Harvesting from Gas Giants

There has been some small amount of discussion here on Earth about mining extraplanetary resources. One subject was the He3 on the moon, which makes an excellent fusion fuel. Another has been precious metals or rare earths on asteroids. These are all high value to weight materials, found on places without deep gravitational wells. The idea would be very simple. Go out and mine them where they are found, ship them back via rocket, and land them somehow on Earth.

An alien civilization might start with such things in their early days of harvesting interplanetary resources, but what about the gas giants? Assuming their solar system has one or more gas giants, like we have Jupiter and Saturn, these planets represent the large majority of planetary mass in the solar system, meaning they might be the location of the largest amount of resources. We do not yet understand the composition of the core of these planets, as their atmospheres are too thick to see though and too dense to fly a probe into and survive. We hardly know the radius of the solid cores, or if indeed there is a well-defined one. We know what we see, the upper atmosphere, which for Jupiter and Saturn, is a banded pattern with a great amount of wind shear and turbulence. The banded patterns likely come from the mixing of lower layers of the atmosphere due to convective circulation forced by rotation, akin to Coriolis forces. The lower layers have a different composition due to the gravity gradient and therefore a different albedo shows where they are circulated upwards.

Is there any way that energy might be harvested from a planet such as Jupiter and brought back to the home planet, such as Earth, with a positive return? There must be a total amount of energy expended to both obtain the energy at the gas giant and then transport it back to the home planet and down to the surface there and this has to be less than the energy provided. These calculations cannot be done now, as we are short a hundred years or so of engineering, but some ideas as to the sources can be guessed. One source is the simplest: hydrogen. Hydrogen brought to a planet with a surplus of oxygen is an excellent fuel, only making water when combusted. One idea for alien planets is that their energy on the surface would be transported and used as hydrogen gas, but whether it is or not, hydrogen for power plants is still a viable concept.

The upper atmosphere of Jupiter and all gas giants is mostly hydrogen, with a bit of helium mixing in. Helium does not much degrade the fuel ability of hydrogen, and might be fairly valuable on the home planet in its own right, so bringing back a bit of an admixture with helium is not a bad idea. Gas giants are gas giants because they are large enough to retain hydrogen for time comparable with the age of the solar system or the universe, so any large enough planet should be mostly hydrogen at the top of its atmosphere. Skimming into that exosphere with a large tank and a large scoop should pick up hydrogen quite nicely. It might be compressed a bit more before shipping it back to the home planet, to whatever degree minimizes the tank mass per hydrogen mass.

The energy cost for shipping some tank back to the home planet is not connected to the energy for a space ship, as shuttle orbits can be used which are immensely efficient, needing only steering propulsion. There would have to be energy expended to bring the tank of hydrogen up to match the velocity of the ship transporting it, and then energy expended when the tank was dropped off into an orbit around the home planet. An empty tank would have to be attached in place of the full tank to the shuttle craft, and this could be managed as a momentum transfer, so that only the energy to decelerate the mass of hydrogen is necessary, plus some energy for steering and aligning orbits.

Once in orbit around the home planet, the hydrogen would have to be brought down to the home planet’s surface. Reusable shuttle craft is one idea, burning some of the hydrogen with atmospheric oxygen to land calmly and gently. We have only begun to perfect the art of reusable surface-to-orbit shuttles, but they obviously can be done. Another more exotic idea is to have a large diameter pipe oriented vertically, from geostationary orbit down to the surface. This idea has been discussed as a space elevator, but it is even simpler to have a hollow pipe though which hydrogen could flow. Obviously, there would be great bending moment on the pipe from the atmosphere, but if that is solved, it would likely be more efficient that reusable shuttles.

Other sources are much more difficult to harvest, but might be possible. There are tremendous wind forces on Jupiter, and likely gas giants in other solar systems. There is no clear way to put something down on the core surface to capture the wind energy and then transform it into a transportable form and bring it up through the entire atmosphere back to orbit where it could be returned to the home planet. Instead, some form of long tether between two floating objects, at different altitude, might serve to provide enough differential wind velocity to harvest power electrically. We have not mastered the art of transmitting power over long distances even on Earth or from Earth orbit to Earth surface, but it does seem to be remotely possible that a tethered platform could transmit power to an orbiting satellite, which would then put it into some form where it could be transported back to the home planet. Batteries are an inefficient form of energy storage, and chemical fuel in the form of carbon bonds is much more energy-dense. Shipping water and carbon dioxide on the shuttle back to the gas giant, where they were transformed to octane or something similar, and then shipping by shuttle back to the home planet would use very little energy in transferring orbits. The octane might be burned in home planet orbit and the exhaust packaged and reshipped to the gas giant.

Another less thought of form of energy harvesting is from the magnetic field of the gas giant. If the alien solar system had rapidly rotating gas giants, they should have strong magnetic fields, although we frankly do not understand the mechanisms for planetary magnetic fields very well. A huge loop of conductor, orbiting at low orbit over the gas giant and oriented perpendicularly to the field lines, should capture a large current which could be transformed and transferred in much the same way as electrical power captured from wind shear forces.

All in all, energy harvesting from a gas giant might be quite possible for an advanced alien civilization, and this would be manifested by large numbers of shuttle craft going to and from the gas giant and the home planet. These might be visible in very limited circumstances. Nothing else in any of these schemes would even conceivably be visible with a giant telescope from our solar system, but if we could better understand the sources of usable energy in an alien solar system, we might better understand the longevity of the civilization and their propensity for star travel.