Astronauts do get taller in space, though the change is only temporary. When a person is in the microgravity environment of space, the constant downward compression of Earth’s gravity is suddenly removed. This change causes the spine to lengthen, resulting in an astronaut temporarily gaining a measurable amount of height. This effect, known as spinal elongation, typically leads to an increase of 1 to 3 inches, or roughly 3% of an astronaut’s pre-flight stature.
The Physics Behind Spinal Elongation
Height gain centers on the spine’s intervertebral discs, which are the soft, fluid-filled cushions separating each vertebra. On Earth, gravity acts like a continuous press, constantly squeezes these discs. This compression pushes fluid out of the discs, which is why people are slightly shorter at the end of the day than they are in the morning.
In microgravity, this persistent axial loading is eliminated, allowing the discs to relax and draw in additional fluid, much like a sponge soaking up water. As all 23 intervertebral discs expand simultaneously, the cumulative effect results in a noticeable lengthening of the entire spinal column. The spine’s natural S-shaped curves, which help absorb shock and distribute weight on Earth, tend to straighten out in the absence of gravity, contributing to the overall height increase.
Beyond Height: Bone and Muscle Changes
The temporary height gain is one aspect of how microgravity affects the musculoskeletal system, which also experiences long-term changes. On Earth, bones maintain their density by constantly reacting to the mechanical stress of supporting body weight and fighting gravity. Without this mechanical load in space, the body begins to resorb bone tissue faster than it can create new bone, leading to a loss of bone mineral density.
Astronauts can lose approximately 1% to 2% of bone mass per month, particularly in weight-bearing bones like the hips and lower spine. Over a six-month mission, this rate of loss can lead to conditions similar to osteoporosis. Skeletal muscles, which no longer need to work against gravity to maintain posture and move the body, rapidly weaken and shrink, a process called atrophy.
This muscle atrophy can result in a loss of 10% to 20% of muscle mass during a typical space mission. To combat these effects on bone and muscle, astronauts must follow a rigorous, daily regimen of resistance and aerobic exercise, often using specialized equipment that simulates the mechanical loading of gravity. Unlike the passive spinal elongation, maintaining bone and muscle mass requires this active intervention throughout the mission.
The Return Trip: Reversing the Growth
The extra height gained in space is not permanent and begins to reverse almost immediately upon returning to a gravitational environment. As soon as an astronaut is back on Earth, the downward pull of gravity reasserts its influence. This force compresses the spine once again, gradually squeezing the excess fluid out of the intervertebral discs.
The process of height reversal is remarkably quick, with most astronauts returning to their pre-flight height within a short period, often within hours or days. This rapid re-compression of the spine and the re-establishment of the natural spinal curves can sometimes lead to temporary discomfort or back pain for the returning crew member.