Energy availability in interstellar space [closed]

Question

This answer by Abulafia on long distance trading makes me think of the concept of “civilizations gone to seed” described by Asimov, as asteroid-based habitats, already used to not having a planet, can simply cast off and wander across to the neighboring solar system.

So, consider a habitat that is stationed a long way from its parent star — specifically, half way between stars as a worst case. How much energy is available?

What is the flux from starlight? Could large enough collectors be useful, and still build-able?

Bussard ramjets, in concept, are supposed to collect hydrogen for fuel. Could a slow moving habitat gather useful quantities of hydrogen to maintain their power needs?

How far do the Oort clouds reach? Are there icy bodies all the way to the half-way point where the clouds of adjacent merge? What is the density distribution of these clouds and how much would that offer such a habitat for hydrogen (for power) and other material?

What other sources have I overlooked?

I’m assuming the stellar neighborhood surrounding our own sun, including the region between this solar system and our immediate neighbors.

Answer 1

The Oort clouds extend out approximately to distances believed to range from 0.8 to 3.2 light years and if this is the same for other stars and the average distance of separation for stars in the vicinity of the solar system is six light years. An asteroid habitat at the mid-point between stars will be three light years (on average) from either star, and barely within the upper bound estimate for the outer reaches of each star's Oort clouds.

A mobile space habitat could harvest resources from the icy planetismals and cometary bodies in the extreme reaches of the overlapping Oort clouds. Their ability ability to convert matter into usable energy will be an important limit on their capacity to survive and thrive in deep space.

The question of available energy resources for interstellar habitats has been considered by Eric R Jones and Ben M Finney's "Fastships and Nomads: Two Roads to the Stars" 1. Their model interstellar habitat is comet-based, but the energy issues are the same. Comets are mainly asteroids with lots more dirty ice.

The greatest obstacle facing potential nomads, is energy which is very scarce in the interstellar deep. Nonrenewable sources would include deuterium to power fusion generators and the kinetic energy (energy of motion) of the comet, which could be extracted from interaction with the galactic magnetic field. Renewable energy sources would include starlight collected with gigantic mirrors and possibly cosmic rays (if anyone can figure out a practical scheme for catching them). Elsewhere we have estimated that the aluminum in a typical comet would be sufficient to build mirrors to collect a few hundred megawatts of starlight. Other more abundant substances may prove to be more applicable for the mirror surfaces. We expect that power levels of 1 megawatt (MW) per person are reasonable and therefore a typical comet could support a few hundred people with starlight.

The use of Bussard technology to collect hydrogen as a resource is probably going to be limited by the amount of power required to drive such systems. Because of the low relative velocities of nomadic habitats to the interstellar medium hydrogen would have to be collected by first ionizing the gas and driving it towards the habitat with vast magnetic fields. It seems likely that two sets of magnetic fields would be used to push the ionized hydrogen together. One magnetic scoop would be based on the habitat itself, while another scoop was situated at a distance and this pushed the ionized gas towards the habitat's collection field.

The main drawback would be the power necessary to implement a dual collection system. If the energy extracted from the hydrogen exceeded the energy cost in powering this Bussard double collector, then it would be feasible. This is speculative.

Jones and Finney are confident that sufficient energy can be extracted from starlight, the habitat's kinetic energy, cosmic rays (speculative), and nonrenewable sources like deuterium to make small spacefaring communities in the interstellar deep viable.

1 Ben R Finney and Eric M Jones (eds) Interstellar Migration and the Human Experience (Berkely: University of California Press, 1985)

Answer 2

a4android has an excellent answer on harvesting in situ resources, and will has mentioned "Vacuum energy" (although the concept is still speculative), so I will suggest another thing people in very deep space might use: beamed energy.

Robert L Forward may well have been the first to suggest using high energy laser beams to propel lightsails across interstellar space. A civilization capable of building lasers that can send terawatts of energy across interstellar space and accurately strike a moving target light years away with 26 Terawatts of energy, then powering a settlement or starship in the Oort cloud would be childs play. Indeed, that amount of energy striking the asteroid would probably make it quite warm and pleasant, even at a distance of a light year, and its slow orbit around the sun would make tracking very easy.

While Forward thought of lasers being powered by soar energy satellites orbiting Mercury and feeding the laser array, it is possible to generate huge quantities of laser energy directly by harnessing the solar photosphere. Using can array of mirrors orbiting the sun and creating a torrid laser cavity around the solar equator provides a means of tapping the beam and illuminating many targets in the plane of the ecliptic, or with some clever control of the mirrors, even objects in highly elliptical orbits around the sun. The author suggests that the amount of energy could be tuned to "grow potatoes on Neptune's moon Triton" so once again, illuminating and warming very deep space colonies and powering civilizations that far away would be rather trivial.

The solar laser would appear as a brilliant spot on the limb of the star

If the civilizations had been colonizing deep space comet nuclei, they might already have giant mirrors for collecting sunlight, so the high energy lasers would be easy to tap. Natural sources of energy like the Deuterium trapped in cometary ice could then be conserved, and fusion reactors ignited in the times the laser was occulted by a passing planet.

The only major piece of infrastructure outside the solar lasers themselves would be a system of deep space mirrors or lenses (as described in Forward's paper) to focus the beam and steer it at the moving targets. Since these would be sharing the orbit of Saturn, they are deep space to us, but for people in the Oort cloud, they would still be brilliant points of light unimaginably far away.

Answer 3

If you have the vacuum of space, you always have vacuum energy. https://www.scientificamerican.com/article/what-is-the-casimir-effec/

Maybe not much energy, but you can multiply that by very, very much vacuum.

Answer 4

Most of our technology is based on in some part taking the mechanical energy of hot things making cold things hot.

There is a technology that I've heard about that is makes the system work the opposite way, If this is true then there would be plenty of energy in interstellar space which, if I remember right, is hotter than in side the solar system. This means that all you'd have to do is make a dual system where mechanical energy can gained from either direction. You would essentially have an infinite supply of energy.

Same with Vacuum Energy.

You could also just have a nuclear powered station since you really don't need much energy to run a space station. If you're in between stars just send a probe out to collect more once in a while, maybe once a century.

You could build solar panels. They'd be highly inefficient, but you'd get something from them.

The biggest expenditure of energy I would suspect is figure a way to cool things down, in which case the problem is you are radiating heat, but it is getting caught in everything around you which means you need to move or push the heat away... doing so however would mean expending gas or heat sink resources which there is a finite limitation to, so even if you can get enough energy, unless you have a cooling system that can work without doing that, you will need to have regular visits a solar system... how often would be based on how much gas/heat sink material you can carry and need which is based on the size of your station or ship, it's crew capacity, the number of things it is doing that can't work in heat, etc.

The point is... we can power cities for thousands of years with old inneficient nuclear reactors so power isn't really an issue. It's cooling that is, and you'll run out of cooling materials long before you run out of power.