The International space station, a testament to humanity’s ingenuity, orbits at 7.66 kilometers a second, taking it ninety minutes to completely circle the earth. The higher you go above the Earth, the less pull you feel from Earth’s gravity, but even for the space station, it still experiences nearly ninety percent of Earth’s gravity — enough so that the orbital velocity has to be blindingly fast in the sideways direction to avoid being pulled down to Earth. For astronauts on the station, they experience what feels like no gravity but is actually just free fall. Free fall is defined as a state where an object only feels the acceleration due to gravity.
To explain free fall, first let’s look at the orbit: the acceleration due to Earth’s gravity is perpendicular to the space station’s velocity, and thus only changes the direction of the velocity. This mechanism is what causes the circular (or elliptical) orbit. However, because the space station’s speed nearly equals the speed of its fall, astronauts end up experiencing free fall, which gives them that weightlessness sensation. The station and all inside are essentially free falling at a tremendous speed around Earth. It’s like a never-ending amusement park ride!
This state of free fall is often called microgravity due to astronauts perceived sense of little to no gravity. As awesome as the idea of floating around a space station seems, our body is primed for gravity, and a lot of biological processes rely on its presence for them to work correctly. Thus, the weightless environment of the station is detrimental to astronaut’s bodies. Muscles and bones tend to atrophy faster up in space, and thus astronauts have a very rigid exercise routine to try to lessen that effect. Problems with the heart can also happen, and due to the redistribution of fluids away from extremities — causing a puffiness in parts of the body like the face — over time this may cause cardiovascular issues. Also, some astronauts experience nausea, disorientation, and other forms of discomfort — it’s often called adaptive space sickness, and it’s their bodies trying to adjust to the perceived lack of gravity. If they vomit, the fluid would float in spheres in front of their face, and if they’re in a space suit, this could result in them choking to death. Also, exposure to space radiation is increased, and even with radiation shielding on the station, any space walks would expose the astronaut, and if they do numerous space walks for prolonged periods of time, could put them in danger of radiation sickness. Due to the effects on the body – many quite negative – astronauts are rotated off the station after a predetermined amount of time — generally around two months. These painful and often disorienting effects worsen the longer a person stays in a weightless environment, which is why there’s a limit on how long anyone can stay on the station.
Other problems include fluid storage since fluids will form spherical blobs that float, thus a leak could end up with blogs of fluid floating in all directions. Much harder to clean up, compared to a spill in a gravity environment, where all the fluid ends up on the floor. Because gravity plays a huge role in the maintenance of our bodies, gravity on a space station could be beneficial, especially for long-term living in space. So, for writers who wish to use space stations in their stories, especially for living areas, it would be best to create one with gravity or at least with a fairly close simulation of gravity.
One way to do this is to build a cylindrical or wheel-like station, where the wheel (or the entire cylinder) rotates. The centrifugal force created from that rotation, which is always perpendicular to the direction of motion, will push people against the outer rim of the wheel; unlike gravity, it always pushes away from the center. This will provide the body with a perceived sense of gravity. It’s not actually gravity, just an illusion of it, but the important part is that it provides a force that pushes us against the rim; this allows a person to walk without floating, and it tricks the body into thinking it’s in a gravity field. However, there’s several major differences between a rotating space station’s simulation of gravity and an actual gravity field.
First, objects will not fall straight down like they would in a gravity field. Instead they will move in a curve due to an effect known as the Coriolis effect.
The Coriolis effect is an force that appears in rotational frames of reference, and it forms at right angles to the motion and axis of rotation; it curves objects and fluids in the direction opposite the station’s spin. This force actually exists on any rotating body, including our Earth, but because of Earth’s size, you can’t see the Coriolis force on a small scale — it’s only apparent on a large scale such as in the ocean or atmospheric currents.
For a person walking on a rotating space station, the Coriolis force would feel like a force either pushing them toward the axis of rotation or away from it, depending on the person’s distance from the actual axis. This effect is most profound within the person’s inner ear, and it can cause dizziness and nausea. Although produced through a different effect, it’s still somewhat similar to what weightlessness can induce: dizziness, nausea, and other vertigo induced sickness.
How can we remedy this? The best way is to adjust the speed of rotation. For a station that’s about the size of the International Space station (length equal to about a football field), most theories suggest two revolutions per minute (rpm) may be optimal to create a small amount of “gravity” and to avoid the worst of the Coriolis effects; however, this is only a tiny fraction of Earth’s gravity. Also, these same theories predict that if the rpm goes above 7, little to no people could adjust, and the station would be unlivable. In order to simulate 1 g (Earth’s gravity), the space station would have to revolve at 30 rpm, but that’s too much for the body to handle in a small station! Thus, in order to have the optimal 2rpm, the station would have to be approximately 447 meters in diameter. How large is that? Five American football fields. This would result in a very massive station, which could prove detrimental, especially if the station needed to alter its orbital path.
The more mass a station has the more fuel needed for movement of any kind — the fuel amount increases exponentially when you add more mass. For a very massive station, the fuel necessary would quickly reach outrageously large levels, especially since fuel also has mass and would add to the mass of the station, thus requiring more fuel. This exact problem is also one of the main reasons why spaceships need to have the smallest amount of mass that’s feasibly possible for that mission. In contrast to a spacecraft, most adjustments on the station would probably be devoted to avoiding orbital decay. Orbital decay results from either collisions with other objects (regardless of how large or small they are, though the smaller the object is the less damage) or from the atmospheric drag from the planet itself — this results from increased sun activity, which heats and expands the upper atmosphere. The gas in our atmosphere resists the velocity of the station, causing a drag force that pushes against the station, slowing it a bit. To compensate, adjustments to the orbit is necessary, and this often requires fuel.
As a side note, an interesting result of the Coriolis effect is that anything dropped in a rotating station, it will drop in a slight curve (larger curve if the rpm is high). Nothing will ever drop straight down like in a gravity field, where gravity pulls the object toward the center of mass. Another interesting effect is the perceived gravity will differ the further you are from the outer rim of the station; those who stand directly on the rim, their feet would feel a higher percentage of the perceived “gravity” than their head. Those that climb a ladder to reach a platform a few meters above the floor of the outer rim would perceive less “gravity” than those who stood directly on the rim. The smaller the station is, the more pronounced these effects.
Is there other theories for gravity “machines?” Ones that avoid problems like the Coriolis effect?
One idea is to put an incredibly dense and massive object in the center of the station or spacecraft. This would create its own gravity field — as in natural gravity. An advantage to this idea is that it would result in a real gravity field, and thus we’d eliminate some of the above quirks and disadvantages of a rotating space station. However, the addition of a massive object would require an outrageous amount of fuel in order move the station into higher (or lower) orbits or for orbital adjustments. Finding a massive object that would fit in the space station, and provide a gravity field somewhat similar to Earth’s, would be difficult. It’d have to be incredibly dense in order for the large mass to fit in a small enough area for storage at the center of the station; however, to produce a state of matter that dense requires either extremely high pressure or extremely low temperatures– thus a containment field would be absolutely necessary in order to hold the object safely. This isn’t easily done with our current technology. Also, the amount of energy needed to maintain that state is quite a bit unfeasible for the creation of a gravity field suitable for long-term habitability.
For a space station, simulating gravity or producing it’s own gravity field would be essentially for long term survival for anyone living on that station. As of right now, with our current technology, we’d be able to pull off the rotating space station, but as for one that produces it’s on gravity field? That’s unlikely at the moment. However, that doesn’t mean we can’t speculate about it in science fiction.