If technology and economy continue apace, then in coming centuries our demand for resources will compel us to devour asteroids, Mercury, Mars, Venus, the moons of Jupiter and Saturn… We will absorb everything we contact, creating vast, spinning habitats and sails which catch the solar wind. Eventually, we will need to harvest the sun itself, with more metal in its core than all the planets combined. Yet, along the way, we will need to consume Jupiter, Saturn, Uranus, and Neptune. I propose a way to do so: Roche Mining.
The Roche Limit
When two planets spin close together, they experience a force that tugs each planet apart, crumbling them. This is because the side of each planet which is closest to the center of orbit wants to spin faster than the parts furthest away from the center. This difference in rotation rate between each side of the planet pulls them, counteracting the gravity that holds the planet together. The orbital distance at which they crumble is the Roche Limit. Spin further inward, and half of each planet spirals into the center, while half is flung outward to form rings of debris.
We will do this to each pair of planets: Jupiter and Saturn, spun together until half of each core is thrown outward in chunks, Uranus and Neptune together, and finally, the sunken core of Jupiter+Saturn spun alongside the similar core of Uranus+Neptune, to toss half of that remaining mass into orbit. The cost of nudging them into destructive orbits is much lower than the cost of hoisting all that gas and rock in a rocket, or on a space elevator.
Most of the gas from these four planets will be harvested even more easily: when the gas giants begin to spiral together, the gas itself is tugged by that same force, the result of the difference in rate of rotation between the upper atmosphere and the surface of the planet. The gas is lofted toward the center of orbit, somewhere between the two planets. At that balancing point, the gravity in each direction cancels, and gas accumulates. Cargo tanks could re-fuel from this floating oasis, completely avoiding the cost of launching out of a planet’s gravity well. (Additionally, when those cargo ships are full and ready to leave, they might perform a gravity slingshot, spinning closer to one of the orbiting planets such that they are flung outward again at a higher velocity, while the planet’s orbital energy is reduced by an equal amount, drawing it closer to the center of rotation. The planet’s potential energy is added to the rocket fuel!)
Fundamentally, this process of drinking from pooled gasses and spinning planets until they tear apart saves us the energy of lifting that matter off-world. Jupiter’s gravity is too intense for rockets to escape, unless they are almost entirely fuel. That launch cost limits the net energy of scooping up gas in a rocket. If Jupiter is instead torn into manageable chunks, after all the gas has been drunk from its surface, then rockets can land, mine the chunks, and escape those smaller gravity wells with little effort.
In essence, Roche Mining utilizes the potential energy of each pair of planets to lift the mass of those planets apart. The potential energy between Jupiter and Saturn is great enough to split both worlds, and to begin that process, we only need to nudge them a little ways, disrupting their orbits and setting them spinning around each other.
Rocks and Rockets
Roche Mining is the most efficient means for converting potential energy into extracted mass, and the impetus comes from a tiny nudge. (The eROI is insanely high.) And, when the two planets are spun very close, just before the Roche Limit is reached, then the force that tears them apart is a counterbalance to gravity — on the outermost pole of each planet, as well as the innermost point, you would experience almost no apparent gravity. Rockets could launch from these places easily, perhaps after laying nuclear explosives which might aid in the fragmentation of the planetary core, when the Roche Limit pulls it in two.
The simplest method for nudging planets is to use Mars as a medium for exchanging energy between Jupiter and Saturn. Saturn will lose potential energy, which Jupiter gains, at almost perfect efficiency. How to nudge: set rockets on the surface of Mars, and aim it toward Jupiter. Swing past the gas giant as a gravitational slingshot, reversed, such that it slows Mars down while imparting energy to Jupiter.
With greater energy, Jupiter enters a higher orbit, circling further from the sun. Nudge Mars onward toward Saturn, and slingshot again, this time to accelerate Mars, while slowing Saturn down. Because Saturn loses energy, it falls closer to the sun. After numerous such sweeps, Mars has transferred energy from Saturn’s orbit to Jupiter’s, and the two are brought close together. You only pay a comparatively small cost, rocketing Mars about the first time, and reorienting periodically, while the slingshot from Saturn powers the rest of the journey.
With comparatively little effort, we would greatly multiply the resources available to us. And some of us today may live to see it: a great molten clod congealing at the center of a disc of debris, arms of crumbling rock and metal spiraling outward, drone rocket-rigs’ thrusters sparkling along the fringes.