Have you ever wondered why astronauts seem to grow in space? It’s not an optical illusion—astronauts really do become taller while in space, typically gaining about 2 inches (5 cm) in height. This intriguing change shows how deeply Earth’s gravity shapes our bodies.
Your Spine Under Pressure
On Earth, gravity constantly compresses our bodies. Our spine is made up of 33 vertebrae separated by water-filled cushions called intervertebral discs that act as natural shock absorbers. As you move under Earth’s pull, these discs compress a little, which is why you might be up to half an inch shorter by evening than at dawn.
When you lie down at night, the pressure eases, and by morning you regain your full height. This daily change is normal—but what happens when gravity is nearly absent?
Zero Gravity, Maximum Growth
In microgravity environments, such as the International Space Station, the pull of gravity disappears. Without gravity’s compression:
- Intervertebral discs expand as they absorb extra fluid
- The spine’s natural curve straightens somewhat
- Postural muscles relax since they no longer battle against gravity
This spinal elongation happens quickly. Astronauts often feel the stretch within a few days in orbit, sometimes experiencing back discomfort as their spines adjust.
“It’s like someone is stretching you from both ends,” said NASA astronaut Mike Hopkins after his 166-day mission. “My height increased by about two inches.”
The Fluid Shift Factor
Spinal expansion isn’t the only reason astronauts appear taller. In microgravity, fluids that normally settle in the lower body shift upward, a process known as “fluid shift.” This contributes to what astronauts call “puffy face and bird legs” syndrome—where the face swells while the legs thin out.
This fluid redistribution not only adds to the height increase but also alters pressure in various body systems, even affecting vision, which scientists are still trying to fully understand.
The Return to Earth: Shrinking Back
The height gain is temporary. Once astronauts return to Earth, gravity compresses their spines again, and most shrink back to their original height within a few days, though the adjustment can be uncomfortable.
NASA astronaut Scott Kelly, who spent a year in space while his identical twin brother stayed on Earth, provided researchers with valuable data. During his mission, Kelly grew about 1.5 inches taller than his Earth-bound brother, only to return to his normal height shortly after landing.
Beyond Height: Why It Matters
This height change isn’t just a curiosity—it has practical implications for space travel:
- Spacesuits need to fit astronauts who grow taller in space
- Spacecraft seating must be designed to accommodate returning astronauts
- For long-duration missions, such as trips to Mars, understanding these changes is even more critical
Research into spinal elongation also benefits Earth-bound medicine by offering insights into spine health, disc degeneration, and potential treatments for back conditions.
The Future of Space Physiology
As humans plan to settle on the Moon or Mars, it becomes increasingly important to understand how different gravitational forces affect our bodies. Would people living on the Moon (with 1/6 of Earth’s gravity) still experience some spinal elongation? How would children born in such environments develop?
Our bodies have evolved under Earth’s gravity for millions of years. By studying how they react in microgravity, we gain valuable insights into our biology and the forces that have shaped human physiology over time.
The next time you look up at the International Space Station streaking across the night sky, remember that the astronauts inside aren’t just floating—they’re also growing taller with every orbit, experiencing a brief yet remarkable transformation.
