 ×1020 ) seconds

P = 3.16×107 seconds

Divide that by 3600 seconds per hour and 24 hours per day and you get 365.68 days  Which is a little off since I used approximations for the mass of the sun (it is really 1.989×1030 kg) and the orbital distance (it’s really 1.4959787×1011 m).

Other uses

You can also use this to calculate the orbital distance if you know the length of the year and the mass of the sun or calculate the mass of the sun if you know the orbital distance and length of a year.  These take a little bit of algebraic manipulation of the equation but everyone had algebra in high school so we’re all good.  If you don’t want to do the math yourself, there are a number of on-line calculators that will do it for you.  This one has a variety of options for the input and output values (although for some reason it doesn’t have kilograms as a option for mass.  This one doesn’t look as nice but does have kilograms as an input option.

Missing Data

Now it’s entirely possible that you don’t have all the information you need.  I mean how often do game supplements list the mass of the primary star.  They rarely even list the orbital distance.  Without those it makes it really hard to get the year length.  However, all is not lost.

Stellar Data

A Hertzsprung-Russel diagram showing different types of stars and their colorsWhile most system and supplements don’t list out the mass of the star they will often give its spectral type or at least it’s color.  And that’s enough for us to estimate the mass.  Of course, the spectral type is best, but even the color can give us some clues.  The reason this works is that the spectral type is basically just a measure of the star’s color.  The color of the star is directly related to the star’s temperature and the temperature is directly related to the mass.  So knowing the color or spectral type allows us to estimate the mass.

Now blue stars are hotter and more massive while red stars are cooler and smaller.  A yellow-orange star like our sun in kind of in the middle.  Now here I’m talking about “normal” or main sequence stars and not things like giants, supergiants, or white dwarfs.  While it’s possible for these type of stars to have planets around them, these types represent stars that are dying (or have died) and they probably won’t have habitable planets around them.  So for main sequence stars, the color can tell us the approximate mass.

Now given the color or spectral type, we just need to look up the mass.  One good site for this is the Atlas of the Universe’s stellar classification page.  You want to look at the “Main Sequence (V) table”.  The lines are colored the color of the star and the mass is given in multiples of the Sun’s mass (1.989×1030 kg) in the 4th column.  This table isn’t very fine grained, however, and only has a few entries.

If you want a super detailed version you can use this Stellar Classification Table.  This table has an entry for every single spectral type and stellar luminosity class.  I didn’t talk about luminosity class but just know that main sequence stars are luminosity class V.  So the Sun is a G2V star.  When looking at this table you will be interested in the ones that end in V.  This table lists the spectral type in the first column and the mass (again as a multiple of the Sun’s mass) in the second column.  A neat thing about this table is that it give RGB colors for the stars in the right most column.  The color of the numbers is the color of the stars as they would appear to our eyes.

So if you have the color or the spectral type, you can use those tables to estimate the mass of the star.

Orbital distance

This one’s a bit harder to estimate without some prior information.  In fact, to do this correctly we need to talk about habitable zones and stellar evolution and that’s a complete post (or two) all by itself that I’ll be writing in the near future.  For now we need some way to make a back-of-the-envelope estimate.

First, the Earth is near the inner edge of the habitable zone for the Sun.  And that zone extends out nearly to Mars.  So if you have a star like the sun, then the habitable planet is going to be about the Earth’s distance.  Maybe a shade closer if the climate is generally warmer and a bit further out if it is cooler.

Second, as the star gets redder it puts out less energy.  This means that the planet has to be a lot closer in order to get the same amount of total energy from the star.  So the habitable zone moves in closer and the orbital distances will be smaller for redder stars.  The opposite is true for bluer stars.  They put out more energy and so the planet would have to be further away to prevent it from getting fried.

The question is how much closer or further away do we need to go?  The energy put out by the star is proportional to the star’s temperature raised to the 4th power.  Which means doubling the temperature by a factor of 2 increase the energy output by a factor of 16.  Similarly reducing the temperature by a factor of 2 reduces the energy output by a factor of 16.

So that’s how we make our estimate of the distance.  Given the star’s spectral type or color, both of the linked tables give us its temperature.  Take the ratio of the star’s temperature to that of the Sun’s and square it, then square it again (that gives you the ratio to the 4th power).  That value is what you should multiply the Earth’s orbital distance by to get a (roughly) similar temperature range.  Then tweak it as desired for a slight climate variation.  And now you have the orbital distance and can figure out the length of the year.

And Now You Know

That’s it.  Even if we have just the color of the star and the knowledge that the planet is habitable, we can make a reasonable estimate of the length of its year.  The more we know about the system, the better our calculation will be, but even just the stellar color can get us something.

Did that all make sense?  Or was it all Greek?  Are there any tricks you use?  Are there any topics mentioned that you’d like me to go into more detail on?  Let me know in the comments below.