First published February 21, 2022.
Could people one day live in a giant spinning structure orbiting Ceres, the largest object in the asteroid belt? Finnish physicist Pekka Janhunen thinks it is at least physically possible. In a preprint first posted to arXiv in November 2020, he sketched a "megasatellite": a vast disc-shaped frame carrying many rotating habitats, built from material mined on Ceres and lifted into orbit by a space elevator. Each habitat would spin to create gravity close to Earth's, letting its residents live with gardens, trees and breathable air far from home.
The idea drew wide media attention in 2021 and again in early 2022. Here is what the paper actually proposes, why Ceres was chosen, and the large assumptions that stand between a thought experiment and a real settlement.
Who proposed it and where
The paper, titled "Terraforming the dwarf planet: Interconnected and growable Ceres megasatellite world", is by Pekka Janhunen of the Finnish Meteorological Institute in Helsinki, who is also listed with Aurora Propulsion Technologies and the University of Tartu in Estonia. Janhunen is best known in space circles for the electric solar wind sail, a propellant-free propulsion concept.
The work appeared on arXiv, a widely used repository where researchers share papers before or without formal peer review, and it is filed under popular physics. Several revisions followed; the fifth version was posted in May 2021. At that stage it had not been published in a journal. That does not make it wrong, but it does mean readers should treat it as a detailed proposal rather than an established result.
Why Ceres?
Ceres was discovered by Giuseppe Piazzi in 1801 and reclassified as a dwarf planet in 2006. It is about 940 kilometres across and holds roughly 40 percent of the asteroid belt's mass. NASA's Dawn spacecraft orbited it from 2015 until 2018, mapping water ice and finding ammonium-bearing salts and clays, including in the bright deposits of Occator Crater.
Those ammonia compounds are the key to Janhunen's choice. Earth-like air is mostly nitrogen, and nitrogen is surprisingly scarce on many rocky bodies. In the paper he explains that Ceres was picked chiefly because it "has nitrogen, which is necessary for the settlement's atmosphere." He adds that large carbon-rich asteroids could be alternatives, but suitable ones are roughly as far from Earth as Ceres anyway.
Ceres also has very low surface gravity, about one thirty-fifth of Earth's, and spins once every nine hours or so. Together these make a space elevator far more practical there than on Earth, which matters because lifting millions of tonnes of material into orbit would otherwise be ruinously expensive in energy.
How the megasatellite would work
Rather than building on Ceres itself, where gravity is too weak for people to stay healthy over a lifetime, the settlement would orbit it at about 100,000 kilometres. The design has several layers:
- Habitats: pairs of cylinders, each habitat about 10 km long with a radius of 1 km, rotating roughly once every 66 seconds to produce near-Earth gravity on the inner surface.
- Frame: the cylinders would be attached to a large disc-shaped structure by passively safe magnetic bearings, so the habitats can spin while the frame stays steady.
- Light: flat and parabolic mirrors would gather sunlight and channel it into the habitats, since sunlight at Ceres is much weaker than at Earth.
- Soil and shielding: about 1.5 metres of soil for gardens and trees, with most of the mass going into thick radiation shielding.
In Janhunen's figures, each twin-cylinder habitat would house about 56,700 people at a density of 500 per square kilometre, giving each person about 2,000 square metres of living area. Most of the roughly 10,000 tonnes of material needed per resident would be shielding and soil. Because new habitats could be added to the frame over time, the settlement is described as "growable", eventually able to house far more people than live on Earth.
Travelling between habitats
Moving from one cylinder to another would mean leaving the spinning section, so passengers would be weightless for much of a short trip. The paper acknowledges that weightlessness causes nausea for some people on first exposure, and speculates that residents raised with occasional zero-g trips might cope better. It also discusses options for keeping gravity at the stations. The upside, in the author's view, is that internal travel needs no propellant, which helps make a closed, fully recycling economy possible.
How long it might take
The paper offers rough physical limits rather than a schedule. Assuming a self-expanding industrial base that could double its capacity every four months, it estimates about a decade to bootstrap the power system and space elevator, and roughly another twelve years to build the first habitats and shields. Janhunen himself notes that real timelines would almost certainly be longer, given the technology and logistics involved.
The big open questions
The concept is careful engineering on paper, but it rests on assumptions that are far beyond current capabilities:
- Getting there. Ceres averages about 414 million kilometres from the Sun, and no crewed mission has gone further than the Moon.
- Self-replicating industry. The fast doubling times assume highly automated mining and manufacturing that does not yet exist.
- A working space elevator. Even at Ceres, building and maintaining a tether more than a thousand kilometres long is unproven.
- Long-term stability. The paper notes that the orbit is expected to be stable but had not yet been simulated in detail.
- Human health. The effects of spending a lifetime in rotating artificial gravity, with frequent short spells of weightlessness, are unknown.
Put simply, nobody is building this. Its value lies in showing which problems would have to be solved, and in which order, for large settlements beyond Earth to make sense.
The science fiction connection
Many readers noticed echoes of The Expanse, the novel series by James S. A. Corey and its television adaptation, in which Ceres is a major hub of human life in the asteroid belt. In the story, people live inside Ceres itself, which has been spun up to give some gravity, and they depend on water and ice from the belt. Janhunen's design takes a different route, keeping people in orbit where spin gravity can match Earth's, but the shared idea is that the belt's resources could support whole communities far from our planet.
The proposal also belongs to a longer tradition of rotating space colonies, from Gerard K. O'Neill's cylinder designs in the 1970s to more recent studies. What makes the Ceres version distinctive is its focus on nitrogen, on growth over time and on using a dwarf planet's weak gravity to make construction cheaper.
Sources: Janhunen, "Terraforming the dwarf planet" (arXiv:2011.07487); full paper (PDF); Wikipedia: Ceres (dwarf planet); Futurism.
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