Friday, November 20, 2015

Interstellar Nomads – Part 5 – Sensors

Interstellar nomads comprise an alien civilization or several who have mastered long-duration interstellar flight and have simply decided not to get off the ships, but just keep traveling. This makes sense in a way, as they have done everything possible to make the starships reliable, problem-free, and durable, but also to provide the passengers a comfortable habitation. They may have done extensive speciation to adapt their life-forms to interstellar travel. If they arrived at some planet that we call habitable, they might call it un-inhabitable as it has unpleasant aspects such as gravity and an atmosphere, which they do not need or want in their current incarnation. Such a planet would just be dirt for their use, or more likely, since they do not favor mining in a deep gravity well, just something littering up the solar system. It would have been better for them if the planet had never consolidated out of asteroids.

How long an alien nomad ship would remain in a solar system depends on the resources that the solar system offers them. They could leave immediately after stocking up with enough resource, including energy, to get them to the next promising solar system, but why would they? The essence of their existence is that they are enjoying life aboard what might be a palatial space cruiser, and whether that cruiser is traveling at some fraction of lightspeed to another destination star, or it is simply orbiting one star for an extended period, makes little difference in their happiness and enjoyment of their life aboard the cruiser.

It will be generations between stars, so these nomads are not tourists, going from star to star to gawk at the planets it has, and see if they have bigger canyons or deeper oceans or funnier creatures or whatever the last one had. They see the ship as planet-bound aliens see their planet. A planet for its inhabitants is a place to live, and in fortunate places, to enjoy life. For nomads, the cruiser is the same thing. That is the center of their attention, not the refueling depots, called asteroids, that they visit out of necessity. So, orbiting around some star which has plentiful asteroids, they might just stay there until the resources get short.

There is an altenative situation, in which there are inhabitants on one of the planets who also do some traveling in space, perhaps only interplanetary, and they have some desire to interact with the nomad ship. Perhaps they regard it has hostile or simply insist on it not taking all their resources, and raise objections to its presence. Then it would be time to leave as soon as refueling was done. But since the universe is so old, and stars live so long, and evolution takes such a lengthy period, the likelihood of a coincidence of visiting a star and having an alien civilization on one of the planets just into interplanetary exploration is vanishingly small.
If the reasons for being nomads were universal, all civilizations will abandon their home planets and cruise the stars. This means there is a short window of time between asymptotic technology being achieved and the transformation of the species into a nomadic one.

When it is time to leave, the nomadic ship has to make a determination of where to go next. They need sensors to find another suitable solar system within their travel radius. This means, most likely, some large aperture devices capable of determining many details of the other solar system. It could be deployed soon after the ship arrived at a destination solar system, so that they could leave as soon as they wanted to, knowing just where to go next.

The sensors would be measuring aspects of nearby stars and their accompanying solar systems for the presence of available resources, which likely means low gravity places to get minerals. Any star high up enough in the stellar sequence produces photons, if they consume them. Otherwise, if they want to find a source of deuterium, for example, the sensor would have to be able to see if it could be found there based on direct signatures, in other words, seeing some absorption lines of it in an atmosphere of one of the planets, or on indirect signatures: where there is one thing they don’t use, there is always one they do use accompanying it.

If they use scooping ships to gather gas from the outer atmosphere of a certain type of planet, for the purpose of extracting some element or isotope they need, their knowledge of how atmospheres form and are retained would give them the knowledge to connect what they can see with what they need to know. An atmosphere with hydrogen lines would have deuterium, especially if the sun exhibits the usual ratio of emissions from these two isotopes.
Thus, a sensor with a large aperture, perhaps ways to block the sun’s output when they are looking at planets, with spectroscopic capability, in other words, just what we have today scaled up to a larger size, would probably be sufficient for them.

One implication of the requirement for these sort-of-ordinary sensors is that they would likely not be made biologically. Biology is not so precise as to be able to grow such sensors. Neither would a biological network be suitable for processing the data from it, so this facility of the ship, as probably many others, requires some mechanical/electronic components. This adds to the list of things they need to find on their asteroid depots, and to the things they need to be able to extract, purify, and adapt to the uses for both sensors and computation.

So, sensor design for a nomad ship for solar system scanning is probably not difficult, but it does impose a new list of resource demands, as well as manufacturing demands, assembly demands, maintenance demands, and computation demands. So far, no obvious barriers pop out.

The ship would also need navigation sensors, to ensure it could make the trip to a destination solar system, and to ensure that it could avoid any obstacles along the way. The existence of rocks in space of varying sizes is not clear, but hitting any large one might be more damaging that they wanted to tolerate. This means some sort of avoidance sensor is needed. This is likely a much more demanding requirement than the star-tracker needed for navigation. Rocks in interstellar space are cold and therefore not good infrared sources. Finding enough power for an active sensor may be a serious problem, as the sensor would need to be able to detect small objects at large distances and then track them to determine their potential for collision. The earlier the object is found and tracked, the less fuel is needed to adjust the trajectory. The degree of adjustment needed is related to how accurately the position and course of the rock is, as it would be necessary to evade the whole zone of uncertainty surrounding the presumed location of the rock at the time of closest approach.

Thus, scanning sensors and navigation sensors are likely not worth exploring deeper, but if there really are rocks in interstellar space, avoidance sensors might present an interesting problem for the nomad ship. A second implication of the sensor discussion is that the most observable feature of a nomad ship in one’s own solar system might be the scanning sensor. It might have a large size and a large albedo, and perhaps some curious aspects, such as glinting. More things to look into.

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