When Bones Float Away: Why Astronauts Lose Bone Density in Space

Space travel captures human imagination like few other achievements. Yet living in space presents serious physiological challenges, one of the most significant being rapid bone density loss. Understanding why this happens not only protects astronauts on long missions but also helps scientists better understand osteoporosis and bone health here on Earth.

What Happens to Bones in Space?

Bones are living tissues that constantly remodel themselves in response to mechanical stress. As we grow, exercise and gravity place loads on our skeleton, stimulating bone formation and maintaining density.

The bones most affected by gravity — particularly the long bones of the legs, pelvis and lower spine — contain large amounts of trabecular (spongy) bone. This porous structure is lightweight but strong, designed to absorb mechanical stress efficiently.

In microgravity, however, these loads are greatly reduced. The body interprets this as excess bone no longer being needed and begins breaking it down. Astronauts can lose roughly 1–2% of bone mass per month, particularly in weight-bearing regions such as the hips.

Understanding Microgravity

Despite the term “zero gravity”, astronauts actually experience microgravity, not a complete absence of gravity. The International Space Station and its crew are in continuous free fall around Earth, creating the sensation of weightlessness.

On Earth, gravity constantly stimulates bones through standing, walking and movement. In microgravity, this stimulation largely disappears, which disrupts normal bone maintenance.

Why Does Bone Loss Occur?

Bone remodelling involves three main cell types:

• Osteocytes – embedded bone cells that sense mechanical strain

• Osteoclasts – cells that break down bone tissue

• Osteoblasts – cells that build new bone

• 

  In microgravity:

  • Osteocytes detect reduced mechanical loading

  • Osteoclast activity increases (more bone breakdown)

  • Osteoblast activity decreases (less bone formation)

This imbalance leads to net bone loss.

Research also suggests increased activity of the signalling molecule RANKL (receptor activator of nuclear factor-κB ligand) in microgravity, promoting osteoclast formation and accelerating bone resorption.

Put simply: without regular mechanical stress, the body shifts from maintaining bone to recycling it.

How Do Space Agencies Reduce Bone Loss?

High-Intensity Exercise

Astronauts aboard the ISS exercise for about 2–2.5 hours daily using specialised equipment such as:

• Advanced Resistive Exercise Device (ARED)

• Cycle Ergometer (CVIS)

• Treadmill with harness systems

These simulate weight-bearing activity to maintain bone and muscle health.

ARED Machine

CVIS Machine

Nutritional Countermeasures

Diet plays an essential role in skeletal maintenance. Astronauts focus on:

• Calcium for bone mineralisation

• Vitamin D for calcium absorption

• Omega-3 fatty acids, which may reduce bone resorption

Adequate nutrition supports osteoblast activity and helps maintain skeletal integrity.

Pharmacological Treatments

For longer missions, exercise and diet alone may not fully prevent bone loss. Some astronauts use medications such as:

• Alendronate

• Zoledronic acid

Conclusion

Spaceflight-induced osteopenia remains a significant challenge in human space exploration. Reduced mechanical loading disrupts the balance between bone formation and resorption, leading to rapid bone loss.

Through exercise, nutrition, medical interventions and ongoing research, space agencies continue working to safeguard astronaut health. As missions become longer and more ambitious, understanding and mitigating bone loss will be essential — not only for space travel, but also for advancing bone health here on Earth.

SOURCES

• NASA Human Integration Design Handbook (Bone Loss in Space)

• European Space Agency (ESA) — Bone Loss and Countermeasures in Microgravity

• LeBlanc et al. (2007). Bone mineral and lean tissue loss after long-duration space flight

• National Institutes of Health (NIH) — Osteoporosis and Bone Biology resources

• Royal Osteoporosis Society (UK) educational materials