Oort Clouds

Something missing from a lot of the world generation stuff for Traveller is the matter of Oort Clouds. Our solar system has one (possibly two), and it’s thought that a lot of star systems will also have them. Basically, they are swarms of comets in the very outer limits of a solar system – from thousands of AU to tens of thousands of AU. The outer cloud is actually a shell that surrounds the whole solar system, whilst the inner cloud tends to be more of a doughnut in shape.

From a Traveller RPG perspective, a watery comet means a potential source of free fuel. Given the distance from the main world to the clouds though, which would involve the good part of a year of travel (and enough of a distance for time dilation to kick in) they really don’t make sense to use for anything travelling to the system’s main world.

If you just want to just hop across star systems though, it’s probably less dangerous than stopping at gas giants and refuelling there (just of running into pirates is lower due to the larger distances involved), and cheaper than buying fuel at the main port. Since the Deepnight Revelation campaign is going to be exploring lots of uncharted worlds, I’ve decided that I want to include Oort Clouds in the world generation process, which means I need to figure out how that impacts things in Traveller.

A post like this would normally fall into my WorldGen category, but since I’m more interested in it’s affect on the game, I’m looking at it more from the perspective of running a Traveller game and how to use it there.

Part of the way the Deepnight campaign will work, is that the mission needs to survey star systems at long distances, building up a picture of the path ahead. The closer they get, the more detail they’ll be able to get on worlds. From a distance though, they’ll just be picking up star types and maybe the presence of gas giants. The campaign guide has all sorts of useful information on how this works ‘in game’, though I’m modifying those rules so I can automate them. However, the basic idea remains the same. The Deepnight Revelation will scan dozens of parsecs ahead and build up a picture of the star systems on their route, so it can explore interesting looking ones and find a suitable path.

Part of that ‘find a suitable path’ will be about finding stopping points to refuel at. In Traveller, starship travel involves make ‘Jumps’ of a few parsecs, then refuelling with Hydrogen before another jump can be made. Refuelling can be done by extracting hydrogen from water, or by skimming gas giants. So the campaign assumes the expedition will be looking for systems with gas giants or water worlds so refuelling stops can be made. If every system has an outer cloud of water-ice comets, this search for hydrogen potentially becomes a lot easier.


The first thing is that we haven’t been able to ‘see’ Oort Clouds around other star systems – it’s just an assumption that because we seem to have one, therefore other systems do as well. Even discovering objects in ours is really hard, so my first assumption is that Oort Clouds are very hard to detect from multiple light years away, making them difficult to survey.

You might be able to detect roughly where one is around a system from a few parsecs distance, but detecting individual objects within the cloud will be difficult unless those objects are exceptionally large. This makes jumping directly to an individual object very hard to do.

Thinking about it, the asteroid belt was discovered until after individual asteroids (or ‘planets’, later ‘dwarf planets’ – Pluto wasn’t the first planet to get downgraded, it was just the most popular) had been discovered. So maybe if a cloud has individual large bodies in it, these might be spotted in a long distance scan before the cloud itself is.

The exact position and nature of the object won’t necessarily be known though.


Jumping into the middle of an Oort Cloud is all well and good, but finding anything in reach is going to be tricky. There are estimated to be trillions of bodies over a kilometre in size within our Oort Clouds, which sounds like a lot. But consider that the ‘inner’ cloud might be a torus some 10,000AU from the star, with a thickness (minor radius) of about 5,000 AU. The ‘outer’ cloud is a shell potentially tens of thousands of AU thick.

For even a trillion bodies just in the inner shell, each one will have almost 10 cubic AU of space to itself, so you’re looking at maybe 10 AU between objects. Even at the 6g thrust of the Deepnight Revelation (I’m ignoring the ‘Deep Space Manoeuvre Drive’, since I don’t like it), that’s a couple of days to get to the nearest one if they jump out within the cloud at random.

And that trillion number needs to be split across the entire cloud, so it could be tens of AU to the nearest decent sized snowball. Smaller ones will be closer, but very hard to detect and may not be suitable for refuelling from.


So once an object has been discovered, and a ship has reached it, what then? Does the snowball contain potential fuel of good enough quality to be used? If we assume that the snowballs are impure, possibly with lots of carbon and other deposits, the ability to use them as a fuel source might be limited. They might have a limit on the amount of hydrogen that can be obtained, or purifying it may be harder than usual.

Which gives a plausible reason for why it might not be so easy to just jump into the middle of an Oort Cloud, find something and use it for fuel.


So how many systems have an Oort Cloud? Most systems will, but the frequency goes down both for larger stars and smaller stars. The distance of the clouds from their star increases with the mass of the star, until it reaches a point where the comets are more likely to be disrupted by nearby stars, flinging them out into space. Smaller stars may have had less matter from which to form a halo of comets.

Multi-star systems will also tend to be less stable, especially if the stars are a large distance from each other. A Far Binary (100s or 1000s of AU distance between the stars) will probably not have Oort Clouds, whilst Close Binaries (< 1 AU from each other) probably will.


For a game perspective, I want oort clouds to be present and available, but not to be a way of guaranteeing fuel availability in every system, far away from the risks of pirates and politics. The Deepnight books have some guides on how easy it is to find fuels sources in deep space, though the time involved look a bit on the low side. I’ll probably keep to using distances, and make sure the numbers are large enough to make it generally preferable to try to use a gas giant or other more ‘traditional’ Traveller resource for refuelling. Whatever I do in Deepnight I’d prefer to keep using consistently with other games (and I’d like to do Pirates of Drinax at some point), so I need to consider the implications of using them in less exploratory campaigns.

So I don’t have exact numbers yet on how I’ll define them, but I have some vague ideas.

Samuel Penn