Mind-Bogglingly Big

“Space … is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space” —Douglas Adams, The Hitchhiker’s Guide to the Galaxy.

From Atlas Obscura’s article on the Maine Solar System Model.

Northern Maine is home to an interesting throwback to the old roadside attractions: the Maine Solar System Model. Last summer I finally had a chance to visit this 3-D scale model of the solar system, which demonstrates the enormous distance between planetary bodies. The entire inner system is easily walkable, with the model Earth being exactly 1 mile from the model Sun. The outer system, however, is best traversed with a car, with the model Pluto being 40 miles away. It’s not a bad way to spend an afternoon and has helped me imagine intra-system travel in Traveller.

Starships in Traveller have two types of drives. The jump drive (J-drive), usually used for travel between star systems, allows faster-than-light movement of up to six parsecs. Regardless of distance, the duration of travel is generally one week. The maneuver drive (M-drive) provides constant acceleration and is usually used for intra-system travel. In a typical trip through normal space a ship uses its M-drive to constantly accelerate until it reaches the halfway point, and then uses the M-drive to decelerate throughout the second half of the trip, arriving at the destination with a velocity of zero.

In our recent Traveller adventure “The Professor’s Charter,” the PCs needed to pilot their starship the Starjammer from the planet Desolation in the inner system to a position in the outer system where the derelict liner Speedwell was expected to be found.

Because the Desolation system is a known haven for pirates, smugglers, and otherwise bad actors, I wanted this voyage to occur in normal space for dramatic reasons. That meant the time required to reach the Speedwell needed to be less than one standard week; otherwise, the quickest and safest way to get there would be to just use the Starjammer’s jump drive.

The Traveller Book (1982) and the Mongoose Traveller Core Rules (2022) provide handy formulas for calculating intra-system travel time, distance, and acceleration. The Starjammer’s M-drives provide constant 3g acceleration, which is 29.41995 m/s². Knowing this it was easy to place the Speedwell 1,916,956,472 km away from Desolation. For context, this is roughly equivalent to the distance from Earth to the midpoint between the orbits of Saturn and Uranus. With constant 3g acceleration, the Starjammer would need 141.86 hours to reach the Speedwell. (Had the PCs wished to use their jump drive, which would have taken an average of 160 hours, I certainly would have let them.) 

While distances in the millions or billions of kilometers are probably impossible to comprehend, players can readily understand travel times of hours, days, or weeks. Players can also follow progress when it is expressed as percent complete: “you’re 30% there, you’re halfway there, you’re 90% there, etc.”

If kilometers are simply too small to be useful for most descriptions of intra-system distance, other units are available. The astronomical unit (AU), which roughly represents the distance between the Earth and the sun, is 149,597,871 km and is often used to express orbital position within a star system. If Earth’s orbit is 1 AU, Pluto’s average orbit is 39 AU.

A couple of handy units of distance to also consider are the light-second (299,792 km) and light-minute (17,987,548 km), which represent the distance traveled by light in a vacuum in 1 second or 1 minute, respectively. The benefit of expressing distances in light-seconds or light-minutes is that this also tells us the lag time in communications and sensors.

With these units in hand, here’s an overview of relevant distances in the Desolation system:

The Desolation System

Location	Distance from Dross
	Kilometers	Light-Seconds	Light-Minutes	Astronomical Units
Dross (M2 V primary)	-	-	-	-
Desolation (B401200-E)	35,900,000	120	2.00	0.24
Dross 100D Limit	38,600,000	129	2.15	0.26
Halfway Point	958,478,236	3,197	53.29	6.41
Speedwell (Type M subsidized liner)	1,952,856,472	6,514	108.57	13.05

Note that Desolation orbits inside the 100D limit of its star, meaning that incoming starships need to jump outside that limit and then travel to the main world, likely adding a few hours to travel time. This jump masking normally only occurs with red main sequence stars like Dross.

The Traveller Map data indicates that the Desolation system also has two gas giants and nine additional planets, but as these were irrelevant to the Professor’s Charter I didn’t bother to determine the locations of these other objects.

One thing I did not quite grasp when prepping or running the Professor’s Charter was how paltry Traveller sensor ranges are beside the enormous distances within a star system. Even after years of playing the game, my expectations are still biased by Star Trek, where “long-range scans” can seemingly pinpoint and track individual ships from parsecs away.

The Traveller Book states that “Ordinary or commercial starships can detect other ships out to a range of about one-half light-second,” or about 150,000 km. “Military and scout starships have detection ranges out to two light-seconds,” or about 600,000 km. “Once a vessel has been detected, it can be tracked by anyone up to three light-seconds,” or about 900,000 km (76). For context, this is not quite the safe jump distance from a Size 6 world.

The Mongoose Traveller Core Rules provides a more nuanced series of sensor range bands, but these are notably shorter in distance than the guidelines from The Traveller Book. The longest range band is Distant, defined as 50,000+ km, only about one-sixth of a light-second and beyond.

MgT High Guard addresses this by setting an upper limit on the Distant band of 300,000 km, or about 1 light-second. This distance is also “the maximum practical range that attacks can be made” (26). High Guard also introduces two new range bands: Very Distant (up to 5,000,000 km) and Far (over 5,000,000 km). These changes bring MgT sensors more in line with CT sensors.

But in any case, Traveller sensors can detect only out to a miniscule distance against the scale of a star system. The upper limit of Distant is “only” 1 light-second, while Very Distant ranges from 1 to 16.68 light-seconds. Sixteen light-seconds is roughly the safe jump distance from a typical small gas giant, while the safe jump distance from a large gas giant might be twice that.

This implies that a lone commercial ship traveling through an outer system is a little like a blind man moving through a vast and featureless desert, only able to sense what is immediately around him. As the Maine Solar System Model scale is 1:93 million, the Distant range band would only extend out 10-½ feet. If there is trouble out there, he won’t know it until it is on top of him. Conversely, trouble won’t be able to sense the blind man any earlier.

The following table shows the journey of the Starjammer as it moves from Desolation to the Speedwell:

Travel to the Speedwell

Location	Trip Completion	Distance (km)		Velocity	Time (Hours)
		Traveled	Remaining	(km/s)	Elapsed	Remaining
Start	0.00%	0	1,916,956,472	0	0.00	141.86
First Report	2.24%	42,865,200	1,874,091,272	1,588	15.00	126.86
Halfway	50.00%	958,478,236	958,478,236	7,507	70.93	70.93
Far Range	99.48%	1,906,956,472	10,000,000	767	134.61	7.25
Very Distant Range	99.74%	1,911,956,472	5,000,000	542	136.74	5.12
Distant Range	99.98%	1,916,656,472	300,000	133	140.61	1.25
First Contact	99.9948%	1,916,856,472	100,000	77	141.14	0.72
Very Long Range	99.9974%	1,916,906,472	50,000	54	141.35	0.51
Long Range	99.9987%	1,916,931,472	25,000	38	141.50	0.36
Medium Range	99.9995%	1,916,946,472	10,000	24	141.63	0.23
Short Range	99.9999%	1,916,955,222	1,250	9	141.78	0.08
Close Range	100.0000%	1,916,956,462	10	2	141.84	0.02
Adjacent Range	100.0000%	1,916,956,471	1	0	141.86	0.00
End	100.0000%	1,916,956,472	0	0	141.86	0.00

The halfway point to the Speedwell is about 958 million km from Desolation, roughly equivalent to traveling from Earth to a point between the orbits of Jupiter and Saturn. At the halfway point the Starjammer is moving at 27,025,200 km (!) per hour, or a little over 2.5% of light speed (c). In order to avoid zipping by the Speedwell, the Starjammer needs to begin decelerating at the same rate it had been accelerating.

As can be seen, the Starjammer has to complete over 99% of its trip before the Speedwell comes within even the most distant range band. And the decelerating Starjammer needs only one hour and fifteen minutes to move through all seven range bands in the Core Rules.

If pirate ships had been waiting around the Speedwell and were expecting the Starjammer, they could probably get in a few shots before the Starjammer could react. And if any kind of battle occurred, it would take nearly two hours before radio transmissions reached Desolation, and then days before any ships could investigate. This is why commercial ships in Charted Space are so often armed: help is often very, very, very far away.

A prosperous, high population system with a Class A or B starport and multiple inhabited secondary worlds might well have sensor nets around all major planetary bodies. These nets might consist of automated satellites and stations posted in orbit and to the various LaGrange points, monitoring and reporting traffic around each planet out to their respective 100D limits. This information would be continuously relayed throughout the system, though more distant stations would have an inherent time lag. A sensor net like this would allow commercial ships to keep tabs on other in-system ships.

Such blanket coverage is likely the exception rather than the rule, and even then will only see a small fraction of space in the system. Ships operating outside these 100D limits would be effectively invisible to system control, and an asteroid belt would be a nightmare to cover. A system with a Class C or D starport might have a sensor net around the main world and maybe a gas giant or two, while a system with a Class D or X starport might be lucky to have a sensor net around the main world.

In Traveller, the isolation of distant worlds even with magical tech like the maneuver drive perhaps helps explain why most development in a star system tends to coalesce around a single world. It’s just too much hassle to try and work resources in the outer system when one can just use the jump drive to trade with a nearby system. From a referee’s perspective, this means that most systems are largely unexplored, or at least unexploited, even if inhabited for thousands of years. Which means most systems have plenty of adventuring potential in the outer system, even if the inner system seems thoroughly developed.

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