A couple of posts in the past week by Centauri Dreams’ Paul Gilster have considered the question of a far-future colonization of the ice dwarfs of the Kuiper belt, the cloud of icy bodies orbiting beyond Neptune’s orbit of which Pluto is the most prominent member. Inspired by Karl Schroeder’s blog post about the distinction between habitable and colonizable worlds, and by a recent talk given by engineer Ken Roy, Gilster speculates. In “Interstellar Expansion: Colonizing Ice Dwarfs”, he sets things up.
Pluto is a case in point. Here we have a surface that appears to be a shell of nitrogen ice covering water ice. When New Horizons gets to the Pluto/Charon binary in 2015, one thing to look for is an equatorial bulge that could have been left over from the early days of Pluto’s formation. No bulge makes the case for stretching of the ice shell over Pluto’s lifetime, strengthening the possibility some are noting that the ice dwarf could contain an ocean beneath about 165 kilometers of crust, an ocean that may be just as deep as the crust is thick
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Build a settlement on an ice dwarf in the outer system and you are not only creating space for living and doing science, but also building the technologies that will one day be used in interstellar colonization missions. But Roy noted that the science fictional image of a domed city in a harsh landscape just won’t work here. Induce Earth-class atmospheric pressure inside such a dome and even a small one (1000 feet in radius) would require a four-inch thick layer of steel to keep the dome intact. Moreover, ice dwarfs have but feeble gravity, creating medical issues from muscle atrophy to bone problems, loss of body mass, sleep disturbance and more. A better choice, then, is to move inward, creating the colony deep within the ice dwarf itself.
At 160 meters, the ceiling of a colony hollowed out within Pluto would be fully supported by the air pressure inside. Artificial light would be essential, of course, and we still have a gravity problem, for Pluto’s gravity is only 6.7 percent that of the Earth — a 200 pound person on Earth weighs but 14 pounds on Pluto. Roy suggests a rotating torus in this setting could provide living and working spaces at 1 Earth gravity. At 1 revolution per minute, a 1790-meter torus supported by maglev rails could accommodate, by Roy’s estimation, 10,000 people living in conditions that would more or less resemble the worldships so often imagined by science fiction writers.
We’re assuming technologies that can create large rotating structures in low-gravity environments, with the ability to move spacecraft at velocities of 0.001 c to build and supply the colony. We’re also assuming proven fusion power plants and considerable expertise in mining and construction. We would put these tools to work to extract local silicates and metals from the surface and, perhaps, rock from buried impactors. We would be working in an environment rich in H2O, but also in methanol, hydrogen cyanide, formaldehyde, ethanol, ethane and long-chain hydrocarbons, all within a salty ice mantle.
In “Resources Between the Stars”, Gilster concedes that these icy worlds are likely to be very poor in the metals and other heavier elements necessary for life, never mind civilization. He turns to rogue planets, too.
We know little about these worlds, but it’s assumed that great numbers of them are out there, doubtless the result of gravitational interactions in young solar systems that caused them to be ejected. Dorian Abbot and Eric Switzer (University of Chicago) call these ‘steppenwolf’ planets because they ‘exist like a lone wolf wandering over the galactic steppe.’
Louis Strigari (Stanford University) has estimated that as many as 105 objects larger than Pluto exist for every main sequence star. If that’s anywhere like the case, then rogue planets ranging between the size of Ceres and Jupiter should be out there in abundance, and we can hope to put some constraints on their numbers through future gravitational microlensing surveys and even exoplanet transit studies, which may catch a rogue planet’s transit. Some studies show that radiogenic heating from the planetary core could keep an ocean under crustal ice liquid for billions of years even out here, where there is no star to provide warmth.
Deep space is not without resources, as we’re learning every day. Roy told the audience in Huntsville that cometary objects from the Kuiper Belt to the Oort Cloud should offer CO2, ammonia, methane, oxygen, carbon and nitrogen, while we can exploit asteroids for silicates and metals. We can only imagine what resources might be available in unattached worlds moving between the stars. This is all work for a civilization that has built a thriving deep space infrastructure, but then, thinking about the future is what we do here.