I realised something.

While reading the core rulebook I always assumed that the relevant distance for jumping into a system was the diameter of the systems star times one hundred, and I got the impression that it would take a few days to get from safe jump distance to the spaceport. However, while I was doing some calculations based on @paul-gilfedder great answer to this question I realised that Earth is orbiting outside of our stars diameter times one hundred.


I had been assuming that if Earth had a spaceport it would take 68h with a G1 drive to get from the spaceport to a safe jump distance, and that that was pretty standard for any solar system. However, if I understand this correctly it should take only 6h 20min to get from the jump point to Earths potential starport since it is only 1.276.000 km and not 140.000.000 that I assumed.

After this longwinded intro I get to my question: When the source material talks about a system and it's main planet/starport is there any information included that lets me know what the travel time from safe jump distance to the spaceport is, or a way to infer it from the UPW? Makes a huge difference if the planets orbit is inside our outside of the stars 100D. For instance safe jump distance to Venus would be 32.000.000 km, which is 25 times that of Earth, because of this.

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    \$\begingroup\$ I'm assuming you're asking about MGT2 based on your other questions, and I've added that tag. Please use the appropriate tag for future questions so that people don't have to be aware of previous questions to be able to answer. \$\endgroup\$ Commented Feb 29, 2016 at 0:06
  • \$\begingroup\$ This question could just as easily be applied to most versions of Traveller. \$\endgroup\$
    – YogoZuno
    Commented Aug 26, 2022 at 21:47

3 Answers 3


To the best of my knowledge, no edition of Traveller has ever forced players take into account the sun's diameter. Possibly this is just an oversight, but having read the original CT LBBs and seen the detail the designers went into, I suspect it's actually just because it practically never applies. I can imagine a few reasons why this might be the case (habitable worlds are often outside of a sun's hundred diameter limit, the gravitational force of a sun is constant enough that you can take it into account when calculating a jump, the designers just decided it wasn't worth it, the 100 diameter rule of thumb breaks down when applied to masses that large), but all such guesses are speculation; The important thing is that no edition of the game that I'm familiar with bothers with this particular detail. I therefore recommend you ignore it; It doesn't add much to play in any case.

And yes, there is a way to work out the hundred-diameter limit for any given world: Just look at its UWP. The second digit is "Size" and corresponds to a particular planetary diameter; I don't know which edition you're using, but there should be a table detailing which size values correspond to what diameters in the rules for generating worlds.

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    \$\begingroup\$ Size values for worlds represent the approximate diameter in thousands of miles, so if you're using metric like a sensible Traveller player would, multiply the size digit by 160,000 to get the safe jump distance in kilometers. \$\endgroup\$ Commented Dec 20, 2016 at 15:46

There have been discussions on Citizens of the Imperium forums and the Traveller Mailing List going back decades on the question of what the 100D really means. Marc was cagey about expressing exactly what the physics was (likely because 100D was an easy number for a game and was made up without any real deep idea of what the physics was - it just worked in a gaming sense to prevent people jumping in on top of planets and jumping out from them).

Does it mean 100 diameters of the planet you are leaving?

Does it have to get you 100 diameters out from the star or stars? (Contrary to GMJoe's answer, depending on star type and the habitable zones, you can have habitable worlds far inside the solar 100D limit)

If you had a binary, trinary, etc. system, the 100D a limit for each star might need understood.

Some large satellites or other large bodies in the inner zone may also be an issue at times.

There was talk about using tidal force as a proxy for 100D and looking for it to drop off to 1% from a system object. That would be (if I recall) tying it to gravity.

Marc Miller was quoted somewhere I do not recall at this moment but he indicated that it was not a gravity thing though -> it is some property of matter instead of mass.

This leads into other discussions of jump shadowing (if I recall the term) - I want to jump from A to B in system or between systems - if there's a large body with a large mass shadow being cast that my jump will take me through... what happens? Do you get forcefully precipitated from jumpspace? Or blow up? Or nothing? Some argued that certain jump failures (misjumps, damage, missing ships, etc) could perhaps be a result of not correctly navigating around these sorts of effects. But there is no official view I'm aware of on these ancillary issues (although I bet 'simpler is better' would be the answer if we asked Marc).

I suspect the simple answer of 100D from the main planet is enough, but other ways to look at it could offer a lot of interesting scenario ideas.

I think a good way to go is simply do both the primary star and the planet you are leaving and that covers most cases. If you want to go real simple, just the planet you are leaving.

I know citations would help, but some of this stuff goes back over 15 years and some of the actors and the threads or sites may not even be around anymore.

  • \$\begingroup\$ One of the problems with jump shadowing is the complexity. Do you have to calculate the 100D limit of EVERY body in EVERY system that is even remotely from A to B to see if any object (asteroids, small planetoids, moons, comets, stars, planets, and I'm likely missing other options) impacts the jump's ability to a) happen at all or b) happen without a misjump or some form of damage or time effect or whatever? If that's what you'd need to do, impractical would be an understatement. \$\endgroup\$ Commented Sep 3, 2022 at 4:41
  • \$\begingroup\$ Jump shadowing is a massive Gordian knot wrapped around a tesseract.... then folded into a manifold. At least if you do it with any rigor and stiff rules trying to include everything that could apply. \$\endgroup\$ Commented Sep 3, 2022 at 4:43

Is there any information included that lets me know what the travel time from safe jump distance to the spaceport is, or a way to infer it from the UWP?

Not in the rules.

But yes, you can make a reasonable GUESS based solely on the UWP. If your players aren't astronomers they will be fine with it -- in other words, this is a very rough approximation that makes big assumptions about the primary star.

If the world has a decent atmosphere (5, 6, 7, or 8), then you can assume it's at least a bit outside of the star's 100D limit. Thus travel time to the jump point is 100D from the planet -- a matter of several hours as you noted. By the way, a shortcut for this is to look for the "Ag" (agricultural) or "Ri" (rich) or "Ga" (garden world) trade codes.

If the world is a "hellworld" and it's not the satellite of a gas giant, then it's likely well inside the star's 100D limit, and will require a couple of days journey (for everything but red dwarf stars).

If the world is a "rockball" with little or no atmosphere, then roll 1D: on 1-3 it's an inner world and subject to long travel times. On 4-6 it's an outer world and not subject to the star's 100D limit.

I'm not an astronomer or astrophysicist, so this won't stand up to scrutiny, but here's an additional useful assumption. If you also know that your mainworld is around a giant star (I, II, or III) then you can adjust your expectations further. Giant stars will most likely have the mainworld inside its 100D limit and travel times will be more like a week.

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    \$\begingroup\$ If you have any of the games' iterations of system building (enhanced system creation), you'll end up knowing the mass of your sun. You could do the 100D proxy using tidal force instead of 100D directly. I don't recall you getting the size of the sun itself, but it might - some of the expanded systems (World Tamer's Handbook, World Builder Handbook, Book 6 Scouts from the original LBBs, MegaTraveller Referee's Manual, etc) might give you more, less or different. Really, go with what you figure and the game will still move along. \$\endgroup\$ Commented Sep 3, 2022 at 4:46

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