How well will interstellar ploughs work?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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?

Answer 4

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.