Given all the logistics involved, it's unlikely that humanity will ever see our way outside the Solar System to colonise exoplanets. But the possibility of settling elsewhere inside the Solar System isn't so far-fetched.
NASA is planning an outpost on the Moon. Crewed missions to Mars aren't far off. Heck, we even already have humans living off-planet (albeit for temporary stints) on the International Space Station.
So is there anywhere else in the Solar System that humans could make our home? Well, according to physicist and astrobiologist Pekka Janhunen of the Finnish Meteorological Institute in Finland, dwarf planet Ceres isn't entirely implausible.
Ceres is an interesting chunk of rock. It hangs out in the asteroid belt between Mars and Jupiter, and with its 952-kilometre (592-mile) diameter, it's considered both the largest known asteroid in the Solar System, and the only dwarf planet closer to the Sun than Neptune.
Why Ceres? It ticks a lot of desirable boxes, Janhunen thinks.
"The motivation," he writes in a preprint paper published on arXiv, "is to have a settlement with artificial gravity that allows growth beyond Earth's living area, while also providing easy intra-settlement travel for the inhabitants and reasonably low population density of 500 [people per] square kilometre."
Mars and the Moon, he argues, might not be the best places for human colonies, because their natural gravity is so different from Earth's. We know that astronauts face health problems when returning to Earth from a low or zero-G environment; we have very little idea of the effects of growing to maturity in low gravity.
An alternative to the planetary colony model is an artificial space colony, orbiting the Sun - a space station spinning to generate enough centrifugal force to mimic one g: Earth gravity.
This would be logistically awful, too. If the population grows too large for one settlement, multiple settlements may be required. If multiple colonies are in orbit around the Sun, they could drift apart, creating other problems, such as inter-settlement travel. If they're orbiting a common body, collision avoidance becomes a problem.
Janhunen's solution is quite neat, really, at least in concept: use Ceres as a base around which the spinning settlement nodes could orbit, connected by a fixed frame.
This wouldn't just solve the problem of keeping the settlement nodes together without the potential of collision, but would also neatly solve the problem of materials, since they could be gathered directly from the dwarf planet. Nitrogen is of particular importance, Janhunen said, since it makes up so much of Earth's atmosphere.
But we also know that Ceres is pretty salty, and recent research suggests that it also might have a lot of water below the surface. Solar panels on the dwarf planet's surface could easily power a space elevator to the satellite.
"Lifting the materials from Ceres is energetically cheap compared to processing them into habitats, if a space elevator is used," Janhunen explains. "Because Ceres has low gravity and rotates relatively fast, the space elevator is feasible."
Radiation shielding, he said, could be built from 80 percent silicate regolith (rock from Ceres) and water. The habitats would be divided into rural and urban spaces, with a soil depth of 1.5 metres up to 4 metres as needed for trees and gardens.
(P. Janhunen, arXiv, 2020)
Because Ceres is so far from the Sun, mirrors could be used to direct sunlight towards the habitat, for the purpose of growing crops, for illumination, and for solar power. These mirrors would be hinged on one side of the disc-shaped satellite, like a makeup compact, and could be adjusted to collect the most sunlight as the dwarf planet moves around the Sun.
"We use a disc geometry for the megasatellite because its symmetry eliminates tidal torque so that reaction wheels are not needed to maintain attitude," Janhunen writes.
"The habitats are illuminated by natural sunlight. The sunlight is gathered onto the disk by two planar mirrors inclined at a 45-degree angle and concentrated to desired intensity by parabolic mirrors."
This could be grown, as needed, by simply adding more habitats at the edges of the first one, to potentially millions of habitats, for a lifestyle that could, perhaps, be even better than life on Earth.
After all, there would be no natural disasters or undesirable weather, and its modularity would mean that it could just keep on growing with the population. In principle, Ceres could support, Janhunen believes, 10,000 times Earth's current population.
Of course, it's all very speculative, and yet to be tested. In addition, Janhunen notes that orbital artificial gravity is still a goal that is yet to be realised.
For that matter, so are space elevators, giant mirrors, and radiation shielding sufficient to protect a space colony. Orbital simulations for Ceres, and the logistics of transporting that many humans out past Mars, are also factors yet to be considered.
However, once these kinks are ironed out, it would only take around 22 years to build a human satellite in orbit around Ceres, Janhunen calculates.
"The overall level of difficulty of executing this project is probably similar to settling Mars," he writes.
"The delta-v and triptime to Ceres are longer, but on the other hand one avoids planetary landings and the atmospheric weather and dust. On Ceres it requires some effort to lift the materials to orbit using the elevator, but it is energetically cheap. Once the materials are in high Ceres orbit, the thermal environment is uniform and energy is easy to get due to absence of eclipses."
Certainly worth thinking about, right?
Janhunen's paper, written under the framework of the Finnish Centre of Excellence in Research of Sustainable Space, is available on arXiv.
H/T: Phys.org