Astronauts returning from a six-month ISS tour lose ~1–2% of bone mass per month and need a year of rehab. A future Mars crew might come home unable to stand. What if instead of immediately throwing them into 1g, we transitioned them through a building-sized centrifuge that simulates Moon gravity (0.165g) — letting bones and cardiovascular systems re-adapt gradually?

The geometry problem. Centrifugal acceleration is a = ω²r. To feel comfortable, the rotation rate should stay below ~2 rpm — above that, the Coriolis force on head movement triggers brutal vertigo (NASA's tolerance studies put the hard wall around 3 rpm). At 2 rpm, ω = 0.209 rad/s, so for a = 1.62 m/s² (lunar g):

r = a/ω² = 1.62 / 0.0438 ≈ 37 meters

That's a wheel 74 m in diameter — roughly a 25-story building laid on its side. Push to 3 rpm and radius drops to 16 m, but you'll have nauseous patients. Call it a 40 m radius drum for margin.

Structural loads. The rim moves at v = ωr ≈ 8.4 m/s — a fast jog. Hoop stress in a thin rotating ring is σ = ρv². For a steel deck (ρ = 7850 kg/m³), σ ≈ 550 kPa, which is laughable — 1/700th of structural steel's yield. The centrifuge isn't stress-limited; it's moment-limited. The whole 40 m wheel has to spin without wobble, which means precision bearings the size of subway tunnels and active balancing as patients walk around inside (a 70 kg person moving 5 m off-axis creates an unbalanced moment of ~350 kg·m).

Power budget. A 40 m radius habitat ring 8 m wide and 4 m tall, built from aluminum and composites at ~500 kg/m² of floor, masses about 2π × 40 × 8 × 500 ≈ 1,000 tonnes. Plus patients, equipment, water, air: call it 1,500 tonnes. Moment of inertia I ≈ MR² = 1.5×10⁶ × 1600 = 2.4×10⁹ kg·m². Spinning up to 2 rpm in 30 minutes requires:

τ = I·α = 2.4×10⁹ × (0.209/1800) ≈ 280 kN·m
P_peak = τω ≈ 58 kW

Trivial. Steady-state power is just bearing friction and air drag — maybe 100 kW, less than a single MRI machine. The whole facility could run on a rooftop solar array.

What it actually does to bones. Wolff's law says bone remodels to mechanical load. Bedrest studies (the standard Earth analog) show bone loss begins reversing at as little as 0.3–0.5g of axial loading for a few hours daily. Lunar 0.165g is below that threshold, so you'd need to graduate patients: start at 0.3g (radius 67 m at 2 rpm, or 30 m at 3 rpm), then ramp. Better design: a variable-radius track where patients walk inward or outward to dial their effective gravity, like an exercise prescription written in geometry.

The catch. Coriolis. At 2 rpm, dropping a coffee cup from chest height makes it land 4 cm sideways. Pouring water curves visibly. Patients adapt in days — Skylab and ground studies confirm it — but the rehab facility itself becomes a slightly surreal place where stairs are subtly canted and you can't trust your inner ear during the first week.