Monday, November 30, 2015

Interstellar Nomads – Part 7 – Perils

Let me spoil this post by saying that it looks like a huge ship of interstellar nomads could handle all the perils the galaxy is going to throw at them. Sorry. Keep reading if you want to know the details.

Several posts in this blog attended to the perils that an alien civilization would face. These were not the social perils like revolution and war; these catastrophes were treated separately from the natural disasters that the alien civilization might encounter, or in some cases, would certainly encounter. This post just talks about the equivalent set of natural disasters that nomads about a permanent spaceship might have to deal with.

There really is only one. Something hits the ship. Something big. Just like asteroids can come in and level a portion of the surface of a planet, and send shock waves through the atmosphere of the rest of it, extinguishing many species, a big enough rock or iceball that a ship runs into would wreck it. It isn’t so much the size of the rock that makes a difference, it is the differential speed. The ship, if it is in the phase of traveling between solar systems, is moving at a fraction of light speed, perhaps 0.1% or even 1%. This means that anything impacting it has a lot of kinetic energy, seen in the rest frame of the ship.

The question is, why would they steer their ship into such a rock? Obvious answer: they didn’t see it in time to maneuver the ship to a non-collision trajectory. Why wouldn’t they see it? There are two explanations: one that they weren’t looking because of equipment failures and the other is that their equipment couldn’t see far enough out to find the rock.

No matter what they do, there will be times when they are flying blind. If they have five equivalent forward-looking sensors, and each one is down 0.1% of the time due to unforeseen and unplanned outages, all five will be down 10 to the minus fifteen of the total time. If they travel for a hundred million years, this is about thirty seconds. Thirty seconds is probably a tiny fraction of the response time they allow themselves to take care of maneuvering. In short, they are flying blind so little it doesn’t matter a whit.

What about not being able to see far enough? In other words, here are a group of genius engineers who build a ship to go so fast their best sensor cannot see obstacles in front of it. Well, slow it down.

There is another aspect to this, and that is that detection and tracking ranges for active sensors depend on the reflectivity of the object being detected and tracked. This means a small black (in all used wavelengths) object lying dead ahead would not be seen very far out. If the ship is large, the time for it to change course and miss something in front of it is also large. It is a question of the size of the maneuvering propulsors that are used to change direction. If there aren’t any, and the direction of the ship is changed by manipulating the direction of the discharged propulsive material from the main propulsor pushing the ship forward, the ability of the ship to maneuver fast is limited. Having side-facing propulsors of sufficient size is what is needed. They need to be located so they can provide some sideways thrust to the center of mass of the ship rather than being located at one end, where they would simply change the direction of the ship, leaving the main propulsor to then get the ship off the previous course onto a new safe one.

This is a question of ship design. There is a maximum size of space rock they can allow to hit the ship without major damage which would endanger its ability to reach the next stopping point, a solar system where repairs can be made. This maximum is controlled by the hull thickness, which adds to the weight of the ship and therefore to the needed size of propulsors, both main and side. On the other side of this tradeoff is the size and power of the forward-looking sensors, which essentially serve the purpose of providing maneuver time. The required maneuver time is controlled by the ratio between the equivalent side thrust, no matter how provided and the mass of the ship. Unfortunately, this tradeoff is a difficult one, as maneuver time is inversely proportional to speed of the ship and only proportional to the fourth root of the sensor power. Hard to push something much higher when you are fighting a fourth power relationship.

This means passive defenses against impact, such as a sacrificial layer on the front of the ship, and active defenses, meaning something like a laser cannon, are likely to be key players in the design of the ship. One concept is the combination of a laser scanning device with a destructive mode of operating. The cannon would scan for objects, using full power to maximize detection distance, and then operate as a deflector or disruptor in close. A very high power device would be needed for this. It would be interesting to see if the power needs of the ship aren’t really hotel and thrust components, as is almost always the initial guess, but actually sensor and disruptor power.

Another design factor is the location of critical components as far away from likely impact areas, principally the nose, as possible. If hotel functions were destroyed, the ship would be a goner. If main propulsion was all destroyed, the ship could not decelerate at the next destination solar system, and so eventually it would also perish.

The design considerations here are generic to starship design, not solely for a nomad cruise ship. The only principal difference is that the cruise ship is no burdened with the large baggage load of vehicles to descend to the surface of a planet. There do not seem to be any additional perils that would face a nomad ship when it was engaged in parking in some asteroid belt of a destination solar system, extracting resources and accumulating energy. There are design dangers, but natural dangers, no. Asteroid impact might be considered one, if the belt was dense, but if a space rock could be detected and dealt with while traveling at fractional light speed, an asteroid heading for a collision with the ship could easily be handled.

At this point in Earth’s progress toward astronomical knowledge, we don’t have much of an idea of how many rocks might be residing in interstellar space. Up to recently, we didn’t know there were rogue planets, which may have been because we didn’t know if there were planets at all around most solar systems. That uncertainty has disappeared, but all the rest remain. This information would be most interesting for contemplating a nomad ship design.

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