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Navigation in Traveller

This article originally appeared in the May/June 2020 issue.

This series will explore a number of concepts with navigation in Traveller. The rules are often unclear and the science and/or reasoning behind them not ideal. In your own Traveller universe, you can (of course) ignore or alter these as you see fit. The previous articles in this series covered the 100-diameter limit, running vs standing jumps, and calculating jump vectors; future planned topics are in-system navigation and exploratory navigation.

Part 4: Jump Masking and Interstellar Courses

Jump masking occurs when a planet or star’s 100D limit is large enough to block the line of travel from a source system to the destination planet. The distance inside of a stellar system is small compared to interstellar distances, but planets are small on any system scale. For the most part we will see that normally only a star’s 100D limit will be an issue for any jump.

It would seem that one would have to consider the positions of all the planets in the system to determine if there is a masking issue. Likely this is due to how we think of the solar system as a horizontal disk and would image other systems the same way. While all planets orbit in more or less the same plane, and for the most part all of the stars also orbit the galaxy in the same plane, these planes don’t line up. In fact consider images of the Milky Way. The disk is always shown at an angle to the horizon in most photographs. This is because the Solar system’s orbits are in fact inclined 60° to the galactic plane.

If a ship is entering a system from the “top” all the planets are completely distinct vectors, it is only when entering the system near “edge-on” would vectors have a significant chance of intersecting another body. So with two systems at arbitrary angles to the galactic plane, the chance of a jump between them hitting one of the systems edge-on is insignificant. Moreover the possibly interfering planet needs to be at the right point in its orbit to be in the way. To take our system. If one were to jump to Earth and Jupiter just happened to be at the correct point in its orbit < 1% of the time (0.44% of the circumference), then the angle of the jump vector to the plane of the solar system would also have to be within 2.55°, which given 360° of possible angles is (2.55/360) = 0.7%, combined with chance that is it in the correct point in its orbit gives a total of 0.003%. Even if Jupiter was in the orbit of Mars and masking 28.3° of the approach vectors with its 100D limit, the chance that a random vector to Earth would be masked is (28.3/360)×1.5% (smaller orbit) = 0.1%.

So planets can be effectively ignored unless the story specifically calls for it, however stars have huge 100D limits and are a serious issue. Again using the Solar system, the Sun’s 100D limit is 143,000,000 km and the Earth only orbits at 149,000,000 km. This means that the Sun blocks 147° or 40.8% of any random vector to Earth. Six months later it would be blocked from another 40.8%, meaning that 18.4% are completely clear. In our case the habitable zone around the Sun is more than the 100D limit, but for some stars the habitable zone is completely within the 100D limit. GURPS Traveller Starships has a nice table for this which indicates that the habitable zone for G5 and cooler stars (K, M) will always be inside the 100D limit, while that for O and most B stars is always outside. That doesn’t mean that the star can’t mask the planet when it is behind, just that no masking occurs when the planet is on the near side. The Sun (being G2) and A, F and some B stars have a habitable zone partially inside and outside the stellar 100D limit.

Thus travel to planets in the habitable zone of the G5-G9, K and M stars will be always be a trip to the 100D of the star. Remember though, not all destinations are in the habitable zone, and the smaller K and M stars have much smaller 100D limits as well. Also note that these limits on spectral classes only apply to main-sequence stars (92.5% of all stars, plus 6% white dwarfs and 0.5% sub-dwarfs which have even smaller 100D limits). What happens when a star masks the planet? The course needs to be set to the nearest point on the star’s 100D limit to where the planet is. If the planet is on the near side ~45% of the time, then it should be only a matter of travelling the difference of distance between the planet’s orbit and the 100D stellar limit. Likely only millions of km, or perhaps 12-20 hours. (This is where having accurate system maps help). However if the planet is near the edge of the sphere or behind the star then the extra travel time becomes 100s of million km or several days of extra travel.

Ideally the destination system is fully defined with stellar class and orbital bands defined and the referee can just look up the system details, but if that level of detail is not available, then we can come up with an approximation for in-system travel time. The stars with habitable zones well outside the 100D limit (O, B, A) make up less than 1% of all stars. F and G are 3% and 7.6% respectively. While M stars make up 76% of the stellar population that doesn’t mean that they make up that fraction of destinations; M stars have such a small habitable zone that the likelihood of a habitable planet there is small. If we take most of the M stars (90%) out of consideration and run some numbers we would still expect 75% of the habitable zones to be completely masked. Adding in the numbers for partial and habitable zones completely outside the star’s 100D limit. This means that some 83.6% of all planets will be always masked, 3% to be never masked by the star, while 13.4% are masked while only on the opposite side of the star (e.g. Earth). [Without removing the M stars the numbers would be 94.6/0.9/4.5] This means that 97% [99.1%] of trips to worlds in the habitable zone would have extended delays for half their orbit.

While not all destinations are in the habitable zone at all, that should be obvious from the planetary UPP. Size 0 worlds and ones with 0 or 1 atmosphere are likely outside of the habitable zone. Those with a Hydrographic digit 2+ are most definitely in the habitable zone however. Even if the planet is outside of the star’s masking area, as will be discussed in the next section, the ship likely can’t target it anyways and will have to aim for the star.

Is the destination world masked?
2D Masking status
2-7 Destination is masked and on opposite side of star; long delay required while maneuvering through normal space
8,9,11 Destination is masked, but on near side of star; small delay incurred while maneuvering through normal space
10 Destination is not currently masked
12 Destination is never masked (from this origin)

Interstellar Courses

Plotting a course to the destination planet is just a matter of looking it up in the ephemeris and having the computer plot a course to intersect the 100D limit (as we saw above - presumably of the star) or set the distance to come out just past the star. Having the data available means we know exactly where the star is currently, irrespective of visual information in relation to the star the ship is currently at. However is it never as easy as it seems.

To compute a vector to a star the computer needs the precise angle (from the previous article ±500 mas [milli-arc seconds] or 1/7200 of a degree to hit a G star’s 100D limit at 1 pc) and to get that one needs to know precisely where the starship is in relation to the source star so the computer can calculate the angle to the destination.

Some sources indicate that jumps need to be made at an exact point in time. Assuming the ship doesn’t change orientation, or at least the jump field doesn’t change orientation, travelling a few thousand km extra will have negligible impact on where the ship emerges from jumpspace. The ship will appear the same few thousand km different than expected at the destination. Angular accuracy is the primary concern.

As the star looks the same from all sides and the planets move, how does one know where the ship is in relation to the source star and which direction is which? There are at least three methods:

Ideally the ship is at a high-tech world that has several navigation satellites in orbit around the star. Just like a GPS, by analysing the signals from at least four of them the ship’s exact position can be determined.

The star charts can have planetary information. As long as the ship can identify and accurately measure the angles to at least 3 planets its position can be determined.

Using “standard candles” – distant super bright stars far enough away within the galaxy that their movement can be ignored, or better intergalactic quasars. Identifying and measuring the angles to these can determine a highly accurate position. The more that are measured the higher the accuracy. [Cepheid variable stars and pulsars can also be used this way. –ed.]

Identifying these objects in a sky full of stars takes some time, so jumps can’t be done immediately after arriving in a system. The bad effects of mis-identifying a reference are so severe (appearing 1000s of AU out of position) that ships will wait the hour or so it would take to orient themselves.

As mentioned in the previous article, the angular measurement necessary to travel to a nearby (1pc) stellar 100D limit is around 75-1000 mas depending on the size of the star. A default telescope has about a 1000 mas resolution, but we only need to know where the local star is and a number of reference point and the more measurements the better the average becomes. A regular ship may need to measure 15 or more reference points identify its exact position, while a military ship 3 or so.

While these measurements are taking place the ship needs to maintain a known speed on a well-defined course (such as a straight line or a fixed orbit) – no acceleration or maneuvering while the measurements are taking place. That means that jumping while the manoeuver drive is on (“running jump”) is extremely detrimental to navigation.

Targeting a star, as can be seen, is harder than regular optical sensors can easily determine, but hitting the 100D limit for a size 8 planet would require ±8.5mas of accuracy—10 to 20 times more accurate measurements than the star. Optics cannot really get better without large apertures. While optical interferometry can make smaller optics act as if they are larger, there are issues with vibration and unequal heating. The accuracy that jumping in Traveller requires is so precise; targeting a size 1 planet would require that the ship be orientated within the wavelength of ultraviolet light or the size of a HEPA filter opening, and at 3 or more parsecs, we are getting to navigational accuracy less than the size of computer memory cells, So it is just as well that over half the jump journeys will end at the star’s 100D limit. [To explain the analogy – imagine a computer memory chip is directly in front of the starship aiming the ship at memory cell 1576545 will hit the size 1 planet 3pc away, but aiming at one of its neighbours will mean missing].

Also in Traveller, there is a custom known as “jump dimming” where the ships lights are dimmed just before jump. There is likely another reason for this besides the one explanation in the starship operators’ guide: reducing energy levels below what is required for the manoeuver drives.

Consider a grain of sand (0.01g) travelling at planetary speed (30 km/s) at an impact point away from the centre of mass of the starship. This impact could cause the ship to rotate ever so slightly. At 4500J of energy 12m from the centre of mass hitting a scout ships, gives a torque of about 13 arcsec/s* over 1ms collision time = 13mas, but in space there is nothing stopping that rotation so until the ship can fire its thrusters the angular change will continue. 13 mas at 2pc distance is 2,000,000 km off target – at 40m (the length of the ship) it is 0.002 mm; would in fact the ship even perceive such a small change of orientation (½ the width of a strand of spider silk)?

It is not just collisions with high-speed objects ships are not fixed in place or under any effects of gravity at the jump point. Movement within the ship, or shifting cargo can cause effects to the ships orientation – imperceptible to people, but significant to the course. Lifting a 1 ton cargo crate 2.2 meters at a distance from the centre of mass would have the same effect as the grain of sand. So jump dimming is a method of letting the crew/passengers know that all movement should be minimalized between the final course measurement and the start of jump.

This seems to paint a dark picture of interstellar travel, but having the star charts (Ephemeris) allows for the possibility of travel. 90% of the time the ship is aiming to intersect with (and stop at) the 100 diameter limit of the star which should be within the optical measuring limits of any ship. While aiming at even large planets is extremely more difficult, thanks to the ephemeris the exact distance, well within a million km, will be known and even if the ship misses the planet it will still come out close by. Intersecting the 100D limit of any but the largest planets, particularly ones more than 1pc distant is a dream for any navigator.