Floating in the microgravity environment of the space station, astronauts cannot distinguish up and down thanks to gravity like on the ground, so they have to apply many orientation measures.
For humans, the sense of orientation is important. Getting lost on a flat surface is troublesome, but not being able to tell up from down is even worse. Normally, gravity helps ensure that humans on Earth don’t have such problems. But what happens to astronauts in space?
The International Space Station (ISS) and Tiangong operate at an altitude of about 400 km. At such a small distance compared to the radius of the Earth, the gravitational force is about 90% as strong as it is at sea level. However, astronauts on the space stations do not feel this gravitational force because the stations are in free fall. They do not crash into the ground because they are moving so fast. If Earth’s gravity suddenly disappeared, they would be flung into space.
During training, astronauts experience weightlessness while free-falling in a KC-135 0-G aircraft. Once in space, this state is maintained. Since the entire space station is in free-fall, astronauts do not feel like they are falling but are floating with other objects in the station.
However, humans have evolved within a gravitational field for millions of years, so living without one can be uncomfortable. For many, this leads to disorientation and nausea.
The environment in orbit was previously called “zero gravity.” But today, experts use the more accurate term “microgravity,” because even though Earth’s gravitational influence is nullified, everything with mass has gravity. So astronauts still experience very small gravitational forces from the space station and from each other.
These forces are so weak that the human body doesn’t really register them. Furthermore, the space station wraps around the astronaut and pulls him in all directions, even more strongly toward the side with the most mass. So in orbit, they can’t tell up from down by gravity like they can on the ground.
Astronaut Josh Cassada looks through one of the Cupola’s seven windows. Photo: NASA
Interestingly, the human body uses both the inner ear’s ability to sense gravity and acceleration and the eye’s ability to record position relative to other objects to orient itself. Therefore, although astronauts cannot feel up and down, they can still observe with their eyes.
One way to differentiate up and down in a space station is to have everything oriented in the same way. For example, the ISS is designed so that most of the light comes from one direction. That direction would be the ceiling, and the opposite direction would be the floor. Message boards on the walls would be confusing if they were oriented randomly, so they are often placed in the same way. This encourages astronauts to keep their heads up toward the ceiling most of the time.
The second way to tell up from down is externally. Using the Sun or stars for orientation would be a hassle, but using the Earth is much more convenient. Like the Moon, the ISS always keeps the same side facing Earth, but that doesn’t mean it doesn’t rotate. Instead, the station rotates with the same period it orbits the Earth—about once every 90 minutes.
Maintaining a fixed face has many benefits beyond providing consistent orientation for astronauts. For example, radio transmitters don’t need to move around the station as much. Plus, the same gravitational forces that keep the moon facing the planet also act on the satellite. It’s easier not to try to fight those forces.
In 2010, the ISS was equipped with the Cupola – a module with 7 windows providing a panoramic view of the blue planet. Previously, astronauts had to rely on small windows to observe. The fact that these windows are scattered in many places on the station also helps astronauts know the direction of the Earth, or the direction “down”.