How Much Bone Mass Do Astronauts Lose in Space?

Astronauts lose roughly one to two percent of their hip and spine bone mass for every month spent in orbit — meaning a single six-month stay aboard the International Space Station can strip away as much bone density as twenty years of normal aging on Earth.

Why Does Bone Density Disappear in Microgravity?

On Earth, bones are constantly loaded by the body’s weight. That mechanical stress is the signal that tells bone-building cells — called osteoblasts — to keep doing their job. In microgravity, that load vanishes entirely. Without it, osteoblasts go quiet. But the cells responsible for breaking bone down, called osteoclasts, keep running at full speed. The result is a one-way street: resorption without rebuilding. The rate of loss actually outpaces what happens on Earth during severe osteoporosis.

How Astronauts Fight Back: Two Hours a Day

To counter this, astronauts aboard the ISS spend roughly two hours each day on a resistive exercise machine capable of simulating up to 600 pounds of load. The Advanced Resistive Exercise Device (ARED), installed in 2008, made a significant difference. Many crews since then have returned to Earth with bone density close to their pre-flight levels — a real improvement over earlier missions. But density is only part of the story.

The Hidden Damage: Bone Structure Doesn’t Fully Recover

Even when overall bone density appears to recover, the bone’s internal architecture — its fine trabecular structure — continues to thin. That microstructural loss doesn’t always come back. A 2022 study tracked seventeen astronauts for a full year after they returned from the ISS. Nine of them — more than half — had still not fully recovered their leg bone density twelve months after landing. The bones looked closer to normal on a scan, but the scaffolding inside remained compromised.

What This Means for a Mars Mission

A round-trip journey to Mars would take roughly three years. At the rates observed on ISS missions, the cumulative bone loss could push astronauts past NASA’s own flight-qualification thresholds — the medical standards that determine whether a person is cleared to fly again. An astronaut physically capable of reaching Mars might be permanently grounded the moment they returned home. The bones that carried them there could be the reason they never flew again.

Can Drugs Help?

The same class of medications used to treat osteoporosis on Earth — bisphosphonates — has been tested on ISS crew members. When combined with the resistive exercise regimen, this approach largely held bone loss in check during ISS-length missions. The biology is not insurmountable. But whether these interventions scale to a three-year deep-space journey, far from medical support and resupply, remains an open and urgent question.

The Bigger Picture

Space medicine has solved, or at least managed, many of the threats the human body faces in orbit. Bone loss is one where significant progress has been made — but not enough for the missions that come next. Rockets can already reach Mars. The skeleton has to keep up.