Floating in the microgravity environment of the space station, astronauts cannot distinguish up from down due to the absence of gravity, as they would on Earth. Therefore, they have to employ various orientation techniques.
For humans, the ability to orient themselves is crucial. Getting lost on a flat surface is already troublesome, but not being able to tell which way is up or down is even worse. Typically, gravity ensures that people on Earth do not encounter such issues. But what happens to astronauts in space?
Astronaut Peggy Whitson floating in the International Space Station (ISS). (Photo: NASA)
The International Space Station (ISS) and the Tiangong space station operate at an altitude of about 400 km. With such a small distance compared to the Earth’s radius, gravitational force is nearly 90% of that at sea level. However, astronauts on space stations do not feel this gravitational force because the stations are in free fall. They do not crash down to Earth because they are moving at extremely high speeds. If Earth’s gravity were to suddenly vanish, they would drift off into space.
During training, astronauts experience weightlessness while in free fall on a KC-135 0-G aircraft. This state is maintained once they reach space. Since the entire space station is also in free fall, astronauts do not feel as though they are falling; instead, they float alongside other objects in the station.
However, humans have evolved for millions of years in a gravitational field, so living without gravity can be disorienting. For many, this results in disorientation and nausea.
The environment in orbit used to be referred to as “zero gravity.” But today, experts use the more accurate term “microgravity.” Even when the effects of Earth’s gravity are minimized, everything with mass still has gravitational pull. Therefore, astronauts still experience a very slight gravitational force from the space station and from each other.
These forces are so weak that the human body does not really register them. Furthermore, the space station surrounds the astronauts and pulls them in all directions, even pulling more strongly toward the area with the greatest mass. Consequently, in orbit, they cannot distinguish up from down based on gravitational forces like they can on the ground.
Astronaut Josh Cassada looking through one of the 7 windows of the Cupola. (Photo: NASA).
Interestingly, the human body uses both the ability to sense gravity and acceleration from the inner ear, as well as the capability to recognize relative position to other objects through sight for orientation. Therefore, astronauts, although unable to feel up and down, can still observe visually.
One method to distinguish up from down in the space station is to arrange everything in the same orientation. For example, the ISS is designed so that most of the light comes from one direction. This direction will be considered the ceiling, while the opposite direction is the floor. Notice boards can create confusion if oriented randomly on walls, so they are usually placed in a consistent style. This helps encourage astronauts to regularly maintain their orientation with their heads pointing toward the ceiling.
The second method to distinguish up from down is to rely on the outside. Using the Sun or stars for orientation can be quite complicated, but using Earth is much more convenient. Like the Moon, the ISS always keeps one face fixed toward Earth, but that does not mean it does not rotate. Instead, the station rotates with the same period as it orbits Earth—about 90 minutes per orbit.
Maintaining this fixed orientation provides many benefits beyond offering consistent orientation for astronauts. For example, radio transmission devices do not need to move much within the station. Additionally, the gravitational forces that keep the Moon oriented towards the blue planet also affect artificial satellites. Everything becomes easier if one does not have to fight against those forces.
In 2010, the ISS was equipped with the Cupola—a module with 7 windows that provides a panoramic view of the blue planet. Previously, astronauts had to rely on small portholes for observation. The scattered placement of these portholes throughout the station also helps astronauts identify the direction of Earth, or the direction of “down.”
