Each mark is a habitat placed by its characteristic size on a logarithmic scale. Colour shows feasibility tier; hollow marks are order-of-magnitude estimates. Select a name to open its entry.
Every entry lists who proposed it and when, its size and population, how it makes gravity, light and shielding, and what stands between it and reality.
Every habitat is a power budget: life support, industry, light for crops, and the radiators that dump it all as heat. The ladder shows energy per kilogram from batteries to annihilation, with the kinetic energy of fast travel marked for comparison.
How much collector, fuel and radiator a population needs. Waste heat is radiated at 300 K with emissivity 0.9 (≈ 413 W per m² of one-sided radiator).
Getting there is a rocket-equation problem. Carry your own reaction mass and exhaust speed decides everything; leave it behind (sails, beams, launchers, tethers) and power, track length or material strength decides instead.
Mass ratio m₀/m₁ for a chosen Δv, using the relativistic form m₀/m₁ = exp[(c/vₑ)·artanh(Δv/c)], which reduces to Tsiolkovsky’s at everyday speeds. Rendezvous missions need the Δv twice over (relativistically, twice the rapidity).
| System | Exhaust speed | Mass ratio | Propellant share | Verdict | Lifts off Earth? |
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Sort by any column. Area is shown as multiples of Earth’s total surface (510 million km²).
Every proposal, paper, novel and milestone cited on this page — habitats, power sources and propulsion — in order.
Marshall T. Savage’s The Millennial Project: Colonizing the Galaxy in Eight Easy Steps (1992; second edition 1994 with an introduction by Arthur C. Clarke, illustrated by Keith Spangle) strings several habitats on this page into one programme. Each step is meant to produce the money, technology and people for the next, over a thousand years. The bar on each step shows how far from home it reaches.
Critics noted that the book is dense with engineering detail (OTEC plants, spirulina yields, asteroid tables) but says little about how governments and societies would change to carry it out; Savage replied that one book cannot script a millennium. Its population argument is Malthusian, and its motive rests on reading the Fermi paradox as evidence that we are alone and must spread life ourselves. Its weakest technical link is the transparent water-filled membrane hull that Asgard, Avalon and the Solaria colonies all depend on.
Almost every habitat here makes gravity by spinning. Radius fixes the spin rate, and the spin rate fixes both comfort and the material strength needed to keep the rim from flying apart: a free-spinning hoop needs specific strength σ/ρ ≥ g·r, before payload and safety margin.
| Material | σ/ρ (J/kg) | Largest radius at these settings | Holds? |
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The ring turns at the real rate for this radius. A person on the rim is drawn to scale when the radius is small enough to see one.
Each habitat gets a tier for the hardest thing it needs, and a 0–10 score within the scale of this page. The score is a judgement, not a measurement; the tier is what matters.