How Gravity Affects Astronauts

The development of the human body was shaped by the conditions on Earth’s surface, including gravity. On Earth, muscles and bones grow strong by working against gravity. Fluids in the body, mainly blood, are pulled down­ward by gravity.

Astronauts in orbit are pulled by gravity almost as strongly as they are on Earth, but the effect of gravity vanishes in orbit because the astronauts and their spacecraft are in a state of free fall. The combination of their speed with the downward pull of gravity means that their curving fall exactly matches the curve of the Earth’s surface. Astronauts in their spacecraft fall without getting any closer to the ground. This effect explains why astronauts float in space.

Gravity tells us which way is up and which way is down. Down is the direc­tion in which gravity pulls us, so down is toward the center of Earth. Up is the opposite direction. When the effect of
gravity is removed, up and down have no meaning. Astronauts sometimes have to take a moment to figure out which is the floor and which is the ceiling because they lose their sense of up and down. When they are in space, they can work, eat, or sleep just as comfortably with their heads pointing at the floor as any other way.

An astronaut’s body is affected by spaceflight. Without gravity to push against, muscles waste away and bones lose calcium. Fluids that are normally pulled downward spread out through the body, making astronauts’ faces fatter and their legs thinner. The balance mechanism in the ear does not work properly, so astronauts can feel dizzy and sick for the first days of a mission.