The Humern empire uses Photonic Railways to transport its unimaginably vast cargo containers from one solar system to the next. These ships can reach truly staggering velocities (>0.8c even for 'short' journeys) before finally stopping their acceleration phase.

Each ship on the railway is a series of huge, very precisely crafted mirrors, and as such any form of impact could be catastrophic. The ships that form the 'train' follow one another along a very finely controlled series of lasers. Each ship reflects the light for their own propulsion back to the base station, and the light for the next ships further along the line. Needless to say the Humerns have some pretty impressive precision mirrors, laser technology and material engineering skills.

Even with all their technological prowess interstellar gases and debris can still cause an issue for the photonic railways, and so the Humerns use 'Ploughs'. A Plough is a large blast shield (usually 10x the diameter of the following ships and designed to be thick enough that it will survive the trip) and set of massive gyroscopes, designed to either absorb or deflect interstellar gas and debris away from its more fragile followers. It requires a lot of laser stations at the sending end to speed it up, and often the Ploughs are allowed to 'crash' into elliptic orbits around the target star for capture using more conventional means rather than being slowed by laser stations at the destination (or sometimes they're just flung into the interstellar void).

The question is this: How many Ploughs will be required to clear (and maintain) a path through the somewhat inaccurately named 'void'? Will a large fleet when the railway is established be enough to clear the path for generations to come, or will each train require its own Plough to clear the way?

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    $\begingroup$ Won't the photonic ploughs block the laser beams propelling the train of ships? I admit this version of beam propulsion confuses me. If the mirrors reflect light back to the base station, then how does the light reach the next ship further down the line? $\endgroup$
    – a4android
    Commented Jan 25, 2018 at 11:34
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    $\begingroup$ I recommend looking at this interesting concept of beamed propulsion: centauri-dreams.org/?p=38164 SailBeams are cool! Also, do a search on the Centauri Dreams website for beamed propulsion and you will have a gold mine of articles on beam propulsion. $\endgroup$
    – a4android
    Commented Jan 25, 2018 at 11:41
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    $\begingroup$ @a4android: That was the purpose of the previous question about photonic railways. The eventual solution was to have 'clusters' of mirrors on each ship that were individually angleable. With sufficiently precise (read 'hand waved') angling you can bounce a laser 'through' one ship to hit the next, which then bounces it back 'through' the first ship before it hits the base station again. You can then put as many trains in the line as you have available pairs of mirror clusters, you just need some insane maths and ridiculous material physics. $\endgroup$
    – Joe Bloggs
    Commented Jan 25, 2018 at 11:43
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    $\begingroup$ OK. I should go back to previous question for an explanation of your photonic railways. $\endgroup$
    – a4android
    Commented Jan 25, 2018 at 11:52
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    $\begingroup$ This sounds like the ideas of a mad mathematician, after a night of heavy drinking with a slightly sadistic mirror maker. I like it! $\endgroup$
    – Burki
    Commented Jan 25, 2018 at 13:58

4 Answers 4


Given that you'll be cutting what are essentially straight lines across the orbital plane of the galaxy you're going to need to plough the line continuously in realtime at the head of each train and even then you are going to have incidents. The greater the angle the line cuts across the mean orbit of the galactic arms the worse the situation will be when it comes to lateral impactors getting at the train behind the plough, so lines directly outbound from the core to the rim are at greatest danger from lateral debris strikes behind the shield while lines that run retrograde near orbital lines are the safest. Running prograde near orbital lines will be very dangerous; during the acceleration phase debris can "catch up" to the back of the train because it's moving slower than the mean motion of the material around it. In the case of tangential and prograde lines there is also the possibility of late phase occlusions where debris gets between the launch station and the train after the plough has cleared the line and blocks laser light emissions from reaching the train at all. Retrograde lines are safer but not immune to this phenomenon.

In short you're looking at a plough at the front of every train, a shield behind some of them that you can shed fairly early in the run, and a steady, small, but significant lose rate in incidents ranging from slight damage to single containers to the cascade demolition of whole trains. Space is huge and very sparsely scattered with matter, the rate of loss will be staggeringly low as a percentage of traffic but it will be there.

  • $\begingroup$ D'ya know: I hadn't even considered galactic orbitals. These trains will have to take account of the currents of space. That's pretty darn cool. $\endgroup$
    – Joe Bloggs
    Commented Jan 25, 2018 at 19:51
  • $\begingroup$ Given your last statement the rate of loss will be staggeringly low as a percentage of traffic but it will be there, why would we even need a plow in this case? $\endgroup$
    – kingledion
    Commented Jan 26, 2018 at 2:08
  • $\begingroup$ @kingledion I would, there are still major benefits to clearing the line, it will reduce loses, it just won't eliminate them. $\endgroup$
    – Ash
    Commented Jan 26, 2018 at 13:00

Give each train its own plow

High-velocity impacts with lightweight projectiles don't follow the intuitive pattern of a bullet drilling through anything in its way, requiring very thick armor to prevent penetration. Rather, the faster the projectile, and the lighter it is, the more the impact resembles a small explosion. At relativistic speeds, and with individual atoms of interstellar medium, the result is an explosive burst of radiation that does not penetrate no matter how thin the shield is. Mitigating this radiation and sustaining the ablation rate of constant bombardment are the primary concerns.

Thus, the plow does not need to be incredibly thick or enormously massive. On a trip to Alpha Centauri (4.4ly) at up to 0.2c, a spacecraft would appear to suffer only half a millimeter of ablation to a quartz shield. The individual particles explode on contact with the shield, blasting out microscopic divots, but not penetrating through to affect the spacecraft behind them. As long as the shield is thick enough to survive ablation over the duration of the journey, it will suffice.

At 0.8c, the amount of energy released in each impact will be considerably greater than at 0.2c, but the required shielding still isn't excessive. If the rate of ablation scales linearly with the kinetic energy of collision, this implies around a centimeter of ablation for the 4.4 light-year distance.

Each train can be given a shield just a few centimeters thick and slightly larger in diameter than the train itself- there's no need for it to be ten times the size of the train, or for it to be the size of a spacecraft in its own right. A simple, disposable shield made of easily-available materials will suffice, can be recycled and reformed into new shields at the destination, and in the context of interstellar travel is so cheap it's hardly worth accounting for.

  • $\begingroup$ The impact physics lab you linked has "gun capable of propelling objects... over 33,000 feet per second." This is 0.0003c, a tiny tiny fraction of the speed the ship is going. Don't assume that this can tell us anything about relatavistic speeds. $\endgroup$ Commented Jan 25, 2018 at 16:11
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    $\begingroup$ At that kind of speed, will the impact of particles on the shield create a fusion reaction? Even if it doesn't what is to stop bits of this very thin shield breaking off in an impact and then impacting the ship at some fraction of lightspeed? $\endgroup$ Commented Jan 25, 2018 at 16:12
  • $\begingroup$ According to the paper I linked, neither of those is a realistic concern given the physics of relativistic impact. Thin shields have been proposed for relativistic interstellar travel for several decades so if you're aware of any new research indicating their unsuitability I'll update this answer. $\endgroup$
    – Catgut
    Commented Jan 25, 2018 at 18:01
  • $\begingroup$ upvote just for the reference paper. Previously, I asked a question about collecting space dust on an intergalactic/interstellar generation ship, and this points to a good theoretical analysis. Thank you. $\endgroup$ Commented Jan 26, 2018 at 1:43

You don't need ploughs.

Two parallel beams, going in opposite directions, left on continuously. The cargo vessel has a little bit of maneuvering ability (mirrors), so halfway there, it drifts into the other beam and begins to decelerate. Where it could have a collision with a returning ship at relativistic velocities.... OK. Four beams, two pairs, or maybe two lanes in each of two beams, if the beam was wide enough.

A debris object (particle, pebble, nut, bolt, wrench, etc.) that floats around in a beam is accelerated in the direction of travel. The smaller its mass, the higher the acceleration (barring shadows cast by the vessel). The geometry and composition of the side of the debris object receiving the beam would affect velocity components normal to the axis of the beam, so you'd want to make sure that objects wouldn't drift from one beam to the other somehow.

So anyway, the beams plough the road. You'd maybe want to leave the beams on for awhile when the system is first booted, to clear the space. Debris objects would be entering the destination system at relativistic velocities, so you'd want to point the beams above the plane of the destination system's ecliptic. Even so, given undefined object geometries and such, it seems there would be some spread or divergence from the more collimated beam. Since incoming cargo vessels would have a bit farther to travel, they could perhaps use mirrors or similar to establish an approach vector to an unloading station, and leave the beam earlier.

So what happens when an object drifts into a beam? The drifter would be lit up. Forward sensors on the cargo vessel might be able to detect its position (blue-shifted infrared?). If the beam width were large compared to the width of the cargo ship (its beam, heh), it might have enough room to establish a minor course correction possibly months ahead.

Probably off-topic details:

  • Civilizations downstream from the cargo vessel's system might become annoyed at the rain of relativistic rocks on their turf, which all seem to come from a very precise point in the sky.
  • The political relationship of both the source and destination systems would have to be completely harmonious over an indefinite number of decades, even at fine detail. Some dissident group tows something (perhaps a long rod of tungsten) tied to a course-correction unit, into the beam, and they can nuke places in the destination system. How would you police that volume of space?
  • $\begingroup$ This isn't a bad idea, but it needs to weigh costs in the balance. The cost of operating the lasers to clear the path vs. the cost of the ploughs and their replacements. I expect you'd need to maintain your first assumption of a beams fore and aft (propulsion and the "plow") and how much having a foreward beam will slow the traffic (or, how much the aft beam must be strengthened to account for the destructive fore beam). The answer is good, but to be great it needs this analysis. $\endgroup$
    – JBH
    Commented Jan 29, 2018 at 20:20

Use the laser beam the train rides on to clear the lane just before sending the train.

Just send a big burst followed by the train. Anything solid that's moving just in front of a train will probably make things worse as it will disintegrate and create more junk to fly though.

  • $\begingroup$ That’s an astonishingly simple solution, but it would have to be a constant beam or debris/gases wouldn’t get pushed out of the way before the train hit them. Certainly worth thought though.. $\endgroup$
    – Joe Bloggs
    Commented Jan 26, 2018 at 10:15
  • $\begingroup$ I mean what good is having, what is essentially a supper laser if you cant blast stuff with it. There is only a 0.2c difference in speed. $\endgroup$ Commented Jan 26, 2018 at 11:11
  • $\begingroup$ the super laser has to move things out of the way of the train, the train is constantly accelerating. Either the laser has to constantly accelerate the obstruction too or eventually the train will catch up and hit it. $\endgroup$
    – Joe Bloggs
    Commented Jan 26, 2018 at 12:13
  • $\begingroup$ I was thinking more that it would just vaporize it, maybe it's just sci-fiction ... lol. But it was the simplest solution I could think of $\endgroup$ Commented Jan 26, 2018 at 22:47
  • $\begingroup$ The laser could absolutely vaporise whatever is in front of it, but hitting even the thinnest vapour at relativistic speeds leaves you having a Bad Day. $\endgroup$
    – Joe Bloggs
    Commented Jan 27, 2018 at 11:09

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