Microgravity
& the Body

The human body is uniquely designed to live in Earth’s gravity. In space, the body begins to adapt to the microgravity environment.

NSBRI’s science and technology program is addressing ways to reduce or eliminate many of the changes to the body that impact an astronaut’s ability to perform well in space and that might impact their health after returning to Earth. The NSBRI program also looks at ways to enhance countermeasures already in place on long missions.

Here’s a quick introduction to how the body reacts to life in space.

Bones
In microgravity, astronauts no longer walk to get to different parts of the spacecraft, they float. This means that the bones in the lower part of the body that typically bear weight – the legs, hips and spine – experience a significant decrease in load bearing. This reduction leads to bone breakdown and a release of calcium, leaving the bone more brittle and weak. The release of calcium can also increase the risk of kidney stone formation and bone fractures. To put it in perspective, postmenopausal women who are untreated for bone loss can lose 1 to 1.5 percent of bone mass in the hip in one year while an astronaut can lose the same amount of hip bone mass in a single month. On missions outside Earth’s orbit, radiation exposure may also impact bone loss.

Muscles
Extended spaceflight results in less work for the legs and back. As a result, the muscles can begin to weaken or atrophy, and this could lead to fall-related injuries and accidents during exploration missions. Astronauts currently exercise to help maintain their muscle mass, but nutritional interventions designed to reduce the muscle loss may one day be added as a complement to the exercise program.

Fluid Shift
In space, blood is redistributed with a little more in the upper part of the body and a little less in the lower extremities. While in space, astronauts often have a puffy face due to this fluid shift and legs that are smaller in circumference. The fluid shift to the head can also lead to a feeling of congestion.

Cardiovascular System
Although the cardiovascular system generally functions well in space, the heart doesn’t have to work as hard in the microgravity environment. Over time, this could lead to deconditioning and a decrease in the size of the heart. There is also a concern that space radiation may affect endothelial cells, the lining of blood vessels, which might initiate or accelerate coronary heart disease.

The Spine: Taller in Space
Astronauts get a bit taller in space. On Earth, the disks between the vertebrae of the spinal column are slightly compressed due to gravity. In space, that compression is no longer present causing the disks to expand. The result: the spine lengthens, and the astronaut is taller. One possible side effect is back pain that may be associated with the lengthening of the spine.

Inner Ear and Balance System
On Earth, a complex, integrated set of neural circuits allows humans to maintain balance, stabilize vision and understand body orientation in terms of location and direction. The brain receives and interprets information from numerous sense organs, particularly in the eyes, inner ear vestibular organs and the deep senses from muscles and joints. In space, this pattern of information is changed. The inner ear, which is sensitive to gravity, no longer functions as designed. Early in the mission, astronauts can experience disorientation, space motion sickness and a loss of sense of direction. Upon return to Earth, they must readjust to Earth’s gravity and can experience problems standing up, stabilizing their gaze, walking and turning. These disturbances are more profound as the length of microgravity exposure increases. The changes can impact operational activities including approach and landing, docking, remote manipulation, extravehicular activity and post-landing normal and emergency egress.

Sleep and Performance
Many factors – the loss of a 24-hour day/light cycle, a confined environment and work demands – can impact an astronaut’s ability to work well in space. In addition, exploration crews will have to shift their “body clocks” from the Earth day/night cycle to that of their destination. Scientists hope to help the crew increase their alertness and reduce performance errors through improvements to spacecraft lighting, sleep schedules and the scheduling of work shifts.