October 2026 · Technical note

Simulation as the Ultimate Test Ground

Every countermeasure, habitat and treatment for the Moon and Mars has to be tested somewhere first. We are building that place: a simulation of the human body, trained on data from ultrasound imaging and spacewalk training, and extrapolated to gravities few people have ever lived in.

01 · The problem

There is nowhere on Earth to test for partial gravity

Twelve people have walked on the Moon, for a few days each. No one has lived in Martian gravity. Programs heading to a permanent lunar base will design habitats, exercise and medical care for conditions that have almost no human data behind them.

The test grounds that exist each hold one piece of the problem. A parabolic flight produces partial gravity for seconds at a time. A bed rest study removes load for weeks but keeps Earth gravity. A station gives months of weightlessness to a handful of people a year. None of them can be run at 0.17 g or 0.38 g for months, with enough people to see the range of how bodies respond.

Test groundGravityDurationVolume

Parabolic flight0 g, 0.17 g, 0.38 gSeconds per parabolaA few flights a year

Head-down bed rest1 g, unloaded posture60 to 90 days12 to 24 people per study

Station crew studies0 gAbout six monthsA handful of people a year

Rodent unloading1 g, unloaded limbsWeeksHigh, but not human

SimulationAny value from 0 g to 1 gHours to run a missionUnlimited runs

Three test conditions shown as samples: Earth at 1 g, Mars at 0.38 g, the Moon at 0.17 g — Fig. 1. Three gravities, one body. Today only the first can be studied for long.

02 · The data

Two instruments that measure the body as it works

A simulation is only as good as what it learns from. We are building it on two streams of data that the Lab already works with, chosen because both can be collected repeatedly from the same person, on Earth and in flight.

Stream AUltrasound imagingMuscle thickness, cortical bone, heart function, blood flow and the optic nerve sheath, measured again and again without radiation. A record of how tissue changes over weeks and months.

Stream BSpacewalk trainingA haptic training rig that loads the body like a spacesuit task: force, reach, joint torque, fatigue and timing, recorded on every session. A record of how the body performs under load.

Ultrasound shows what the body is. Training data shows what the body can do. Together they connect a change in tissue to a change in capability, which is the question a mission planner actually asks.

Scattered measurements converging into a single model — Fig. 2. Every scan and every training session becomes a data point in the same model.

03 · The method

From one gravity to every gravity

Step 1MeasureCollect ultrasound and training data on Earth, in analogs and in flight.

Step 2FitLearn how each tissue responds to load, per person and across people.

Step 3ExtrapolateTreat gravity as a dial and predict 0.38 g, 0.17 g and 0 g.

On Earth every measurement is taken at 1 g, but the load on a tissue still varies: rest, exercise, bed rest, training under added or reduced force. The model learns how bone, muscle and the heart respond across that range, then predicts what happens at loads Earth cannot produce for long.

Each prediction carries its uncertainty. Where the data is dense, close to 1 g and close to 0 g, the range is narrow. In between, at lunar and Martian gravity, it widens, and that is exactly where the next measurement is worth the most. The model tells us which flight, analog or training session to run next.

Predicted response across gravity from 0 g to 1 g, measured points in blue, predicted ranges in grey — Fig. 3. Illustrative. The range is narrowest where data exists and widest at lunar and Martian gravity.

04 · The use

What can be tested before anyone flies

Countermeasures. How much exercise, of what kind, keeps bone and muscle at a given gravity, and for how long a crew can stay before the loss matters.

Habitat and tasks. Whether a layout, a ladder or a surface task is safe for a body that has spent three months at a sixth of Earth's gravity.

Treatments. Which drug, dose and timing to carry into a scarce bed rest or flight slot, instead of finding out there.

Bone is the clearest example. A bisphosphonate combined with exercise reduced bone loss in station crews (LeBlanc et al., Osteoporosis International, 2013). Amgen and UCB flew mice treated with a sclerostin antibody on the last Space Shuttle mission in 2011, and a sclerostin antibody, romosozumab, was later approved for osteoporosis. Each of those studies took years and a flight. A simulation lets the same question be asked first, at any gravity, and sends only the strongest candidates to flight.

05 · The limits

What the model cannot do yet

A model trained mostly at 1 g and 0 g can be confidently wrong in between. We treat every partial gravity prediction as a hypothesis to be tested, not a result, and publish the uncertainty with it.

Bodies also differ. Sex, age, genetics and fitness change how a person responds to unloading, and the early datasets will be small. The model will be most useful first for ranking options, which countermeasure or which dose is better, and only later for predicting a single person’s outcome.

For the wider argument, see Simulation and the Next Era of Human Spaceflight.