By Don Lincoln, Ph.D., University of Notre Dame
Video clips of astronauts on the International Space Station doing flips and all sorts of things makes it very clear that they are working in a place where there is no gravity. There’s only one problem. That this is not even close to being true. Is there gravity inside the International Space Station? Then how do we explain what we see in the videos?
(Image: NASA/Public domain)
Weight of a Person on Space Stations
Let’s work it out. The radius of the Earth is about 6,400 kilometers and the International Space Station orbits about 400 kilometers straight up, or at an orbital radius of about 6,800 kilometers.
Remember that Newton’s law of gravity says that the force between two objects, in this case a person and the Earth, is equal to G times the person’s mass times the Earth’s mass divided by the squared distance between the person and the center of the Earth.
Now to show you that gravity does exist on the space station, we can look up the numbers or we can be smart and set the problem up as a simple ratio. We want to know the weight of a person on the surface of the Earth compared to the weight of a person in the space station, there are some constants in the problem. The mass of the person and the Earth doesn’t change and neither does G. So, we don’t have to worry about those things. Instead, we remember that a weight is a force.
What we can do is we can set-up the two equations in the following way. Let’s take a person with a weight of 150 pounds on the surface of the Earth and let’s have our unknown as the weight of a person on the International Space Station. The two equations will be the 150 pounds equals G times the masses divided by the 6400 kilometers squared and the unknown weight on the space station being the same G times masses divided by 6800 kilometers squared.
So, we can take the ratios and the G and masses cancel out and we get the weight on the space station divided by 150 pounds equals 6400 divided by 6800 all squared. Doing the arithmetic, we find that a person who weighs 150 pounds on Earth will weigh 133 pounds on the space station.
It’s incorrect to say that they won’t weigh anything. So how do we understand what we see with our own eyes? The astronauts definitely look like they are weightless. Or are the conspiracy theory crew right after all and it’s all a fake? No, definitely not. We shouldn’t have even asked such a ridiculous question. But, what’s the explanation?
This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, Wondrium.
The International Space Station is in Free Fall
Believe it or not, the explanation is that both the space station and the astronauts are literally falling. If you stopped the space station in its orbit or, if it was just lifted straight up 400 kilometers right after it was originally built, it would fall straight back to Earth, much like Alan Eustace or Felix Baumgartner did when they jumped off a platform 40 kilometers above the Earth’s surface.
The space station and everything in it is also moving sideward. So, what is really happening is that the station is constantly falling toward the Earth, but it’s constantly missing.
Now this way of thinking isn’t new. It’s something that Isaac Newton came up with. He thought about shooting a cannonball horizontally. As you know, it will fly horizontally as it falls. Shoot it faster and it will go further. Shoot it even faster and it will go further still. Eventually, he reasoned, the ball will go fast enough that the curvature of the Earth comes into play and the ball will be able to orbit the Earth. The same thing is going on with the space station and the astronauts. They are constantly falling and missing the Earth.
Learn more about the myths of orbital motion.
Zero Gravity or Free Fall?
In fact, the correct word you should use is that the astronauts are not in zero gravity, but rather in free fall. They are most definitely not in zero gravity. As far back as Galileo, we realized that objects of different mass fall at the same speed. So, the space station and the astronauts fall together.
An astronaut once told us about certain unexpected things they encounter in space. He taught us something that takes this whole free fall understanding to an even higher level. He said that if you sat completely still in the shuttle, you would slowly drift forward toward the nose of the ship.
That’s because the shuttle was in low enough orbit that there was a little air drag. Even though the space shuttle was mostly in free fall, the air slowed the shuttle down just a little bit. The astronauts were shielded from the drag and so they didn’t slow down and the net effect is that if you sat there for a while, you’d drift towards the front of the spacecraft.
Of course, you generally don’t need to worry about such little things. One of the brilliant features of science is the ability to simplify a problem by ignoring tiny effects. But if you want to know how a precise answer, eventually ignoring those things will lead you astray and you’ll end up believing something not quite right, like the statement that planetary orbits are perfect ellipses. It just goes to show you that there’s always something to learn in science.
Learn more about what the world gets wrong about science.
Common Questions about Gravity in Space Stations
Yes, there is gravity on the International Space Station. Based on Newton’s law of gravity, which says that the force between two objects, in this case a person and the Earth, is equal to G times the person’s mass times the Earth’s mass divided by the squared distance between the person and the center of the Earth, we can calculate that a person who weighs 150 pounds on Earth will weigh 133 pounds on the space station.
The reason astronauts float inside the space station is that both the space station and the astronauts are in free fall. As far back as Galileo, we realized that objects of different mass fall at the same speed. So, the space station and the astronauts fall together, which creates the illusion of zero gravity or weightlessness.
The basic concept of free fall was first demonstrated by Isaac Newton with the canonball experiment, in which, theoretically, when the canonball is shot out fast enough to bring the curvature of the Earth into play, the ball will be able to orbit the Earth. The same thing happens with the space station and the astronauts. They are constantly falling and missing the Earth.
Yes, astronauts are in free fall on space stations. There is gravity on space stations, and that’s the only force acting on astronauts. Since the only force working on the space station itself is also gravity, both the astronauts and the space station are in free fall together.
