Illustration: Stellar Amenities Lab
The crew is the limit
Commercial competition is driving down the cost of launch and opening new options for stations and landers. On longer missions, the limit increasingly becomes the crew. In weightlessness, astronauts lose bone mineral density at about 1 to 1.5 percent per month in weight-bearing bones, muscle and cardiovascular conditioning decline, sleep is disrupted, and some crew members develop changes to the eye and optic nerve. NASA’s Human Research Program tracks these as risks to crew health and performance, and for several of them the underlying mechanisms are still not fully understood.
A year of aging in a month
Astronauts in orbit lose bone at about 1 to 1.5 percent per month, a loss that takes an older adult on Earth about a year. Within weeks they also lose muscle and heart fitness, and many crew members on long missions develop swelling at the back of the eye.
Figure 1. Bone loss in weight-bearing bones. A month in orbit removes about as much bone as a year of aging on Earth.
NASA has formalized these changes in its own risk framework. Its Human System Risk Board tracks 29 crew health risks and grades the evidence behind each one. After sixty years of human spaceflight, most measurements still happen before and after a mission, so habitat layouts, exercise equipment and medical kits rest on small studies and inherited designs.
Why now
In August 2026 the White House National Security Science and Technology Strategy listed crewed spaceflight enablers among its critical and emerging space technologies, and Executive Order 14369 set a national goal for the initial elements of a permanent lunar outpost by 2030. Crews at that outpost will spend months in partial gravity, where the effects on bone, muscle and circulation are still largely unmeasured.
What it means on Earth
Spaceflight unloads the skeleton and muscles and shifts fluid toward the head within days. In compressed form, it reproduces changes that clinicians treat in immobilized, critically ill and aging patients. NASA has used head-down bed rest for decades as the ground analog for these changes, and we treat the link between orbit and Earth as a hypothesis to test with data.
| Change in spaceflight | Related condition on Earth | Clinical measure |
| Bone loss from skeletal unloading, about 1 to 1.5 percent per month at the hip and spine | Osteoporosis; disuse bone loss after immobilization or spinal cord injury | Ultrasound bone markers; head-down bed rest as the ground analog |
| Atrophy of antigravity muscles such as the calf and thigh within weeks | Sarcopenia; muscle wasting in intensive care; recovery after surgery | Ultrasound muscle thickness and cross section; force output |
| Headward fluid shift and slowed or reversed flow in the jugular vein | Venous thrombosis risk; fluid management in heart failure | Doppler ultrasound of the internal jugular vein |
| Swelling of the optic disc and changes behind the eye | Raised intracranial pressure | Optic nerve sheath diameter by ultrasound |
| Loss of strength, coordination and endurance | Frailty and fall risk; grip strength predicts mortality in aging populations | Grip force, left and right asymmetry, fatigue |
Table 1. Each change in spaceflight, the related condition on Earth, and a clinical measure used to compare the two.
Using measures clinicians already apply on Earth, such as ultrasound muscle thickness, optic nerve sheath diameter and grip strength, means each crew measurement has a matched measurement on the ground. Findings from orbit can then be compared directly with bed rest volunteers, rehabilitation patients and aging cohorts.