The constraint
On the International Space Station (ISS) a serious medical event can be met with a deorbit and landing inside a few hours. From the lunar surface, a return needs a launch window, a transit, and a recovery. Apollo 11 lifted off from the surface on 21 July 1969 and splashed down on 24 July, about three days, with no wait for a window; NASA's original Artemis III surface plan, the profile the first landing on Artemis IV inherits, had the crew spend up to five days in lunar orbit after leaving the surface before Orion heads home. Four days is optimistic rather than typical.
That changes the medical philosophy completely. The question is not how to stabilise somebody for evacuation, it is how to treat them in place for long enough that evacuation becomes optional. It also changes the selection criteria: the crew are chosen partly on the absence of conditions that could deteriorate quickly. Two of the four are crew medical officers, as on the station, where two crew members receive about 40 hours of dedicated medical training before a six month mission.
What actually goes wrong in space
The risk list is not the one intuition suggests. Trauma is rare. What dominates is a set of physiological changes that are slow, predictable and cumulative.
Bone and muscle at one sixth g
This is the largest open question in the entire architecture, and it is genuinely unknown.
In weightlessness, crews lost bone mineral density at roughly one to one and a half percent per month in the weight bearing skeleton on the earlier station exercise devices; with the advanced resistive exercise device and good nutrition the loss at hip and spine falls to about 0.3 to 0.5 percent a month, but it does not stop. The Moon is not weightless: at 0.166 g there is a real load, and the question is whether it is enough to arrest the loss, slow it, or barely affect it.
There is no data. No human has spent more than 75 hours at one sixth gravity, Apollo 17's surface stay. Every long duration figure we have comes from orbit, at zero g, and every partial gravity figure comes from animal centrifuge studies. This base is, among other things, the experiment that answers the question.
One hundred and eighty days is not chosen for operational convenience. It is chosen because it is roughly the point at which orbital bone loss data becomes serious, and because it keeps the first crews inside the range where, if partial gravity turns out not to help at all, the losses are still recoverable on return. If the Moon turns out to be protective, later rotations get longer. If it does not, they do not.
Countermeasures
| Countermeasure | Time cost | Evidence | Note |
|---|---|---|---|
| Resistive exercisethe primary one | 1.5 h/day | Strong, from the ISS advanced resistive exercise device (ARED): loss cut to about a third | A flywheel device, because free weights need weight |
| Cycle ergometercardiovascular | 0.5 h/day | Strong | Also the fitness test baseline |
| Bisphosphonatespharmacological | none | Seven ISS crew on weekly alendronate with ARED: the expected loss at spine, hip and pelvis was attenuated | Carried, not routinely used |
| Vitamin D and calciumdietary | none | Strong | No sunlight reaches the skin, inside or in a suit |
| Fresh foodthe greenhouse | none | Indirect | Vitamin content that does not survive storage |
| Circadian lightinghabitat wide | none | Strong | At the rim the sun circles the horizon for months, so the clock is entirely artificial |
Two hours a day of exercise per crew member, two and a half with setup and stowage as the station schedules it six days a week, is eight to ten crew hours across four people, nearly half of the twenty crew hours a day the module table allots to work. It is the single largest scheduled activity on the base after sleep, and it is non negotiable.
Telemedicine, at 2.6 seconds
A 2.6 second round trip is fine for consultation and useless for guidance during a procedure. A flight surgeon on Earth can look at an ultrasound image and discuss it; the station's Advanced Diagnostic Ultrasound in Microgravity (ADUM) experiment showed non physician crew, guided from the ground, producing diagnostic quality scans. They cannot talk somebody's hands through anything time critical.
So the model is asynchronous: the crew medic performs, records everything, and the ground reviews. Diagnostic imaging is uplinked and read on Earth within the hour. Decision support runs locally, on the base, for anything that cannot wait for a reply.
The kit
| Category | Contents | Mass | Note |
|---|---|---|---|
| Diagnosticsimaging and labs | Ultrasound, portable analyser, vitals suite | 42 kg | Ultrasound is the workhorse: no consumables, no radiation, and remote guidance proven on the ISS |
| Traumastabilisation | Airway, haemorrhage control, splinting, sutures | 28 kg | Stabilise for return, not definitive repair |
| Pharmacy180 day supply | Analgesia, antibiotics, cardiac, psychiatric | 34 kg | Shelf life under radiation is the limiting factor |
| Dentaltemporary | Temporary filling, extraction, analgesia | 6 kg | Disproportionately important |
| Countermeasuresexercise | Resistive flywheel, ergometer | 310 kg | By far the heaviest medical item, and the most used |
| Monitoringcontinuous | Dosimeters, sleep and activity | 8 kg | Dosimeters read against the open surface rate, first measured by the Lunar Lander Neutron and Dosimetry (LND) instrument on Chang'e 4 |
| Total | 428 kg | Three quarters of it is the exercise hardware |
The masses are allocations for the concept; NASA's Exploration Medical Capability element has not published a kit mass for a lunar surface stay of this length.
Psychological health
Four people, 372 cubic metres, 180 days, no way home. Every long duration analog reports the same set of stressors: monotony, confinement, the absence of natural light and privacy, and interpersonal friction that has nowhere to dissipate.
The countermeasures are unglamorous and they are designed into the architecture rather than bolted on. Private berths with a door and a light, at the 2.1 cubic metres of the station's crew quarters. The cupola, which in the built increment shares the open surface exposure of every other module, will be the one unshielded volume once the regolith cover is placed, at 13 mSv a year in the radiation document's design case, and is defended on exactly these grounds. Meals taken together, scheduled as a task. The greenhouse, which crews reliably describe as the best place on any analog station. And a circadian lighting schedule, because at the rim the sun circles the horizon for months at a time and the body needs a clock from somewhere.
What is not yet known
Whether one sixth gravity arrests bone loss, slows it, or barely affects it is genuinely unknown: no human has spent more than 75 hours at it, and every long duration figure comes from orbit. The partial gravity effect on the ocular changes seen in orbit is also unknown, and the chart's 1.1 ocular events per rotation is probably low against the orbital record. The evacuation time is a planning figure: Apollo 11 came home in about three days from surface liftoff, NASA's surface mission profile allows up to five days in lunar orbit before the return leg, and no crew has yet been evacuated from the Moon for a medical reason, so the seven to ten day case is an estimate. The expected event counts are scaled from a six person station crew a few hours from a hospital, not from a surface crew days from one. The kit masses are allocations. Pharmacy shelf life under the surface radiation environment, 13.2 micrograys an hour in silicon by the Chang'e 4 measurement, about 60 microsieverts an hour as the dose equivalent its team reported, is the limiting factor on the 180 day supply and has not been tested at that rate.
Increment plan
Increment 1, 30 day rotations
Short rotations with the storm shelter as the only shielding and a trauma focused kit. The physiological risk of a 30 day stay is well characterised from orbital experience.
Increment 2, 180 day rotations
The regolith cover over the modules, the exercise hardware and the complete pharmacy. This is the increment where crew health becomes the limiting subsystem rather than power.
Increment 3, the bone density experiment
Continuous densitometry through a full rotation, which is the first partial gravity data anyone will have. This determines whether rotations three and beyond get longer.
Increment 4, extended rotations
Conditional entirely on what increment three finds. Stated as conditional, because the honest answer today is that nobody knows.
Sources and further reading
Every figure in this document traces to one of these. Agency documents and peer reviewed work first, reporting only where it is the primary record.